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. Mon, 06 Feb 2012 18:10:47 +0100 http://www.medworm.com/index.php?rid=5569048&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201290002 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5569048 Sun, 01 Jan 2012 05:00:00 +0100 5569048 http://www.medworm.com/index.php?rid=5569047&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201290001 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5569047 Sun, 01 Jan 2012 05:00:00 +0100 5569047 http://www.medworm.com/index.php?rid=5569046&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201290000 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5569046 Sun, 01 Jan 2012 05:00:00 +0100 5569046 http://www.medworm.com/index.php?rid=5542330&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100083 We present a hierarchical Bayesian model (HBM) to estimate the growth parameters, production, and production over biomass ratio (P/B) of resident brown trout (Salmo trutta fario) populations. The data which are required to run the model are removal sampling and air temperature data which are conveniently gathered by freshwater biologists. The model is the combination of eight submodels: abundance, weight, biomass, growth, growth rate, time of emergence, water temperature, and production. Abundance is modeled as a mixture of Gaussian cohorts; cohorts centers and standard deviations are related by a von Bertalanffy growth function; time of emergence and growth rate are functions of water temperature; water temperature is predicted from air temperature; biomass, production, and P/B are subseq... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5542330 Fri, 23 Dec 2011 05:00:00 +0100 5542330 http://www.medworm.com/index.php?rid=5542329&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100069 AbstractCancer survival is one of the most important measures to evaluate the effectiveness of treatment and early diagnosis. The ultimate goal of cancer research and patient care is the cure of cancer. As cancer treatments progress, cure becomes a reality for many cancers if patients are diagnosed early and get effective treatment. If a cure does exist for a certain type of cancer, it is useful to estimate the time of cure. For cancers that impose excess risk of mortality, it is informative to understand the difference in survival between cancer patients and the general cancer‐free population. In population‐based cancer survival studies, relative survival is the standard measure of excess mortality due to cancer. Cure is achieved when the survival of cancer patients is equivalent to t... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5542329 Fri, 23 Dec 2011 05:00:00 +0100 5542329 http://www.medworm.com/index.php?rid=5542328&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000135 AbstractThis paper focuses on the problem of functional statistical classification of gene expression curves. A local‐wavelet‐vaguelette‐based functional logistic regression approach is presented. This approach is specially suitable for the classification of non‐stationary singular (non‐differentiable) curves. The performance of the methodology proposed is illustrated by implementing it for the classification of yeast cell‐cycle temporal gene expression profiles. A simulation study is also carried out for comparison with other functional classification methodologies. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5542328 Fri, 23 Dec 2011 05:00:00 +0100 5542328 http://www.medworm.com/index.php?rid=5501251&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000164 AbstractA modification of the principal component test is presented. It uses a weighted combination of the sums of squares for different principal components and is thus more powerful in high‐dimensional settings with small sample sizes. Under usual normality assumptions, a rotation test is proposed which enables an exact conditional parametric test. The procedure is demonstrated with microarray data for the bacterial composition in the rhizosphere of different potato cultivars. In simulation studies, the power of the proposed statistic is compared with the competing multivariate parametric tests. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5501251 Wed, 14 Dec 2011 05:00:00 +0100 5501251 http://www.medworm.com/index.php?rid=5501250&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000256 AbstractAnalysis of adverse events (AE) for drug safety assessment presents challenges to statisticians in observational studies as well as in clinical trials since AEs are typically recurrent with varying duration and severity. Routine analyses often concentrate on the number of patients who had at least one occurrence of a specific AE or a group of AEs, or the time to occurrence of the first event. We argue that other information in AE data particularly cumulative duration of events is also important, particularly for benefit‐risk assessment. We propose a nonparametric method to estimate the mean cumulative duration (MCD) based on the nonparametric cumulative mean function estimate, together with a robust estimate for the variance of the estimate, as in Lawless and Nadeau (1995). This ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5501250 Wed, 14 Dec 2011 05:00:00 +0100 5501250 http://www.medworm.com/index.php?rid=5542327&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000107 AbstractTo compare the survival between screen‐detected and clinically detected cancers, we applied a series of non‐homogeneous stochastic processes to deal with leadtime, length bias, and over‐detection by using full information on detection modes obtained from the Finnish randomized controlled trial for prostate cancer screening. The results show after 9‐year follow‐up the hazard ratio of prostate cancer death for screen‐detected cases against clinically detected cases increased from 0.24 (95% CI: 0.16–0.35) without correction for these biases, to 0.76 after correction for leadtime and length biases, and finally to 1.03 (95% CI: 0.79–1.33) for a further adjustment for over‐detection. Adjustment for leadtime and length bias but no over‐detection led to a 24% reduction ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5542327 Tue, 01 Nov 2011 04:00:00 +0100 5542327 http://www.medworm.com/index.php?rid=5501249&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100058 AbstractAn estimator of the hazard rate function from discrete failure time data is obtained by semiparametric smoothing of the (nonsmooth) maximum likelihood estimator, which is achieved by repeated multiplication of a Markov chain transition‐type matrix. This matrix is constructed so as to have a given standard discrete parametric hazard rate model, termed the vehicle model, as its stationary hazard rate. As with the discrete density estimation case, the proposed estimator gives improved performance when the vehicle model is a good one and otherwise provides a nonparametric method comparable to the only purely nonparametric smoother discussed in the literature. The proposed semiparametric smoothing approach is then extended to hazard models with covariates and is illustrated by applica... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5501249 Tue, 01 Nov 2011 04:00:00 +0100 5501249 http://www.medworm.com/index.php?rid=5492727&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100025 AbstractThe classification accuracy of new diagnostic tests is based on receiver operating characteristic (ROC) curves. The area under the ROC curve (AUC) is one of the well‐accepted summary measures for describing the accuracy of diagnostic tests. The AUC summary measure can vary by patient and testing characteristics. Thus, the performance of the test may be different in certain subpopulation of patients and readers. For this purpose, we propose a direct semi‐parametric regression model for the non‐parametric AUC measure for ordinal data while accounting for discrete and continuous covariates. The proposed method can be used to estimate the AUC value under degenerate data where certain rating categories are not observed. We will discuss the non‐standard asymptotic theory, since t... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5492727 Tue, 01 Nov 2011 04:00:00 +0100 5492727 http://www.medworm.com/index.php?rid=5417776&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100062 This article reports on the application of Bayes rule for updating risk prediction tools to include a set of biomarkers measured in an external study to the original study used to develop the risk prediction tool. The procedure is illustrated in the context of updating the online Prostate Cancer Prevention Trial Risk Calculator to incorporate the new markers %freePSA and [‐2]proPSA measured on an external case–control study performed in Texas, U.S.. Recent state‐of‐the art methods in validation of risk prediction tools and evaluation of the improvement of updated to original tools are implemented using an external validation set provided by the U.S. Early Detection Research Network. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5417776 Tue, 01 Nov 2011 04:00:00 +0100 5417776 http://www.medworm.com/index.php?rid=5396991&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100070 We describe a nonparametric Bayesian approach for estimating the three‐way ROC surface based on mixtures of finite Polya trees (MFPT) priors. Mixtures of finite Polya trees are robust models that can handle nonstandard features in the data. We address the difficulties in modeling continuous diagnostic data with skewness, multimodality, or other nonstandard features, and how parametric approaches can lead to misleading results in such cases. Robust, data‐driven inference for the ROC surface and for the volume under the ROC surface is obtained. A simulation study is performed to assess the performance of the proposed method. Methods are applied to data from a magnetic resonance spectroscopy study on human immunodeficiency virus patients. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5396991 Tue, 01 Nov 2011 04:00:00 +0100 5396991 http://www.medworm.com/index.php?rid=5396990&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100008 AbstractWe propose a robust Cox regression model with outliers. The model is fit by trimming the smallest contributions to the partial likelihood. To do so, we implement a Metropolis‐type maximization routine, and show its convergence to a global optimum. We discuss global robustness properties of the approach, which is illustrated and compared through simulations. We finally fit the model on an original and on a benchmark data set. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5396990 Tue, 01 Nov 2011 04:00:00 +0100 5396990 http://www.medworm.com/index.php?rid=5396989&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100073 AbstractFor the all pairwise comparisons for equivalence of k (k≥2) treatments Lauzon and Caffo proposed simply to divide the type I error level α by k−1 to achieve a Bonferroni‐based familywise error control when declaring pairs of two treatments equivalent. This rule is shown to be too liberal for k≥4. It works for k=3 yet for reasons not considered by Lauzon and Caffo. Based on the two one‐sided testing procedures and using the closure test principle we develop valid alternatives based on Bonferroni's inequality. The set H of intersection hypotheses reveals a rich structure, leading to the possibility to present H as a directed acyclic graph (DAG). This in turn allows using some graph theoretical theorems and eases proving properties of the resulting multiple testing problems... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5396989 Tue, 01 Nov 2011 04:00:00 +0100 5396989 http://www.medworm.com/index.php?rid=5396988&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100036 AbstractThis paper discusses multiplicity issues arising in confirmatory clinical trials with hierarchically ordered multiple objectives. In order to protect the overall type I error rate, multiple objectives are analyzed using multiple testing procedures. When the objectives are ordered and grouped in multiple families (e.g. families of primary and secondary endpoints), gatekeeping procedures are employed to account for this hierarchical structure. We discuss considerations arising in the process of building gatekeeping procedures, including proper use of relevant trial‐specific information and criteria for selecting gatekeeping procedures. The methods and principles discussed in this paper are illustrated using a clinical trial in patients with type II diabetes mellitus. (Source: Biome... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5396988 Tue, 01 Nov 2011 04:00:00 +0100 5396988 http://www.medworm.com/index.php?rid=5396987&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190015 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5396987 Tue, 01 Nov 2011 04:00:00 +0100 5396987 http://www.medworm.com/index.php?rid=5396986&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190014 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5396986 Tue, 01 Nov 2011 04:00:00 +0100 5396986 http://www.medworm.com/index.php?rid=5396985&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190013 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5396985 Tue, 01 Nov 2011 04:00:00 +0100 5396985 http://www.medworm.com/index.php?rid=5343624&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900259 In this study, we assess the influence of risk factors on the transitions among three cognitive status: cognitive stability (normal cognition for age), memory impairment, and clinical dementia. We have developed a shared random effects model that not only links the propensity of transitions and to the probability of informative missingness due to death, but also incorporates heterogeneous transition between subjects. We evaluate four approaches using generalized logit and four using proportional odds models to the first‐order Markov transition probabilities as a function of covariates. Random effects were incorporated into these models to account for within‐subject correlations. Data from the Einstein Aging Study are used to evaluate the goodness‐of‐fit of these models using the Ak... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5343624 Fri, 21 Oct 2011 04:00:00 +0100 5343624 http://www.medworm.com/index.php?rid=5343623&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100177 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5343623 Fri, 21 Oct 2011 04:00:00 +0100 5343623 http://www.medworm.com/index.php?rid=5330925&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000247 AbstractIn their review of challenges to multiple testing in clinical trials, Hung and Wang (2010) considered the situation where a treatment is to be compared with an active comparator and the aim is to show non‐inferiority and (if possible) superiority with respect to a primary and a secondary endpoint. This note extends their discussion of this particular situation, taking the sequentially rejective procedure they used for illustration as a starting point. Some alternative multiple testing procedures (MTPs) are considered, and corresponding simultaneous confidence regions are discussed that provide additional information “for free”. The choice may then be based on the properties of these MTPs and corresponding confidence regions. