MedWorm.com provides a medical RSS filtering service. Over 6000 RSS medical sources are combined and output via different filters. This feed contains the latest items from the 'Biometrical Journal' source. Thu, 18 Mar 2010 16:29:45 +0100 http://www.medworm.com/index.php?rid=3346078&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900083 Large contingency tables summarizing categorical variables arise in many areas. One example is in biology, where large numbers of biomarkers are cross-tabulated according to their discrete expression level. Interactions of the variables are of great interest and are generally studied with log-linear models. The structure of a log-linear model can be visually represented by a graph from which the conditional independence structure can then be easily read off. However, since the number of parameters in a saturated model grows exponentially in the number of variables, this generally comes with a heavy computational burden. Even if we restrict ourselves to models of lower-order interactions or other sparse structures, we are faced with the problem of a large number of cells which play the role... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3346078 Mon, 08 Mar 2010 00:00:00 +0100 3346078 http://www.medworm.com/index.php?rid=3346077&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900130 The author compares 12 hierarchical models in the aim of estimating the abundance of fish in alpine streams by using removal sampling data collected at multiple locations. The most expanded model accounts for (i) variability of the abundance among locations, (ii) variability of the catchability among locations, and (iii) residual variability of the catchability among fish. Eleven model reductions are considered depending which variability is included in the model. The more restrictive model considers none of the aforementioned variabilities. Computations of the latter model can be achieved by using the algorithm presented by Carle and Strub (Biometrics 1978, 34, 621-630). Maximum a posteriori and interval estimates of the parameters as well as the Akaike and the Bayesian information criter... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3346077 Mon, 08 Mar 2010 00:00:00 +0100 3346077 http://www.medworm.com/index.php?rid=3301616&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900312 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3301616 Wed, 24 Feb 2010 00:00:00 +0100 3301616 http://www.medworm.com/index.php?rid=3283116&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201090000 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3283116 Wed, 17 Feb 2010 00:00:00 +0100 3283116 http://www.medworm.com/index.php?rid=3283115&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900064 The Cox proportional hazards regression model is the most popular approach to model covariate information for survival times. In this context, the development of high-dimensional models where the number of covariates is much larger than the number of observations ( ) is an ongoing challenge. A practicable approach is to use ridge penalized Cox regression in such situations. Beside focussing on finding the best prediction rule, one is often interested in determining a subset of covariates that are the most important ones for prognosis. This could be a gene set in the biostatistical analysis of microarray data. Covariate selection can then, for example, be done by L1-penalized Cox regression using the lasso (Tibshirani (). Statistics in Medicine 16, 385-395). Several approaches beyond the la... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3283115 Wed, 17 Feb 2010 00:00:00 +0100 3283115 http://www.medworm.com/index.php?rid=3283114&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900057 In order to study family-based association in the presence of linkage, we extend a generalized linear mixed model proposed for genetic linkage analysis (Lebrec and van Houwelingen (2007), Human Heredity 64, 5-15) by adding a genotypic effect to the mean. The corresponding score test is a weighted family-based association tests statistic, where the weight depends on the linkage effect and on other genetic and shared environmental effects. For testing of genetic association in the presence of gene-covariate interaction, we propose a linear regression method where the family-specific score statistic is regressed on family-specific covariates. Both statistics are straightforward to compute. Simulation results show that adjusting the weight for the within-family variance structure may be a powe... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3283114 Wed, 17 Feb 2010 00:00:00 +0100 3283114 http://www.medworm.com/index.php?rid=3246602&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900074 Some personal remarks about Hans van Houwelingen's approach to biostatistics in general are followed by a discussion of his article with Koos Zwinderman and Theo Stijnen outlining a bivariate approach to meta-analysis. It is concluded that this is more radical than many may realise in that it permits inter-trial information to be recovered. This has some advantages but in theory opens the door to bias. It is concluded that in practice the size of this bias is likely to be small. I end with some further personal remarks to Hans. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3246602 Sat, 06 Feb 2010 00:00:00 +0100 3246602 http://www.medworm.com/index.php?rid=3246603&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900039 The Aalen-Johansen estimator is the standard nonparametric estimator of the cumulative incidence function in competing risks. Estimating its variance in small samples has attracted some interest recently, together with a critique of the usual martingale-based estimators. We show that the preferred estimator equals a Greenwood-type estimator that has been derived as a recursion formula using counting processes and martingales in a more general multistate framework. We also extend previous simulation studies on estimating the variance of the Aalen-Johansen estimator in small samples to left-truncated observation schemes, which may conveniently be handled within the counting processes framework. This investigation is motivated by a real data example on spontaneous abortion in pregnancies expo... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3246603 Fri, 05 Feb 2010 00:00:00 +0100 3246603 http://www.medworm.com/index.php?rid=3239035&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900044 Simulation-based assessment is a popular and frequently necessary approach for evaluating statistical procedures. Sometimes overlooked is the ability to take advantage of underlying mathematical relations and we focus on this aspect. We show how to take advantage of large-sample theory when conducting a simulation using the analysis of genomic data as a motivating example. The approach uses convergence results to provide an approximation to smaller-sample results, results that are available only by simulation. We consider evaluating and comparing various ranking-based methods for identifying the most highly associated SNPs in a genome-wide association study, derive integral equation representations of the pre-posterior distribution of percentiles produced by three ranking methods, and prov... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3239035 Thu, 04 Feb 2010 00:00:00 +0100 3239035 http://www.medworm.com/index.php?rid=3283113&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900285 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3283113 Mon, 01 Feb 2010 00:00:00 +0100 3283113 http://www.medworm.com/index.php?rid=3114581&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900076 This study briefly recalls the different types of time-dependent covariates, as classified by Kalbfleisch and Prentice [The Statistical Analysis of Failure Time Data, Wiley, New York, 2002] with the intent of clarifying their role and emphasizing the limitations in standard survival models and in the competing risks setting. If random (internal) time-dependent covariates are to be included in the modeling process, then it is still possible to estimate cause-specific hazards but prediction of the cumulative incidences and survival probabilities based on these is no longer feasible. This article aims at providing some possible strategies for dealing with these prediction problems. In a multi-state framework, a first approach uses internal covariates to define additional (intermediate) transi... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3114581 Wed, 23 Dec 2009 00:00:00 +0100 3114581 http://www.medworm.com/index.php?rid=3088789&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900132 The two-sided Simes test is known to control the type I error rate with bivariate normal test statistics. For one-sided hypotheses, control of the type I error rate requires that the correlation between the bivariate normal test statistics is non-negative. In this article, we introduce a trimmed version of the one-sided weighted Simes test for two hypotheses which rejects if (i) the one-sided weighted Simes test rejects and (ii) both p-values are below one minus the respective weighted Bonferroni adjusted level. We show that the trimmed version controls the type I error rate at nominal significance level [alpha] if (i) the common distribution of test statistics is point symmetric and (ii) the two-sided weighted Simes test at level 2[alpha] controls the level. These assumptions apply, for i... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3088789 Tue, 15 Dec 2009 00:00:00 +0100 3088789 http://www.medworm.com/index.php?rid=3088790&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900082 New therapy strategies for the treatment of cancer are rapidly emerging because of recent technology advances in genetics and molecular biology. Although newer targeted therapies can improve survival without measurable changes in tumor size, clinical trial conduct has remained nearly unchanged. When potentially efficacious therapies are tested, current clinical trial design and analysis methods may not be suitable for detecting therapeutic effects. We propose an exact method with respect to testing cytostatic cancer treatment using correlated bivariate binomial random variables to simultaneously assess two primary outcomes. The method is easy to implement. It does not increase the sample size over that of the univariate exact test and in most cases reduces the sample size required. Sample ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3088790 Mon, 14 Dec 2009 00:00:00 +0100 3088790 http://www.medworm.com/index.php?rid=3070623&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900151 Repeated dose toxicity studies are performed to characterize the toxicological profile of a test compound following repeated administrations. The findings and interpretations from these systemic exposure studies in animals are essential for designing subsequent studies and evaluating the safety of the test item for humans. Blood samples for assessment of systemic exposure are usually collected on day one and at the end of the study with multiple dosings of the compound in between. Restrictions in blood volume often require an incomplete sampling design, in which each animal contributes sample measurements at some but not all time points. In this manuscript we derive an estimator for the ratio of area under the concentration versus time curves (AUCs), a frequently used measure of exposure t... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3070623 Wed, 09 Dec 2009 00:00:00 +0100 3070623 http://www.medworm.com/index.php?rid=3026179&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900028 This article presents a novel algorithm that efficiently computes L1 penalized (lasso) estimates of parameters in high-dimensional models. The lasso has the property that it simultaneously performs variable selection and shrinkage, which makes it very useful for finding interpretable prediction rules in high-dimensional data. The new algorithm is based on a combination of gradient ascent optimization with the Newton-Raphson algorithm. It is described for a general likelihood function and can be applied in generalized linear models and other models with an L1 penalty. The algorithm is demonstrated in the Cox proportional hazards model, predicting survival of breast cancer patients using gene expression data, and its performance is compared with competing approaches. An R package, penalized,... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3026179 Wed, 25 Nov 2009 00:00:00 +0100 3026179 http://www.medworm.com/index.php?rid=3026181&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900030 This article examines this situation with the specific example of HIV dynamics models. We examine the score test statistics for testing the effect of explanatory variables and the variance of random effect in this complex situation. We study type I errors and the statistical powers of this score test statistics and we apply the score test approach to a real data set of HIV-infected patients. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3026181 Tue, 24 Nov 2009 00:00:00 +0100 3026181 http://www.medworm.com/index.php?rid=3026180&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800235 All models for incomplete data either explicitly make assumptions about aspects of the distribution of the unobserved outcomes, given the observed ones, or at least implicitly imply such. One consequence is that there routinely exist a whole class of models, coinciding in their description of the observed portion of the data but differing with respect to their "predictions" of what is unobserved. Within such a class, there always is a single model corresponding to so-called random missingness, in the sense that the mechanism governing missingness depends on covariates and observed outcomes, but given these not further on unobserved outcomes. We employ these results in the context of so-called shared-parameter models where outcome and missingness models are connected by means of common late... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3026180 Tue, 24 Nov 2009 00:00:00 +0100 3026180 http://www.medworm.com/index.php?rid=3007575&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900073 This article concerns the situation where multiple studies have evaluated the same diagnostic test with the same multiple thresholds in a population of non-diseased and diseased individuals. Recently, bivariate meta-analysis has been proposed for the pooling of sensitivity and specificity, which are likely to be negatively correlated within studies. These ideas have been extended to the situation of diagnostic tests with multiple thresholds, leading to a multinomial model with multivariate normal between-study variation. This approach is efficient, but computer-intensive and its convergence is highly dependent on starting values. Moreover, monotonicity of the sensitivities/specificities for increasing thresholds is not guaranteed. Here, we propose a Poisson-correlated gamma frailty model, ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3007575 Thu, 19 Nov 2009 00:00:00 +0100 3007575 http://www.medworm.com/index.php?rid=2966709&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900128 We present a new method to estimate the parameters of a popular semi-parametric ROC model, called the binormal model. Our method is based on minimization of the functional distance between two estimators of an unknown transformation postulated by the model, and has a simple, closed-form solution. We study the asymptotics of our estimators, show via simulation that they compare favorably with existing estimators, and illustrate how covariates may be incorporated into the norm minimization framework. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2966709 Fri, 06 Nov 2009 00:00:00 +0100 2966709 http://www.medworm.com/index.php?rid=2966710&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900038 The problem of constructing a confidence interval for the ratio of two regression coefficients is addressed in the context of multiple regression. The concept of a Generalized Confidence Interval is used, and the resulting confidence interval is shown to perform well in terms of coverage probability. The proposed methodology always results in an interval, unlike the confidence region generated from Fieller's theorem. The procedure can easily be implemented for parallel-line assays, slope-ratio assays, and quantal assays under a probit model. Furthermore, this approach can also be extended to compute confidence intervals based on data from multiple bioassays. The results are illustrated using several examples. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2966710 Thu, 05 Nov 2009 00:00:00 +0100 2966710 http://www.medworm.com/index.php?rid=2889972&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800158 This paper analyzes the power divergence estimators when homogeneity/heterogeneity hypotheses among standardized mortality ratios (SMRs) are taken into account. A Monte Carlo study shows that when the standard mortality rate is not external, that is it is estimated from the sample data, these estimators have a good performance even for small sample sets and in particular the minimum chi-square estimators have a better behavior compared to the classical maximum likelihood estimators. In order to make decisions under homogeneity/heterogeneity hypotheses of SMRs we propose some test-statistics which consider the minimum power divergence estimators. Through a numerical example focused on SMRs of melanoma mortality ratios in different regions of the US, a homogeneity/heterogeneity study is illu... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2889972 Mon, 12 Oct 2009 23:00:00 +0100 2889972 http://www.medworm.com/index.php?rid=2889971&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900025 The evaluation of the antihypertensive effect of multiple antihypertensive drugs using data from an observational study requires adjustment for time-dependent confounders. Marginal structural models (MSMs) have been proposed to address this type of confounding through inverse probability weighting. Generally, the probabilities are estimated using logistic regression models that assume linearity between the logistic link and the predictors, but the linearity might be inaccurate. In this article, we proposed MSMs to assess the blood pressure-lowering effects of combination therapy with olmesartan medoxomil (OLM) plus calcium channel blockers (CCB) (OLM+CCB) in an observational study of OLM, and extended estimation methods of the probabilities for the MSMs using generalized additive models (G... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2889971 Mon, 12 Oct 2009 23:00:00 +0100 2889971 http://www.medworm.com/index.php?rid=2889970&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900056 This article clarifies many of these puzzling results. We show that the neighborhood graph structure, synthesized in eigenvalues and eigenvectors structure of a matrix associated with the adjacency matrix, determines most of the apparently anomalous behavior. We illustrate our conclusions with regular and irregular lattices including lines, grids, and lattices based on real maps. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2889970 Mon, 12 Oct 2009 23:00:00 +0100 2889970 http://www.medworm.com/index.php?rid=2879157&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900210 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2879157 Thu, 08 Oct 2009 23:00:00 +0100 2879157 http://www.medworm.com/index.php?rid=2879156&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900144 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2879156 Thu, 08 Oct 2009 23:00:00 +0100 2879156 http://www.medworm.com/index.php?rid=2875290&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800182 Marginal structural models (MSMs) are an increasingly popular tool, particularly in epidemiological applications, to handle the problem of time-varying confounding by intermediate variables when studying the effect of sequences of exposures. Considerable attention has been devoted to the optimal choice of treatment model for propensity score-based methods and, more recently, to variable selection in the treatment model for inverse weighting in MSMs. However, little attention has been paid to the modeling of the outcome of interest, particularly with respect to the best use of purely predictive, non-confounding variables in MSMs. Four modeling approaches are investigated in the context of both static treatment sequences and optimal dynamic treatment rules with the goal of estimating a margi... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2875290 Wed, 07 Oct 2009 23:00:00 +0100 2875290 http://www.medworm.com/index.php?rid=2827752&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800190 In clinical trials, several endpoints (EPs) are often evaluated to compare treatments in some therapeutic area. Suppose that there are two EPs in a clinical trial. We propose a new set of composite hypotheses for continuous variables, taking the relative clinical importance of the EPs into account. The main hypotheses were formulated to show that a treatment is so superior to the control treatment, which is not necessarily a placebo, in one EP, that the possible non-inferiority of the treatment by at most a certain value in the other EP can be compensated sufficiently, taking the clinical point of view into account. The maximum non-inferiority margin of one EP might not be a biologically unimportant difference in exchange for much superiority of the other EP. This formulation leads to a ne... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2827752 Tue, 22 Sep 2009 23:00:00 +0100 2827752 http://www.medworm.com/index.php?rid=2819215&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900081 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2819215 Sun, 20 Sep 2009 23:00:00 +0100 2819215 http://www.medworm.com/index.php?rid=2803498&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800269 Spatial cluster detection has become an important methodology in quantifying the effect of hazardous exposures. Previous methods have focused on cross-sectional outcomes that are binary or continuous. There are virtually no spatial cluster detection methods proposed for longitudinal outcomes. This paper proposes a new spatial cluster detection method for repeated outcomes using cumulative geographic residuals. A major advantage of this method is its ability to readily incorporate information on study participants relocation, which most cluster detection statistics cannot. Application of these methods will be illustrated by the Home Allergens and Asthma prospective cohort study analyzing the relationship between environmental exposures and repeated measured outcome, occurrence of wheeze in ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2803498 Tue, 15 Sep 2009 23:00:00 +0100 2803498 http://www.medworm.com/index.php?rid=2799967&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800233 We consider a response adaptive design of clinical trials with a variance-penalized criterion. It is shown that this criterion evaluates the performance of a response adaptive design based on both the number of patients assigned to the better treatment and the power of the statistical test. A new proportion of treatment allocation is proposed and the doubly biased coin procedure is used to target the proposed proportion. Under reasonable assumptions, the proposed design is demonstrated to generate an asymptotic variance of allocation proportions, which is smaller than that of the drop-the-loser design. Simulation comparisons of the proposed design with some existing designs are presented. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2799967 Mon, 14 Sep 2009 23:00:00 +0100 2799967 http://www.medworm.com/index.php?rid=2724262&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900161 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2724262 Sat, 22 Aug 2009 23:00:00 +0100 2724262 http://www.medworm.com/index.php?rid=2724264&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900062 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2724264 Thu, 20 Aug 2009 23:00:00 +0100 2724264 http://www.medworm.com/index.php?rid=2724263&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900159 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2724263 Thu, 20 Aug 2009 23:00:00 +0100 2724263 http://www.medworm.com/index.php?rid=2719287&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800123 Diagnostic studies in ophthalmology frequently involve binocular data where pairs of eyes are evaluated, through some diagnostic procedure, for the presence of certain diseases or pathologies. The simplest approach of estimating measures of diagnostic accuracy, such as sensitivity and specificity, treats eyes as independent, consequently yielding incorrect estimates, especially of the standard errors. Approaches that account for the inter-eye correlation include regression methods using generalized estimating equations and likelihood techniques based on various correlated binomial models. The paper proposes a simple alternative statistical methodology of jointly estimating measures of diagnostic accuracy for binocular tests based on a flexible model for correlated binary data. Moments' est... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2719287 Thu, 20 Aug 2009 23:00:00 +0100 2719287 http://www.medworm.com/index.php?rid=2708481&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800215 Inference of haplotypes is important in genetic epidemiology studies. However, all large genotype data sets have errors due to the use of inexpensive genotyping machines that are fallible and shortcomings in genotyping scoring softwares, which can have an enormous impact on haplotype inference. In this article, we propose two novel strategies to reduce the impact induced by genotyping errors in haplotype inference. The first method makes use of double sampling. For each individual, the "GenoSpectrum" that consists of all possible genotypes and their corresponding likelihoods are computed. The second method is a genotype clustering algorithm based on multi-genotyping data, which also assigns a "GenoSpectrum" for each individual. We then describe two hybrid EM algorithms (called DS-EM and MG... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2708481 Sun, 16 Aug 2009 23:00:00 +0100 2708481 http://www.medworm.com/index.php?rid=2708480&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800186 Models for longitudinal data are employed in a wide range of behavioral, biomedical, psychosocial, and health-care-related research. One popular model for continuous response is the linear mixed-effects model (LMM). Although simulations by recent studies show that LMM provides reliable estimates under departures from the normality assumption for complete data, the invariable occurrence of missing data in practical studies renders such robustness results less useful when applied to real study data. In this paper, we show by simulated studies that in the presence of missing data estimates of the fixed effect of LMM are biased under departures from normality. We discuss two robust alternatives, the weighted generalized estimating equations (WGEE) and the augmented WGEE (AWGEE), and compare th... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2708480 Sun, 16 Aug 2009 23:00:00 +0100 2708480 http://www.medworm.com/index.php?rid=2708479&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800157 To classify patients either as resistant or non-resistant to HIV therapy based on longitudinal viral load profiles, we applied longitudinal quadratic discriminant analysis and examined various measures, mainly derived from the Brier Score, to assess the biomarker performance in terms of discrimination and calibration. The analysis of the application data revealed an increase in performance by using longer profiles instead of single biomarker measurements. Simulations showed that the selection of mixed models for the estimation of the group-specific discriminant rule parameters should be based on BIC, rather than on the best performance measure. An incorrect model selection can lead to spuriously better or worse performance as misclassification and classification certainty regards, especial... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2708479 Sun, 16 Aug 2009 23:00:00 +0100 2708479 http://www.medworm.com/index.php?rid=2670468&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800113 This article complements the results in Guilbaud (Biometrical Journal 2008; 50:678-692). Simultaneous confidence regions were derived in that article that correspond to any given multiple testing procedure (MTP) in a fairly large class of consonant closed-testing procedures based on marginal p-values and weighted Bonferroni tests for intersection hypotheses. This class includes Holm's MTP, the fixed-sequence MTP, gatekeeping MTPs, fallback MTPs, multi-stage fallback MTPs, and recently proposed MTPs specified through a graphical representation and associated rejection algorithm. More general confidence regions are proposed in this article. These regions are such that for certain underlying MTPs which are not alpha-exhaustive, they lead to confidence assertions that may be sharper than rejec... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2670468 Mon, 03 Aug 2009 23:00:00 +0100 2670468 http://www.medworm.com/index.php?rid=2670467&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800254 A predictive continuous time model is developed for continuous panel data to assess the effect of time-varying covariates on the general direction of the movement of a continuous response that fluctuates over time. This is accomplished by reparameterizing the infinitesimal mean of an Ornstein-Uhlenbeck processes in terms of its equilibrium mean and a drift parameter, which assesses the rate that the process reverts to its equilibrium mean. The equilibrium mean is modeled as a linear predictor of covariates. This model can be viewed as a continuous time first-order autoregressive regression model with time-varying lag effects of covariates and the response, which is more appropriate for unequally spaced panel data than its discrete time analog. Both maximum likelihood and quasi-likelihood a... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2670467 Mon, 03 Aug 2009 23:00:00 +0100 2670467 http://www.medworm.com/index.php?rid=2670466&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800256 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2670466 Mon, 03 Aug 2009 23:00:00 +0100 2670466 http://www.medworm.com/index.php?rid=2708478&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900154 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2708478 Fri, 31 Jul 2009 23:00:00 +0100 2708478 http://www.medworm.com/index.php?rid=2660321&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900104 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660321 Fri, 31 Jul 2009 23:00:00 +0100 2660321 http://www.medworm.com/index.php?rid=2660331&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800146 A conceptually simple two-dimensional conditional reference curve is described. The curve gives a decision basis for determining whether a bivariate response from an individual is "normal" or "abnormal" when taking into account that a third (conditioning) variable may influence the bivariate response. The reference curve is not only characterized analytically but also by geometric properties that are easily communicated to medical doctors - the users of such curves. The reference curve estimator is completely non-parametric, so no distributional assumptions are needed about the two-dimensional response. An example that will serve to motivate and illustrate the reference is the study of the height/weight distribution of 7-8-year-old Danish school girls born in 1930, 1950, or 1970. (Source: ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660331 Thu, 30 Jul 2009 23:00:00 +0100 2660331 http://www.medworm.com/index.php?rid=2660330&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800162 For time series of count data, correlated measurements, clustering as well as excessive zeros occur simultaneously in biomedical applications. Ignoring such effects might contribute to misleading treatment outcomes. A generalized mixture Poisson geometric process (GMPGP) model and a zero-altered mixture Poisson geometric process (ZMPGP) model are developed from the geometric process model, which was originally developed for modelling positive continuous data and was extended to handle count data. These models are motivated by evaluating the trend development of new tumour counts for bladder cancer patients as well as by identifying useful covariates which affect the count level. The models are implemented using Bayesian method with Markov chain Monte Carlo (MCMC) algorithms and are assesse... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660330 Thu, 30 Jul 2009 23:00:00 +0100 2660330 http://www.medworm.com/index.php?rid=2660329&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800195 Unmeasured confounders are a common problem in drawing causal inferences in observational studies. VanderWeele (Biometrics 2008, 64, 702-706) presented a theorem that allows researchers to determine the sign of the unmeasured confounding bias when monotonic relationships hold between the unmeasured confounder and the treatment, and between the unmeasured confounder and the outcome. He showed that his theorem can be applied to causal effects with the total group as the standard population, but he did not mention the causal effects with treated and untreated groups as the standard population. Here, we extend his results to these causal effects, and apply our theorems to an observational study. When researchers have a sense of what the unmeasured confounder may be, conclusions can be drawn ab... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660329 Thu, 30 Jul 2009 23:00:00 +0100 2660329 http://www.medworm.com/index.php?rid=2660328&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800203 The Cochran-Armitage (CA) linear trend test for proportions is often used for genotype-based analysis of candidate gene association. Depending on the underlying genetic mode of inheritance, the use of model-specific scores maximises the power. Commonly, the underlying genetic model, i.e. additive, dominant or recessive mode of inheritance, is a priori unknown. Association studies are commonly analysed using permutation tests, where both inference and identification of the underlying mode of inheritance are important. Especially interesting are tests for case-control studies, defined by a maximum over a series of standardised CA tests, because such a procedure has power under all three genetic models. We reformulate the test problem and propose a conditional maximum test of scores-specific ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660328 Thu, 30 Jul 2009 23:00:00 +0100 2660328 http://www.medworm.com/index.php?rid=2660327&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800209 This article discusses the use of empirical likelihood functions, a well-developed methodology in the frequentist paradigm, to interpret statistical evidence in nonparametric and semiparametric situations. A comparative review of literature shows that, while an empirical likelihood is not a true probability density, it has the essential properties, namely consistency and local asymptotic normality that unify and justify the various parametric likelihood methods for evidential analysis. Real examples are presented to illustrate and compare the empirical likelihood method and the parametric likelihood methods. These methods are also compared in terms of asymptotic efficiency by combining relevant results from different areas. It is seen that a parametric likelihood based on a correctly speci... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660327 Thu, 30 Jul 2009 23:00:00 +0100 2660327 http://www.medworm.com/index.php?rid=2660326&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800253 In doubly interval-censored data, the survival time of interest is defined as the elapsed time between an initial event and a subsequent event, but the occurrences of both events cannot be observed exactly. Instead, only right- or interval-censored observations on the occurrence times are available. Our purpose is to develop a generalized log-rank-type test for comparing survival functions of several groups. For the same problem, Sun (The Statistical Analysis of Interval-censored Failure Time Data, Springer, New York) suggested a nonparametric test based on the estimated marginal survival functions of the two related events. We consider a new method using uniform weights, which depend only on the size of the risk set at each observed time. The proposed method does not require the estimatio... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660326 Thu, 30 Jul 2009 23:00:00 +0100 2660326 http://www.medworm.com/index.php?rid=2660325&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900018 Multiple imputation (MI) has emerged in the last two decades as a frequently used approach in dealing with incomplete data. Gaussian and log-linear imputation models are fairly straightforward to implement for continuous and discrete data, respectively. However, in missing data settings that include a mix of continuous and discrete variables, the lack of flexible models for the joint distribution of different types of variables can make the specification of the imputation model a daunting task. The widespread availability of software packages that are capable of carrying out MI under the assumption of joint multivariate normality allows applied researchers to address this complication pragmatically by treating the discrete variables as continuous for imputation purposes and subsequently ro... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660325 Thu, 30 Jul 2009 23:00:00 +0100 2660325 http://www.medworm.com/index.php?rid=2660324&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800237 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660324 Thu, 30 Jul 2009 23:00:00 +0100 2660324 http://www.medworm.com/index.php?rid=2660323&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900063 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660323 Thu, 30 Jul 2009 23:00:00 +0100 2660323 http://www.medworm.com/index.php?rid=2660322&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900103 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2660322 Thu, 30 Jul 2009 23:00:00 +0100 2660322 http://www.medworm.com/index.php?rid=2633821&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800154 We have considered a Bayesian approach for the nonlinear regression model by replacing the normal distribution on the error term by some skewed distributions, which account for both skewness and heavy tails or skewness alone. The type of data considered in this paper concerns repeated measurements taken in time on a set of individuals. Such multiple observations on the same individual generally produce serially correlated outcomes. Thus, additionally, our model does allow for a correlation between observations made from the same individual. We have illustrated the procedure using a data set to study the growth curves of a clinic measurement of a group of pregnant women from an obstetrics clinic in Santiago, Chile. Parameter estimation and prediction were carried out using appropriate poste... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2633821 Wed, 22 Jul 2009 23:00:00 +0100 2633821 http://www.medworm.com/index.php?rid=2704996&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900037 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2704996 Tue, 07 Jul 2009 23:00:00 +0100 2704996 http://www.medworm.com/index.php?rid=2704995&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800201 Motivated by the needs of scientists using flow cytometry, we study the problem of estimating the region where two multivariate samples differ in density. We call this problem highest density difference region estimation and recognise it as a two-sample analogue of highest density region or excess set estimation. Flow cytometry samples are typically in the order of 10 000 and 100 000 and with dimension ranging from about 3 to 20. The industry standard for the problem being studied is called Frequency Difference Gating, due to Roederer and Hardy (). After couching the problem in a formal statistical framework we devise an alternative estimator that draws upon recent statistical developments such as patient rule induction methods. Improved performance is illustrated in simulations. While mot... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2704995 Tue, 07 Jul 2009 23:00:00 +0100 2704995 http://www.medworm.com/index.php?rid=2704994&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800128 The ROC (receiver operating characteristic) curve is the most commonly used statistical tool for describing the discriminatory accuracy of a diagnostic test. Classical estimation of the ROC curve relies on data from a simple random sample from the target population. In practice, estimation is often complicated due to not all subjects undergoing a definitive assessment of disease status (verification). Estimation of the ROC curve based on data only from subjects with verified disease status may be badly biased. In this work we investigate the properties of the doubly robust (DR) method for estimating the ROC curve under verification bias originally developed by Rotnitzky, Faraggi and Schisterman (2006) for estimating the area under the ROC curve. The DR method can be applied for continuous ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2704994 Tue, 07 Jul 2009 23:00:00 +0100 2704994 http://www.medworm.com/index.php?rid=2704993&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800228 Harrell's c-index or concordance C has been widely used as a measure of separation of two survival distributions. In the absence of censored data, the c-index estimates the Mann-Whitney parameter Pr(X>Y), which has been repeatedly utilized in various statistical contexts. In the presence of randomly censored data, the c-index no longer estimates Pr(X>Y); rather, a parameter that involves the underlying censoring distributions. This is in contrast to Efron's maximum likelihood estimator of the Mann-Whitney parameter, which is recommended in the setting of random censorship. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2704993 Tue, 07 Jul 2009 23:00:00 +0100 2704993 http://www.medworm.com/index.php?rid=2704992&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800222 This paper extends the multilevel survival model by allowing the existence of cured fraction in the model. Random effects induced by the multilevel clustering structure are specified in the linear predictors in both hazard function and cured probability parts. Adopting the generalized linear mixed model (GLMM) approach to formulate the problem, parameter estimation is achieved by maximizing a best linear unbiased prediction (BLUP) type log-likelihood at the initial step of estimation, and is then extended to obtain residual maximum likelihood (REML) estimators of the variance component. The proposed multilevel mixture cure model is applied to analyze the (i) child survival study data with multilevel clustering and (ii) chronic granulomatous disease (CGD) data on recurrent infections as ill... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2704992 Tue, 07 Jul 2009 23:00:00 +0100 2704992 http://www.medworm.com/index.php?rid=2704991&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800236 We assessed complementary log-log (CLL) regression as an alternative statistical model for estimating multivariable-adjusted prevalence ratios (PR) and their confidence intervals. Using the delta method, we derived an expression for approximating the variance of the PR estimated using CLL regression. Then, using simulated data, we examined the performance of CLL regression in terms of the accuracy of the PR estimates, the width of the confidence intervals, and the empirical coverage probability, and compared it with results obtained from log-binomial regression and stratified Mantel-Haenszel analysis. Within the range of values of our simulated data, CLL regression performed well, with only slight bias of point estimates of the PR and good confidence interval coverage. In addition, and imp... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2704991 Tue, 07 Jul 2009 23:00:00 +0100 2704991 http://www.medworm.com/index.php?rid=2704990&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800127 A multivariate probit model for correlated binary responses given the predictors of interest has been considered. Some of the responses are subject to classification errors and hence are not directly observable. Also measurements on some of the predictors are not available; instead the measurements on its surrogate are available. However, the conditional distribution of the unobservable predictors given the surrogate is completely specified. Models are proposed taking into account either or both of these sources of errors. Likelihood-based methodologies are proposed to fit these models. To ascertain the effect of ignoring classification errors and /or measurement error on the estimates of the regression and correlation parameters, a sensitivity study is carried out through simulation. Fina... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2704990 Tue, 07 Jul 2009 23:00:00 +0100 2704990 http://www.medworm.com/index.php?rid=2560251&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800109 The aim of the paper is to apply point processes to root data modelling. We propose a new approach to parametric inference when the data are inhomogeneous replicated marked point patterns. We generalize Geyer's saturation point process to a model, which combines inhomogeneity, marks and interaction between the marked points. Furthermore, the inhomogeneity influences the definition of the neighbourhood of points. Using the maximum pseudolikelihood method, this model is then fitted to root data from mixed stands of Norway spruce (Picea abies (L.) Karst.) and European beech (Fagus sylvatica L.) to quantify the degree of root aggregation in such mixed stands. According to the analysis there is no evidence that the two root systems are not independent. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2560251 Tue, 30 Jun 2009 23:00:00 +0100 2560251 http://www.medworm.com/index.php?rid=2560250&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800159 The sensitivity and specificity of a new medical device are often compared relative to that of an existing device by calculating ratios of sensitivities and specificities. Although it would be ideal for all study subjects to receive the gold standard so true disease status was known for all subjects, it is often not feasible or ethical to obtain disease status for everyone. This paper proposes two unpaired designs where each subject is only administered one of the devices and device results dictate which subjects are to receive disease verification. Estimators of the ratio of accuracy and corresponding confidence intervals are proposed for these designs as well as sample size formulae. Simulation studies are performed to investigate the small sample bias of the estimators and the performan... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2560250 Tue, 30 Jun 2009 23:00:00 +0100 2560250 http://www.medworm.com/index.php?rid=2560249&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800199 The main goal of this paper is to investigate a cure rate model that comprehends some well-known proposals found in the literature. In our work the number of competing causes of the event of interest follows the negative binomial distribution. The model is conveniently reparametrized through the cured fraction, which is then linked to covariates by means of the logistic link. We explore the use of Markov chain Monte Carlo methods to develop a Bayesian analysis in the proposed model. The procedure is illustrated with a numerical example. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2560249 Tue, 30 Jun 2009 23:00:00 +0100 2560249 http://www.medworm.com/index.php?rid=2516889&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900026 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2516889 Wed, 24 Jun 2009 23:00:00 +0100 2516889 http://www.medworm.com/index.php?rid=2504589&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800172 A Markovian susceptible [rarr] infectious [rarr] removed (SIR) epidemic model is considered in a community partitioned into households. A vaccination strategy, which is implemented during the early stages of the disease following the detection of infected individuals is proposed. In this strategy, the detection occurs while an individual is infectious and other susceptible household members are vaccinated without further delay. Expressions are derived for the influence on the reproduction numbers of this vaccination strategy for equal and unequal household sizes. We fit previously estimated parameters from influenza and use household distributions for Sweden and Tanzania census data. The results show that the reproduction number is much higher in Tanzania (6 compared with 2) due to larger ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2504589 Sun, 21 Jun 2009 23:00:00 +0100 2504589 http://www.medworm.com/index.php?rid=2456182&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900049 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2456182 Fri, 05 Jun 2009 04:00:00 +0100 2456182 http://www.medworm.com/index.php?rid=2456185&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200800160 We propose a measure of disassortativeness to summarize contact patterns relevant to the transmission of directly transmitted infections. We discuss the properties of this measure, describe standardization relative to homogeneous mixing, and generalize it to multivariate contact structures. We explore some of its properties and apply our methods to serological surveys of close contact infections and surveys of self-reported social contacts obtained in several European countries. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2456185 Tue, 02 Jun 2009 04:00:00 +0100 2456185 http://www.medworm.com/index.php?rid=2456184&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900035 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2456184 Tue, 02 Jun 2009 04:00:00 +0100 2456184 http://www.medworm.com/index.php?rid=2456183&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900045 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2456183 Tue, 02 Jun 2009 04:00:00 +0100 2456183 http://www.medworm.com/index.php?rid=2165446&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810502 Ranking multivariate ordinal data and applying a non-parametric test is an analytical approach commonly employed to compare treatments. We study three types of ranking and demonstrate how to combine them. The ranking methods rest upon partial orders of the multidimensional measurements or upon the sum of ranks. Since their usage is simple as regards statistical assumptions and technical realization, they are also adapted for health professionals without deep statistical knowledge. Our goal is discussing differences between the approaches and disclosing possible statistical consequences of their usage (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165446 Sun, 08 Feb 2009 05:00:00 +0100 2165446 http://www.medworm.com/index.php?rid=2165457&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710489 Finite mixture models can provide the insights about behavioral patterns as a source of heterogeneity of the various dynamics of time course gene expression data by reducing the high dimensionality and making clear the major components of the underlying structure of the data in terms of the unobservable latent variables. The latent structure of the dynamic transition process of gene expression changes over time can be represented by Markov processes. This paper addresses key problems in the analysis of large gene expression data sets that describe systemic temporal response cascades and dynamic changes to therapeutic doses in multiple tissues, such as liver, skeletal muscle, and kidney from the same animals. Bayesian Finite Markov Mixture Model with a Dirichlet Prior is developed for the i... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165457 Thu, 05 Feb 2009 05:00:00 +0100 2165457 http://www.medworm.com/index.php?rid=2165456&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710493 Longitudinal trials can yield outcomes that are continuous, binary (yes/no), or are realizations of counts. In this setting we compare three approaches that have been proposed for estimation of the correlation in the framework of generalized estimating equations (GEE): quasi-least squares (QLS), pseudo-likelihood (PL), and an approach we refer to as Wang-Carey (WC). We prove that WC and QLS are identical for the first-order autoregressive AR(1) correlation structure. Using simulations, we then develop guidelines for selection of an appropriate method for analysis of data from a longitudinal trial. In particular, we argue that no method is uniformly superior for analysis of unbalanced and unequally spaced data with a Markov correlation structure. Choice of the best approach will depend on t... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165456 Thu, 05 Feb 2009 05:00:00 +0100 2165456 http://www.medworm.com/index.php?rid=2165455&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810487 In some applications involving regression the values of certain variables are missing by design for some individuals. For example, in two-stage studies (Zhao and Lipsitz, 1992), data on "cheaper" variables are collected on a random sample of individuals in stage I, and then "expensive" variables are measured for a subsample of these in stage II. So the "expensive" variables are missing by design at stage I. Both estimating function and likelihood methods have been proposed for cases where either covariates or responses are missing. We extend the semiparametric maximum likelihood (SPML) method for missing covariate problems (e.g. Chen, 2004; Ibrahim et al., 2005; Zhang and Rockette, 2005, 2007) to deal with more general cases where covariates and/or responses are missing by design, and show... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165455 Thu, 05 Feb 2009 05:00:00 +0100 2165455 http://www.medworm.com/index.php?rid=2165454&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810488 Propensity-score matching is increasingly being used to reduce the impact of treatment-selection bias when estimating causal treatment effects using observational data. Several propensity-score matching methods are currently employed in the medical literature: matching on the logit of the propensity score using calipers of width either 0.2 or 0.6 of the standard deviation of the logit of the propensity score; matching on the propensity score using calipers of 0.005, 0.01, 0.02, 0.03, and 0.1; and 5 [rarr] 1 digit matching on the propensity score. We conducted empirical investigations and Monte Carlo simulations to investigate the relative performance of these competing methods. Using a large sample of patients hospitalized with a heart attack and with exposure being receipt of a statin pre... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165454 Thu, 05 Feb 2009 05:00:00 +0100 2165454 http://www.medworm.com/index.php?rid=2165453&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810491 Existing methods for joint modeling of longitudinal measurements and survival data can be highly influenced by outliers in the longitudinal outcome. We propose a joint model for analysis of longitudinal measurements and competing risks failure time data which is robust in the presence of outlying longitudinal observations during follow-up. Our model consists of a linear mixed effects sub-model for the longitudinal outcome and a proportional cause-specific hazards frailty sub-model for the competing risks data, linked together by latent random effects. Instead of the usual normality assumption for measurement errors in the linear mixed effects sub-model, we adopt a t -distribution which has a longer tail and thus is more robust to outliers. We derive an EM algorithm for the maximum likeliho... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165453 Thu, 05 Feb 2009 05:00:00 +0100 2165453 http://www.medworm.com/index.php?rid=2165452&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810492 To test for global linkage along a genome or in a chromosomal region, the maximum over the marker locations of mean alleles shared identical by descent of affected relative pairs, Zmax, can be used. Feingold et al. (1993) derived a Gaussian approximation to the distribution of the Zmax. As an alternative we propose to sum over the observed marker locations along the chromosomal region of interest. Two test statistics can be derived. (1) The likelihood ratio statistic (LR) and (2) the corresponding score statistic. The score statistic appears to be the average mean IBD over all available marker locations. The null distribution of the LR and score tests are asymptotically a 50: 50 mixture of chi-square distributions of null and one degree of freedom and a normal distribution, respectively.We... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165452 Thu, 05 Feb 2009 05:00:00 +0100 2165452 http://www.medworm.com/index.php?rid=2165451&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810495 Kernel smoothing is a popular approach to estimating relative risk surfaces from data on the locations of cases and controls in geographical epidemiology. The interpretation of such surfaces is facilitated by plotting of tolerance contours which highlight areas where the risk is sufficiently high to reject the null hypothesis of unit relative risk. Previously it has been recommended that these tolerance intervals be calculated using Monte Carlo randomization tests. We examine a computationally cheap alternative whereby the tolerance intervals are derived from asymptotic theory. We also examine the performance of global tests of hetereogeneous risk employing statistics based on kernel risk surfaces, paying particular attention to the choice of smoothing parameters on test power (© 2009 WIL... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165451 Thu, 05 Feb 2009 05:00:00 +0100 2165451 http://www.medworm.com/index.php?rid=2165450&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810496 Several previous studies have identified risk factors for childhood mortality in high risk areas, such as Sub-Saharan Africa. Among these are lifestyle factors related for example to nutrition or sanitation. Other factors are related to social class, ethnicity and poverty in general. Few studies have investigated a dependence of these factors by age and season of birth which is the focus in this study. We perform a survival analysis of 9121 children born between 1998 and 2001 in a rural area of western Burkina Faso. The whole population is under demographic surveillance since 1993. All cause mortality is used as the endpoint and follow-up information until the age of five years is available. Recently developed spline regression methods are used for the analysis. Ethnic group, religion, age... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165450 Thu, 05 Feb 2009 05:00:00 +0100 2165450 http://www.medworm.com/index.php?rid=2165449&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810499 Non-inferiority trials are conducted for a variety of reasons including to show that a new treatment has a negligible reduction in efficacy or safety when compared to the current standard treatment, or a more complex setting of showing that a new treatment has a negligible reduction in efficacy when compared to the current standard yet is superior in terms of other treatment characteristics. The latter reason for conducting a non-inferiority trial presents the challenge of deciding on a balance between a suitable reduction in efficacy, known as the non-inferiority margin, in return for a gain in other important treatment characteristics/findings. It would be ideal to alleviate the dilemma on the choice of margin in this setting by reverting to a traditional superiority trial design where a... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165449 Thu, 05 Feb 2009 05:00:00 +0100 2165449 http://www.medworm.com/index.php?rid=2165448&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810500 Most of the available response-adaptive designs in phase III clinical trial set up are not from any optimal consideration. An optimal design for binary responses is given by Rosenberger et al. (2001) and an optimal design for continuous responses is provided by Biswas and Mandal (2004). Recently, Zhang and Rosenberger (2006) [ZR] provided another design for normal responses. Biswas, Bhattacharya and Zhang (2007) pointed out that the design of ZR is not suitable for normally distributed responses, or any distribution having the possibility of negative mean, in general. But they only indicated the problem and bypassed the original problem and set up. In the present paper, we first start with the drawback of ZR. We then provide the appropriate optimal response-adaptive design for normal or co... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165448 Thu, 05 Feb 2009 05:00:00 +0100 2165448 http://www.medworm.com/index.php?rid=2165447&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810501 Neuron experiments produce high-dimensional data structures. Therefore, application of smoothing techniques in the analysis of neuronal data from electrophysiological experiments has received considerable attention of late. We investigate the use of penalized splines in the analysis of neuronal data. This is first illustrated when interested in the temporal trend of a single neuron. An approach to investigate the maximal firing rate, based on the penalizedspline model is proposed. Determination of the time of maximal firing rate is based on non-linear optimization of the objective function with the corresponding confidence intervals constructed based on the first-order derivative function. To distinguish between the curves from different experimental conditions in a moment-by-moment sense,... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2165447 Thu, 05 Feb 2009 05:00:00 +0100 2165447 http://www.medworm.com/index.php?rid=2012432&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710444 The first stage in the analysis of cDNA microarray data is estimation of the level of expression of each gene, from laser scans of hybridised microarrays. Typically, data are used from a single scan, although, if multiple scans are available, there is the opportunity to reduce sampling error by using all of them. Combining multiple laser scans can be formulated as multivariate functional regression through the origin. Maximum likelihood estimation fails, but many alternative estimators exist, one of which is to maximise the likelihood of a Gaussian structural regression model. We show by simulation that, surprisingly, this estimator is efficient for our problem, even though the distribution of gene expression values is far from Gaussian. Further, it performs well if errors have a heavier t... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=2012432 Thu, 04 Dec 2008 05:00:00 +0100 2012432 http://www.medworm.com/index.php?rid=1987161&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810445 Consider a sample of animal abundances collected from one sampling occasion. Our focus is in estimating the number of species in a closed population. In order to conduct a noninformative Bayesian inference when modeling this data, we derive Jeffreys and reference priors from the full likelihood. We assume that the species' abundances are randomly distributed according to a distribution indexed by a finite-dimensional parameter. We consider two specific cases which assume that the mean abundances are constant or exponentially distributed. The Jeffreys and reference priors are functions of the Fisher information for the model parameters; the information is calculated in part using the linear difference score for integer parameter models (Lindsay & Roeder 1987). The Jeffreys and reference pri... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1987161 Wed, 26 Nov 2008 05:00:00 +0100 1987161 http://www.medworm.com/index.php?rid=1987160&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810448 We revisit the heterogeneous closed population multiple recapture problem, modeling individual-level heterogeneity using the Grade of Membership model (Woodbury et al., 1978). This strategy allows us to postulate the existence of homogeneous latent "ideal" or "pure" classes within the population, and construct a soft clustering of the individuals, where each one is allowed partial or mixed membership in all of these classes. We propose a full hierarchical Bayes specification and a MCMC algorithm to obtain samples from the posterior distribution. We apply the method to simulated data and to three real life examples. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1987160 Wed, 26 Nov 2008 05:00:00 +0100 1987160 http://www.medworm.com/index.php?rid=1987159&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810476 Microarrays provide a valuable tool for the quantification of gene expression. Usually, however, there is a limited number of replicates leading to unsatisfying variance estimates in a gene-wise mixed model analysis. As thousands of genes are available, it is desirable to combine information across genes. When more than two tissue types or treatments are to be compared it might be advisable to consider the array effect as random. Then information between arrays may be recovered, which can increase accuracy in estimation. We propose a method of variance component estimation across genes for a linear mixed model with two random effects. The method may be extended to models with more than two random effects. We assume that the variance components follow a log-normal distribution. Assuming tha... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1987159 Wed, 26 Nov 2008 05:00:00 +0100 1987159 http://www.medworm.com/index.php?rid=1987158&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810477 In this paper we introduce a misclassification model for the meiosis I non-disjunction fraction in numerical chromosomal anomalies named trisomies. We obtain posteriors, and their moments, for the probability that a non-disjunction occurs in the first division of meiosis and for the misclassification errors. We also extend previous works by providing the exact posterior, and its moments, for the probability that a non-disjunction occurs in the first division of meiosis assuming the model proposed in the literature which does not consider that data are subject to misclassification. We perform Monte Carlo studies in order to compare Bayes estimates obtained by using both models. An application to Down Syndrome data is also presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Sour... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1987158 Wed, 26 Nov 2008 05:00:00 +0100 1987158 http://www.medworm.com/index.php?rid=1987157&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810479 No AbstractArticle Authors reply (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1987157 Wed, 26 Nov 2008 05:00:00 +0100 1987157 http://www.medworm.com/index.php?rid=1987156&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810481 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1987156 Wed, 26 Nov 2008 05:00:00 +0100 1987156 http://www.medworm.com/index.php?rid=1835749&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810451 Binomial and geometric mixtures can be used to model data gathered in capture-recapture surveys of animal populations, removal surveys of harvest populations, registrations of disease populations, ecological species census, and so on. To compute a nonparametric maximum likelihood estimator for the mixing distribution of heterogeneous capture probabilities, we consider a conditional approach and use a reliable and fast integrative procedure which combines the EM algorithm to increase the likelihood and the vertex-exchange method to update the number of support points. A convergent Newtonian algorithm is used in the M-step of the EM algorithm. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1835749 Tue, 30 Sep 2008 04:00:00 +0100 1835749 http://www.medworm.com/index.php?rid=1601397&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810435 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1601397 Thu, 10 Jul 2008 04:00:00 +0100 1601397 http://www.medworm.com/index.php?rid=1601403&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810432 No AbstractArticle Letter to the editor (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1601403 Wed, 09 Jul 2008 04:00:00 +0100 1601403 http://www.medworm.com/index.php?rid=1601402&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710431 This paper analyzes data arising from a Severe Acute Respiratory Syndrome (SARS) epidemic in Hong Kong in 2003 involving 1755 cases. A discrete time stochastic model that uses a back-projection approach is proposed. Markov Chain Monte Carlo (MCMC) methods are developed for estimation of model parameters. The algorithm is further extended to integrate numerically over unobserved variables of the model. Applying the method to SARS data from Hong Kong, a value of 3.88 with a posterior standard deviation of 0.09 was estimated for the basic reproduction number. An estimate of the transmission parameter at the beginning of the epidemic was also obtained as 0.149 with a posterior standard deviation of 0.003. A reduction in the transmission parameter during the course of the epidemic forced the ef... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1601402 Wed, 09 Jul 2008 04:00:00 +0100 1601402 http://www.medworm.com/index.php?rid=1601401&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710440 Studies of HIV dynamics in AIDS research are very important in understanding the pathogenesis of HIV-1 infection and also in assessing the effectiveness of antiviral therapies. There are many AIDS clinical trials on HIV dynamics currently in development worldwide, giving rise to many design issues yet to be addressed. For example, most studies are focused on short-term viral dynamics and the existing models may not be applicable to describe long-term virologic response. In this paper, we use a simulation-based approach to study the designs of long-term viral dynamics under semiparametric nonlinear mixed-effects models. These models not only can preserve the meaningful interpretation of the short-term HIV dynamics, but also characterize the long-term virologic responses to antiretroviral (A... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1601401 Wed, 09 Jul 2008 04:00:00 +0100 1601401 http://www.medworm.com/index.php?rid=1601400&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710430 The annual percent change (APC) has been used as a measure to describe the trend in the age-adjusted cancer incidence or mortality rate over relatively short time intervals. The yearly data on these age-adjusted rates are available from the Surveillance, Epidemiology, and End Results (SEER) Program of the National Cancer Institute. The traditional methods to estimate the APC is to fit a linear regression of logarithm of age-adjusted rates on time using the least squares method or the weighted least squares method, and use the estimate of the slope parameter to define the APC as the percent change in the rates between two consecutive years. For comparing the APC for two regions, one uses a t-test which assumes that the two datasets on the logarithm of the age-adjusted rates are independent ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1601400 Wed, 09 Jul 2008 04:00:00 +0100 1601400 http://www.medworm.com/index.php?rid=1601399&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810429 No AbstractArticle Authors reply (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1601399 Wed, 09 Jul 2008 04:00:00 +0100 1601399 http://www.medworm.com/index.php?rid=1601398&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810434 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1601398 Wed, 09 Jul 2008 04:00:00 +0100 1601398 http://www.medworm.com/index.php?rid=1444135&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810424 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1444135 Fri, 16 May 2008 04:00:00 +0100 1444135 http://www.medworm.com/index.php?rid=1444139&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710427 This article proposes a new test to detect interactions in replicated two-way ANOVA models, more powerful than the classical F -test and more general than the test of Terbeck and Davies (1998, Annals of Statistics 26, 1279-1305) developed for the case with unconditionally identifiable interaction pattern. We use the parameterization without the conventional restrictions on the interaction terms and base our test on the maximum of the standardized disturbance estimates. We show that our test is unbiased and consistent, and discuss how to estimate the p -value of the test. In a 3 × 3 case, which is our main focus in this article, the exact p -value can be computed by using four-dimensional integrations. For a general I × J case which requires an (I - 1) × (J - 1) dimensional integration f... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1444139 Thu, 15 May 2008 04:00:00 +0100 1444139 http://www.medworm.com/index.php?rid=1444138&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810425 Simultaneous inference is a common problem in many areas of application. If multiple null hypotheses are tested simultaneously, the probability of rejecting erroneously at least one of them increases beyond the pre-specified significance level. Simultaneous inference procedures have to be used which adjust for multiplicity and thus control the overall type I error rate. In this paper we describe simultaneous inference procedures in general parametric models, where the experimental questions are specified through a linear combination of elemental model parameters. The framework described here is quite general and extends the canonical theory of multiple comparison procedures in ANOVA models to linear regression problems, generalized linear models, linear mixed effects models, the Cox model,... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1444138 Thu, 15 May 2008 04:00:00 +0100 1444138 http://www.medworm.com/index.php?rid=1444137&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810420 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1444137 Thu, 15 May 2008 04:00:00 +0100 1444137 http://www.medworm.com/index.php?rid=1444136&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810422 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1444136 Thu, 15 May 2008 04:00:00 +0100 1444136 http://www.medworm.com/index.php?rid=1398156&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710405 Promotion time models have been recently adapted to the context of infectious diseases to take into account discrete and multiple exposures. However, Poisson distribution of the number of pathogens transmitted at each exposure was a very strong assumption and did not allow for inter-individual heterogeneity. Bernoulli, the negative binomial, and the compound Poisson distributions were proposed as alternatives to Poisson distribution for the promotion time model with time-changing exposure. All were derived within the frailty model framework. All these distributions have a point mass at zero to take into account non-infected people. Bernoulli distribution, the two-component cure rate model, was extended to multiple exposures. Contrary to the negative binomial and the compound Poisson distri... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1398156 Fri, 25 Apr 2008 04:00:00 +0100 1398156 http://www.medworm.com/index.php?rid=1398155&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710415 The receiver operating characteristic (ROC) curve is used to evaluate a biomarker's ability for classifying disease status. The Youden Index (J), the maximum potential effectiveness of a biomarker, is a common summary measure of the ROC curve. In biomarker development, levels may be unquantifiable below a limit of detection (LOD) and missing from the overall dataset. Disregarding these observations may negatively bias the ROC curve and thus J. Several correction methods have been suggested for mean estimation and testing; however, little has been written about the ROC curve or its summary measures. We adapt non-parametric (empirical) and semi-parametric (ROC-GLM [generalized linear model]) methods and propose parametric methods (maximum likelihood (ML)) to estimate J and the optimal cut-po... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1398155 Fri, 25 Apr 2008 04:00:00 +0100 1398155 http://www.medworm.com/index.php?rid=1398154&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710418 This paper discusses multivariate interval-censored failure time data that occur when there exist several correlated survival times of interest and only interval-censored data are available for each survival time. Such data occur in many fields. One is tumorigenicity experiments, which usually concern different types of tumors, tumors occurring in different locations of animals, or together. For regression analysis of such data, we develop a marginal inference approach using the additive hazards model and apply it to a set of bivariate interval-censored data arising from a tumorigenicity experiment. Simulation studies are conducted for the evaluation of the presented approach and suggest that the approach performs well for practical situations. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, We... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1398154 Fri, 25 Apr 2008 04:00:00 +0100 1398154 http://www.medworm.com/index.php?rid=1398153&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710419 We present a class of generalized log-rank tests for this type of survival data and establish their asymptotic properties. The method is evaluated using simulation studies and illustrated by a set of real data from a diabetes study. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1398153 Fri, 25 Apr 2008 04:00:00 +0100 1398153 http://www.medworm.com/index.php?rid=1398152&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710426 Relative Survival is the ratio of the overall survival of a group of patients to the expected survival for a demographically similar group. It is commonly used in disease registries to estimate the effect of a particular disease when the true cause of death is not reliably known. Regression models for relative survival have been described and we extend these models to allow for clustered responses by embedding them into the class of Generalized linear mixed models (GLMM). The method is motivated and demonstrated by a data set from the HALLUCA study, an epidemiological study which investigated provision of medical care to lung cancer patients in the region of Halle in the eastern part of Germany. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1398152 Fri, 25 Apr 2008 04:00:00 +0100 1398152 http://www.medworm.com/index.php?rid=1398151&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810421 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1398151 Fri, 25 Apr 2008 04:00:00 +0100 1398151 http://www.medworm.com/index.php?rid=1274217&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710403 In this paper we compare the properties of four different general approaches for testing the ratio of two Poisson rates. Asymptotically normal tests, tests based on approximate p -values, exact conditional tests, and a likelihood ratio test are considered. The properties and power performance of these tests are studied by a Monte Carlo simulation experiment. Sample size calculation formulae are given for each of the test procedures and their validities are studied. Some recommendations favoring the likelihood ratio and certain asymptotic tests are based on these simulation results. Finally, all of the test procedures are illustrated with two real life medical examples. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274217 Mon, 03 Mar 2008 05:00:00 +0100 1274217 http://www.medworm.com/index.php?rid=1274216&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710410 When a new treatment is compared to an established one in a randomized clinical trial, it is standard practice to statistically test for non-inferiority rather than for superiority. When the endpoint is binary, one usually compares two treatments using either an odds-ratio or a difference of proportions. In this paper, we propose a mixed approach which uses both concepts. One first defines the non-inferiority margin using an odds-ratio and one ultimately proves non-inferiority statistically using a difference of proportions. The mixed approach is shown to be more powerful than the conventional odds-ratio approach when the efficacy of the established treatment is known (with good precision) and high (e.g. with more than 56% of success). The gain of power achieved may lead in turn to a subst... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274216 Mon, 03 Mar 2008 05:00:00 +0100 1274216 http://www.medworm.com/index.php?rid=1274215&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710412 In this article we propose a new technique for identifying clusters in temporal point processes. This relies on the comparision between all the m -order spacings and it is totally independent of any alternative hypothesis. A recursive procedure is introduced and allows to identify multiple clusters independently. This new scan statistic seems to be more efficient than the classical scan statistic for detecting and recovering cluster alternatives. These results have applications in epidemiological studies of rare diseases. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274215 Mon, 03 Mar 2008 05:00:00 +0100 1274215 http://www.medworm.com/index.php?rid=1274214&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710406 Case-control designs are widely used in rare disease studies. In a typical case-control study, data are collected from a sample of all available subjects who have experienced a disease (cases) and a sub-sample of subjects who have not experienced the disease (controls) in a study cohort. Cases are oversampled in case-control studies. Logistic regression is a common tool to estimate the relative risks of the disease with respect to a set of covariates. Very often in such a study, information of ages-at-onset of the disease for all cases and ages at survey of controls are known. Standard logistic regression analysis using age as a covariate is based on a dichotomous outcome and does not efficiently use such age-at-onset (time-to-event) information. We propose to analyze age-at-onset data usi... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274214 Mon, 03 Mar 2008 05:00:00 +0100 1274214 http://www.medworm.com/index.php?rid=1274213&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710409 This paper focuses on inferences about the overall treatment effect in meta-analysis with normally distributed responses based on the concepts of generalized inference. A refined generalized pivotal quantity based on t distribution is presented and simulation study shows that it can provide confidence intervals with satisfactory coverage probabilities and perform hypothesis testing with satisfactory type-I error control at very small sample sizes. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274213 Mon, 03 Mar 2008 05:00:00 +0100 1274213 http://www.medworm.com/index.php?rid=1274212&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200810413 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274212 Mon, 03 Mar 2008 05:00:00 +0100 1274212 http://www.medworm.com/index.php?rid=1274211&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710400 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274211 Mon, 03 Mar 2008 05:00:00 +0100 1274211 http://www.medworm.com/index.php?rid=1274210&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710401 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1274210 Mon, 03 Mar 2008 05:00:00 +0100 1274210 http://www.medworm.com/index.php?rid=1216924&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710388 In a randomized clinical trial (RCT), noncompliance with an assigned treatment can occur due to serious side effects, while missing outcomes on patients may happen due to patients' withdrawal or loss to follow up. To avoid the possible loss of power to detect a given risk difference (RD) of interest between two treatments, it is essentially important to incorporate the information on noncompliance and missing outcomes into sample size calculation. Under the compound exclusion restriction model proposed elsewhere, we first derive the maximum likelihood estimator (MLE) of the RD among compliers between two treatments for a RCT with noncompliance and missing outcomes and its asymptotic variance in closed form. Based on the MLE with tanh-1(x) transformation, we develop an asymptotic test proce... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1216924 Fri, 08 Feb 2008 05:00:00 +0100 1216924 http://www.medworm.com/index.php?rid=1176555&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710395 Surveys often contain qualitative variables for which respondents may select any number of the outcome categories. For instance, for the question "What type of contraception have you used?" with possible responses (oral, condom, lubricated condom, spermicide, and diaphragm), respondents would be instructed to select as many of the outcomes that apply. This situation is known as multiple responses. When the data includes stratification variables, we discuss two approaches: (1) the "GEE" approach which uses logit models directly applying the generalized estimating equations (GEE) method (Liang and Zeger, 1986); and (2) the "GMH" approach which extends the generalized Mantel-Haenszel type estimators (Greenland, 1989) to make inferences across multiple responses. These approaches can also be u... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1176555 Fri, 25 Jan 2008 05:00:00 +0100 1176555 http://www.medworm.com/index.php?rid=1176554&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710396 A retrospective likelihood-based approach was proposed to test and estimate the effect of haplotype on disease risk using unphased genotype data with adjustment for environmental covariates. The proposed method was also extended to handle the data in which the haplotype and environmental covariates are not independent. Likelihood ratio tests were constructed to test the effects of haplotype and gene-environment interaction. The model parameters such as haplotype effect size was estimated using an Expectation Conditional-Maximization (ECM) algorithm developed by Meng and Rubin (1993). Model-based variance estimates were derived using the observed information matrix. Simulation studies were conducted for three different genetic effect models, including dominant effect, recessive effect, and ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1176554 Fri, 25 Jan 2008 05:00:00 +0100 1176554 http://www.medworm.com/index.php?rid=1176553&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710398 To search the entire human genome for association is a novel and promising approach to unravelling the genetic basis of complex genetic diseases. In these genome-wide association studies (GWAs), several hundreds of thousands of single nucleotide polymorphisms (SNPs) are analyzed at the same time, posing substantial biostatistical and computational challenges. In this paper, we discuss a number of biostatistical aspects of GWAs in detail. We specifically consider quality control issues and show that signal intensity plots are a sine qua condition non in today's GWAs. Approaches to detect and adjust for population stratification are briefly examined. We discuss different strategies aimed at tackling the problem of multiple testing, including adjustment of p -values, the false positive report... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1176553 Fri, 25 Jan 2008 05:00:00 +0100 1176553 http://www.medworm.com/index.php?rid=1176552&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710399 Mixture cure models have been utilized to analyze survival data with possible cure. This paper considers the inclusion of frailty into the mixture cure model to model recurrent event data with a cure fraction. An attractive feature of the proposed model is the allowance for heterogeneity in risk among those individuals experiencing the event of interest in addition to the incorporation of a cured component. Maximum likelihood estimates can be obtained using the Expectation Maximization algorithm and standard errors are calculated from the Bootstrap method. The model is applied to hospital readmission data among colorectal cancer patients. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1176552 Fri, 25 Jan 2008 05:00:00 +0100 1176552 http://www.medworm.com/index.php?rid=1176551&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710387 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1176551 Fri, 25 Jan 2008 05:00:00 +0100 1176551 http://www.medworm.com/index.php?rid=1176550&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710392 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1176550 Fri, 25 Jan 2008 05:00:00 +0100 1176550 http://www.medworm.com/index.php?rid=1176549&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710397 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1176549 Fri, 25 Jan 2008 05:00:00 +0100 1176549 http://www.medworm.com/index.php?rid=1165643&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710394 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1165643 Mon, 21 Jan 2008 05:00:00 +0100 1165643 http://www.medworm.com/index.php?rid=1165642&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710393 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1165642 Mon, 21 Jan 2008 05:00:00 +0100 1165642 http://www.medworm.com/index.php?rid=1089222&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710390 When making Bayesian inferences we need to elicit an expert's opinion to set up the prior distribution. For applications in clinical trials, we study this problem with binary variables. A critical and often ignored issue in the process of eliciting priors in clinical trials is that medical investigators can seldom specify the prior quantities with precision. In this paper, we discuss several methods of eliciting beta priors from clinical information, and we use simulations to conduct sensitivity analyses of the effect of imprecise assessment of the prior information. These results provide useful guidance for choosing methods of eliciting the prior information in practice. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1089222 Wed, 12 Dec 2007 05:00:00 +0100 1089222 http://www.medworm.com/index.php?rid=1070643&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710391 This paper proposes a novel approach for the confidence interval estimation and hypothesis testing of the common mean of several log-normal populations using the concept of generalized variable. Simulation studies demonstrate that the proposed approach can provide confidence intervals with satisfying coverage probabilities and can perform hypothesis testing with satisfying type-I error control even at small sample sizes. Overall, it is superior to the large sample approach. The proposed method is illustrated using two examples. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1070643 Tue, 04 Dec 2007 05:00:00 +0100 1070643 http://www.medworm.com/index.php?rid=1003091&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710382 Based on uniformly most powerful unbiased (UMPU) tests for two-sided hypotheses and a short note in Lehmann (1959) on critical levels for randomized tests, Meulepas (1998, 1999) proposed (two-tailed) P -values taking into account the randomization constant(s) of the UMPU-tests. While UMPU-tests need an extra uniform observation if randomization is required, the P -values proposed by Meulepas need no extra uniform observation. At first glance, his idea looks very promising in order to define a suitable and powerful P -value. Unfortunately, such P -values are generally too conservative. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1003091 Mon, 05 Nov 2007 05:00:00 +0100 1003091 http://www.medworm.com/index.php?rid=1003090&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710383 This article investigates concepts of concordance and gives a comprehensive statistical treatment for testing concordance of various clinical traits in familial studies. For dichotomous traits, the distribution of this statistic under the null hypothesis of no familial aggregation is obtained by three methods: asymptotic, probability generating function, and permutation. The permutation method is extended to analyze aggregation for non-dichotomous traits and co-aggregations between two traits. We apply the permutation method to analyze the aforementioned multiply-affected IBD family data. Evidence is found for familial clustering of various traits, some of which are not revealed in existing studies. Such analyses provide a basis for investigating the dependence of trait aggregation upon ge... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=1003090 Mon, 05 Nov 2007 05:00:00 +0100 1003090 http://www.medworm.com/index.php?rid=937650&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610375 Disease incidence or mortality data are typically available as rates or counts for specified regions, collected over time. We propose Bayesian nonparametric spatial modeling approaches to analyze such data. We develop a hierarchical specification using spatial random effects modeled with a Dirichlet process prior. The Dirichlet process is centered around a multivariate normal distribution. This latter distribution arises from a log-Gaussian process model that provides a latent incidence rate surface, followed by block averaging to the areal units determined by the regions in the study. With regard to the resulting posterior predictive inference, the modeling approach is shown to be equivalent to an approach based on block averaging of a spatial Dirichlet process to obtain a prior probabili... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=937650 Tue, 09 Oct 2007 04:00:00 +0100 937650 http://www.medworm.com/index.php?rid=937649&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710367 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=937649 Tue, 09 Oct 2007 04:00:00 +0100 937649 http://www.medworm.com/index.php?rid=937648&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710384 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=937648 Tue, 09 Oct 2007 04:00:00 +0100 937648 http://www.medworm.com/index.php?rid=937647&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710385 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=937647 Tue, 09 Oct 2007 04:00:00 +0100 937647 http://www.medworm.com/index.php?rid=864901&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610362 In this report we discuss statistical methods for on-line peak detection. One motive for doing this is the need for health resource planning. Surveillance systems were adapted for Swedish data on laboratory verified diagnoses of influenza. In Sweden, the parameters of the influenza cycles vary too much from year to year for parametric methods to be useful. We suggest a non-parametric method based on the monotonicity properties of the increase and decline around a peak. A Monte Carlo study indicated that this method has useful stochastic properties. The method was applied to Swedish data on laboratory verified diagnoses of influenza for seven periods. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=864901 Tue, 11 Sep 2007 04:00:00 +0100 864901 http://www.medworm.com/index.php?rid=864900&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610377 An estimate of the risk or prevalence ratio, adjusted for confounders, can be obtained from a log binomial model (binomial errors, log link) fitted to binary outcome data. We propose a modification of the log binomial model to obtain relative risk estimates for nominal outcomes with more than two attributes (the "log multinomial model"). Extensive data simulations were undertaken to compare the performance of the log multinomial model with that of an expanded data multinomial logistic regression method based on the approach proposed by Schouten et al. (1993) for binary data, and with that of separate fits of a Poisson regression model based on the approach proposed by Zou (2004) and Carter, Lipsitz and Tilley (2005) for binary data. Log multinomial regression resulted in "inadmissable" sol... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=864900 Tue, 11 Sep 2007 04:00:00 +0100 864900 http://www.medworm.com/index.php?rid=864899&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610379 This paper considers inference methods for case-control logistic regression in longitudinal setups. The motivation is provided by an analysis of plains bison spatial location as a function of habitat heterogeneity. The sampling is done according to a longitudinal matched case-control design in which, at certain time points, exactly one case, the actual location of an animal, is matched to a number of controls, the alternative locations that could have been reached. We develop inference methods for the conditional logistic regression model in this setup, which can be formulated within a generalized estimating equation (GEE) framework. This permits the use of statistical techniques developed for GEE-based inference, such as robust variance estimators and model selection criteria adapted for ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=864899 Tue, 11 Sep 2007 04:00:00 +0100 864899 http://www.medworm.com/index.php?rid=864898&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710360 When a sampling unit doesn't respond to a survey it is termed unit nonresponse. Unit nonresponse may have a dramatic affect on estimation results of interest. Using only those who responded to the survey to calculate the estimate may bias the estimate, known as nonresponse bias. Many approaches have been created in order to account for nonresponse. One such approach is to resample those nonrespondents in a second response "phase" (or more). We build a Bayesian hierarchical model that uses information from multiple response "phases" to estimate the phase specific response rates from I subdomains. This information is simultaneously used to estimate the success rates in those I subdomains. Conditional success rates are then estimated for the first phase respondents, second phase respondents, ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=864898 Tue, 11 Sep 2007 04:00:00 +0100 864898 http://www.medworm.com/index.php?rid=830351&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710370 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=830351 Thu, 30 Aug 2007 04:00:00 +0100 830351 http://www.medworm.com/index.php?rid=826187&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710353 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=826187 Wed, 29 Aug 2007 04:00:00 +0100 826187 http://www.medworm.com/index.php?rid=826191&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610358 Optimal response-adaptive designs in phase III clinical trial set up are gaining more interest. Most of the available designs are not based on any optimal consideration. An optimal design for binary responses is given by Rosenberger et al. (2001) and one for continuous responses is provided by Biswas and Mandal (2004). Recently, Zhang and Rosenberger (2006) proposed another design for normal responses. This paper illustrates that the Zhang and Rosenberger (2006) design is not suitable for normally distributed responses, in general. The approach cannot be extended for other continuous response cases, such as exponential or gamma. In this paper, we first describe when the optimal design of Zhang and Rosenberger (2006) fails. We then suggest the appropriate adjustments for designs in differen... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=826191 Mon, 27 Aug 2007 04:00:00 +0100 826191 http://www.medworm.com/index.php?rid=826190&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610371 Maternity length of stay (LOS) is an important measure of hospital activity, but its empirical distribution is often positively skewed. A two-component gamma mixture regression model has been proposed to analyze the heterogeneous maternity LOS. The problem is that observations collected from the same hospital are often correlated, which can lead to spurious associations and misleading inferences. To account for the inherent correlation, random effects are incorporated within the linear predictors of the two-component gamma mixture regression model. An EM algorithm is developed for the residual maximum quasi-likelihood estimation of the regression coefficients and variance component parameters. The approach enables the correct identification and assessment of risk factors affecting the shor... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=826190 Mon, 27 Aug 2007 04:00:00 +0100 826190 http://www.medworm.com/index.php?rid=826189&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710357 This paper examines exact one-sided confidence limits for the risk ratio in a 2 × 2 table with structural zero. Starting with four approximate lower and upper limits, we adjust each using the algorithm of Buehler (1957) to arrive at lower (upper) limits that have exact coverage properties and are as large (small) as possible subject to coverage, as well as an ordering, constraint. Different Buehler limits are compared by their mean size, since all are exact in their coverage. Buehler limits based on the signed root likelihood ratio statistic are found to have the best performance and recommended for practical use. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=826189 Mon, 27 Aug 2007 04:00:00 +0100 826189 http://www.medworm.com/index.php?rid=826188&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710365 We develop a Bayesian simulation based approach for determining the sample size required for estimating a binomial probability and the difference between two binomial probabilities where we allow for dependence between two fallible diagnostic procedures. Examples include estimating the prevalence of disease in a single population based on results from two imperfect diagnostic tests applied to sampled individuals, or surveys designed to compare the prevalences of two populations using diagnostic outcomes that are subject to misclassification. We propose a two stage procedure in which the tests are initially assumed to be independent conditional on true disease status (i.e. conditionally independent). An interval based sample size determination scheme is performed under this assumption and d... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=826188 Mon, 27 Aug 2007 04:00:00 +0100 826188 http://www.medworm.com/index.php?rid=755944&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710355 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=755944 Wed, 25 Jul 2007 04:00:00 +0100 755944 http://www.medworm.com/index.php?rid=755946&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710352 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=755946 Tue, 24 Jul 2007 04:00:00 +0100 755946 http://www.medworm.com/index.php?rid=755945&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710354 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=755945 Tue, 24 Jul 2007 04:00:00 +0100 755945 http://www.medworm.com/index.php?rid=747426&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610339 This paper proposes an approximative method for maximum likelihood estimation of parameters of Neyman-Scott and similar point processes. It is based on the point pattern resulting from forming all difference points of pairs of points in the window of observation. The intensity function of this constructed point process can be expressed in terms of second-order characteristics of the original process. This opens the way to parameter estimation, if the difference pattern is treated as a non-homogeneous Poisson process. The computational feasibility and accuracy of this approach is examined by means of simulated data. Furthermore, the method is applied to two biological data sets. For these data, various cluster process models are considered and compared with respect to their goodness-of-fit.... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=747426 Fri, 20 Jul 2007 04:00:00 +0100 747426 http://www.medworm.com/index.php?rid=726553&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200610337 Inverse sampling is considered to be a more appropriate sampling scheme than the usual binomial sampling scheme when subjects arrive sequentially, when the underlying response of interest is acute, and when maximum likelihood estimators of some epidemiologic indices are undefined. In this article, we study various statistics for testing non-unity rate ratios in case-control studies under inverse sampling. These include the Wald, unconditional score, likelihood ratio and conditional score statistics. Three methods (the asymptotic, conditional exact, and Mid-P methods) are adopted for P -value calculation. We evaluate the performance of different combinations of test statistics and P -value calculation methods in terms of their empirical sizes and powers via Monte Carlo simulation. In genera... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=726553 Tue, 10 Jul 2007 04:00:00 +0100 726553 http://www.medworm.com/index.php?rid=726552&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710351 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=726552 Tue, 10 Jul 2007 04:00:00 +0100 726552 http://www.medworm.com/index.php?rid=678401&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200710347 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=678401 Mon, 18 Jun 2007 04:00:00 +0100 678401