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5330925 Mon, 17 Oct 2011 04:00:00 +0100 5330925 http://www.medworm.com/index.php?rid=5321664&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100147 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5321664 Fri, 14 Oct 2011 04:00:00 +0100 5321664 http://www.medworm.com/index.php?rid=5355523&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000175 AbstractAnalysis of cumulative incidence (sometimes called absolute risk or crude risk) can be difficult if the cause of failure is missing for some subjects. Assuming missingness is random conditional on the observed data, we develop asymptotic theory for multiple imputation methods to estimate cumulative incidence. Covariates affect cause‐specific hazards in our model, and we assume that separate proportional hazards models hold for each cause‐specific hazard. Simulation studies show that procedures based on asymptotic theory have near nominal operating characteristics in cohorts of 200 and 400 subjects, both for cumulative incidence and for prediction error. The methods are illustrated with data on survival after breast cancer, obtained from the National Surgical Adjuvant Breast and... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5355523 Thu, 01 Sep 2011 04:00:00 +0100 5355523 http://www.medworm.com/index.php?rid=5343622&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100085 AbstractIn epidemiological and clinical research, investigators are frequently interested in estimating the direct effect of a treatment on an outcome that is not relayed by intermediate variables. In 2009, VanderWeele presented marginal structural models (MSMs) for estimating direct effects based on interventions on the mediator. This paper focuses on direct effects based on principal stratification, i.e. principal stratum direct effects (PSDEs), which are causal effects within latent subgroups of subjects where the mediator is constant, regardless of the exposure status. We propose MSMs for estimating PSDEs. We demonstrate that the PSDE can be estimated readily using MSMs under the monotonicity assumption. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5343622 Thu, 01 Sep 2011 04:00:00 +0100 5343622 http://www.medworm.com/index.php?rid=5330924&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000193 AbstractThe effect of the plot shape, number of subplots and their spatial arrangement on the sample variance for spatially explicit point populations is analysed for a simple intensity estimator. We derive the sample variance and covariance for sampling designs involving more than one subplot. Some numerical approximations are also presented. If a clustered point pattern has to be sampled, the best strategy to reduce the sample variance is to consider as many rectangular subplots as possible, for a prescribed total sample area, distributed over a grid. In contrast, if a regular point pattern is to be sampled, then a single circular subplot should be considered. If we assume that the point configuration is Poisson, then we can consider any subplot shape and spatial distribution ensuring no... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5330924 Thu, 01 Sep 2011 04:00:00 +0100 5330924 http://www.medworm.com/index.php?rid=5321663&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100166 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5321663 Thu, 01 Sep 2011 04:00:00 +0100 5321663 http://www.medworm.com/index.php?rid=5191376&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000218 AbstractCharacterizing associations among multiple single‐nucleotide polymorphisms (SNPs) within and across genes, and measures of disease progression or disease status will potentially offer new insight into disease etiology and disease progression. However, this presents a significant analytic challenge due to the existence of multiple potentially informative genetic loci, as well as environmental and demographic factors, and the generally uncharacterized and complex relationships among them. Latent variable modeling approaches offer a natural framework for analysis of data arising from these population‐based genetic association investigations of complex diseases as they are well‐suited to uncover simultaneous effects of multiple markers. In this manuscript we describe application ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5191376 Wed, 31 Aug 2011 23:00:00 +0100 5191376 http://www.medworm.com/index.php?rid=5191375&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000040 AbstractMajor objectives of a clinical trial are commonly stated in a hierarchical order as primary and secondary. The parallel gatekeeping testing strategy provides an opportunity to assess secondary objectives when all or partial primary objectives are achieved. The current available gatekeeping procedures have different pros and cons so users either need to justify the assumption associated with some procedures or tolerate suboptimal power performance of other procedures. By applying the Holm test with a flexible alpha splitting technique, we propose a procedure which (1) is powerful for assessing the primary objectives, (2) can be used when no assumption can be made on the dependency structure of test statistics, and (3) has the full flexibility to allocate user‐preferred alpha to as... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5191375 Wed, 31 Aug 2011 23:00:00 +0100 5191375 http://www.medworm.com/index.php?rid=5191374&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190012 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5191374 Wed, 31 Aug 2011 23:00:00 +0100 5191374 http://www.medworm.com/index.php?rid=5191373&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190011 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5191373 Wed, 31 Aug 2011 23:00:00 +0100 5191373 http://www.medworm.com/index.php?rid=5191372&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190010 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5191372 Wed, 31 Aug 2011 23:00:00 +0100 5191372 http://www.medworm.com/index.php?rid=5155851&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000131 AbstractSequentially observed survival times are of interest in many studies but there are difficulties in analyzing such data using nonparametric or semiparametric methods. First, when the duration of followup is limited and the times for a given individual are not independent, induced dependent censoring arises for the second and subsequent survival times. Non‐identifiability of the marginal survival distributions for second and later times is another issue, since they are observable only if preceding survival times for an individual are uncensored. In addition, in some studies a significant proportion of individuals may never have the first event. Fully parametric models can deal with these features, but robustness is a concern. We introduce a new approach to address these issues. We ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5155851 Tue, 23 Aug 2011 23:00:00 +0100 5155851 http://www.medworm.com/index.php?rid=5155850&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000190 AbstractTwo‐part regression models are frequently used to analyze longitudinal count data with excess zeros, where the same set of subjects is repeatedly observed over time. In this context, several sources of heterogeneity may arise at individual level that affect the observed process. Further, longitudinal studies often suffer from missing values: individuals dropout of the study before its completion, and thus present incomplete data records. In this paper, we propose a finite mixture of hurdle models to face the heterogeneity problem, which is handled by introducing random effects with a discrete distribution; a pattern‐mixture approach is specified to deal with non‐ignorable missing values. This approach helps us to consider overdispersed counts, while allowing for association b... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5155850 Tue, 23 Aug 2011 23:00:00 +0100 5155850 http://www.medworm.com/index.php?rid=5117234&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100120 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5117234 Tue, 09 Aug 2011 23:00:00 +0100 5117234 http://www.medworm.com/index.php?rid=5085320&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100071 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5085320 Sun, 31 Jul 2011 23:00:00 +0100 5085320 http://www.medworm.com/index.php?rid=5085319&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100033 AbstractIn the setting of longitudinal study, subjects are followed for the occurrence of some dichotomous outcome. In many of these studies, some markers are also obtained repeatedly during the study period. Emir et al. introduced a non‐parametric approach to the estimation of the area under the ROC curve of a repeated marker. Their non‐parametric estimate involves assigning a weight to each subject. There are two weighting schemes suggested in their paper: one for the case when within‐patient correlation is low, and the other for the case when within‐subject correlation is high. However, it is not clear how to assign weights to marker measurements when within‐patient correlation is modest. In this paper, we consider the optimal weights that minimize the variance of the estimate... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5085319 Sun, 31 Jul 2011 23:00:00 +0100 5085319 http://www.medworm.com/index.php?rid=5048415&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000142 AbstractIn many studies, the association of longitudinal measurements of a continuous response and a binary outcome are often of interest. A convenient framework for this type of problems is the joint model, which is formulated to investigate the association between a binary outcome and features of longitudinal measurements through a common set of latent random effects. The joint model, which is the focus of this article, is a logistic regression model with covariates defined as the individual‐specific random effects in a non‐linear mixed‐effects model (NLMEM) for the longitudinal measurements. We discuss different estimation procedures, which include two‐stage, best linear unbiased predictors, and various numerical integration techniques. The proposed methods are illustrated using... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5048415 Mon, 18 Jul 2011 23:00:00 +0100 5048415 http://www.medworm.com/index.php?rid=5048416&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100138 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5048416 Sun, 17 Jul 2011 23:00:00 +0100 5048416 http://www.medworm.com/index.php?rid=5155849&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100065 AbstractIn randomized trials, an analysis of covariance (ANCOVA) is often used to analyze post‐treatment measurements with pre‐treatment measurements as a covariate to compare two treatment groups. Random allocation guarantees only equal variances of pre‐treatment measurements. We hence consider data with unequal covariances and variances of post‐treatment measurements without assuming normality. Recently, we showed that the actual type I error rate of the usual ANCOVA assuming equal slopes and equal residual variances is asymptotically at a nominal level under equal sample sizes, and that of the ANCOVA with unequal variances is asymptotically at a nominal level, even under unequal sample sizes. In this paper, we investigated the asymptotic properties of the ANCOVA with unequal slo... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5155849 Thu, 30 Jun 2011 23:00:00 +0100 5155849 http://www.medworm.com/index.php?rid=5124499&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000239 AbstractThe confirmatory analysis of pre‐specified multiple hypotheses has become common in pivotal clinical trials. In the recent past multiple test procedures have been developed that reflect the relative importance of different study objectives, such as fixed sequence, fallback, and gatekeeping procedures. In addition, graphical approaches have been proposed that facilitate the visualization and communication of Bonferroni‐based closed test procedures for common multiple test problems, such as comparing several treatments with a control, assessing the benefit of a new drug for more than one endpoint, combined non‐inferiority and superiority testing, or testing a treatment at different dose levels in an overall and a subpopulation. In this paper, we focus on extended graphical appr... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5124499 Thu, 30 Jun 2011 23:00:00 +0100 5124499 http://www.medworm.com/index.php?rid=5117233&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100052 We present classical and Bayesian implementations of the shared random effects model and highlight the advantages of the latter for making predictions. We then apply the models described to a study of abdominal aortic aneurysms (AAA) to investigate the association between AAA diameter and the hazard of AAA rupture. Out‐of‐sample predictions of future AAA growth and hazard of rupture are derived from Bayesian posterior predictive distributions, which are easily calculated within an MCMC framework. Finally, using a multivariate survival sub‐model we show that underlying diameter rather than the rate of growth is the most important predictor of AAA rupture. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5117233 Thu, 30 Jun 2011 23:00:00 +0100 5117233 http://www.medworm.com/index.php?rid=5085318&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100103 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5085318 Thu, 30 Jun 2011 23:00:00 +0100 5085318 http://www.medworm.com/index.php?rid=5048414&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100042 AbstractEvaluation of impact of potential uncontrolled confounding is an important component for causal inference based on observational studies. In this article, we introduce a general framework of sensitivity analysis that is based on inverse probability weighting. We propose a general methodology that allows both non‐parametric and parametric analyses, which are driven by two parameters that govern the magnitude of the variation of the multiplicative errors of the propensity score and their correlations with the potential outcomes. We also introduce a specific parametric model that offers a mechanistic view on how the uncontrolled confounding may bias the inference through these parameters. Our method can be readily applied to both binary and continuous outcomes and depends on the cov... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=5048414 Thu, 30 Jun 2011 23:00:00 +0100 5048414 http://www.medworm.com/index.php?rid=4991861&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000180 AbstractWe consider the problem of jointly modeling survival time and longitudinal data subject to measurement error. The survival times are modeled through the proportional hazards model and a random effects model is assumed for the longitudinal covariate process. Under this framework, we propose an approximate nonparametric corrected‐score estimator for the parameter, which describes the association between the time‐to‐event and the longitudinal covariate. The term nonparametric refers to the fact that assumptions regarding the distribution of the random effects and that of the measurement error are unnecessary. The finite sample size performance of the approximate nonparametric corrected‐score estimator is examined through simulation studies and its asymptotic properties are als... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4991861 Thu, 30 Jun 2011 23:00:00 +0100 4991861 http://www.medworm.com/index.php?rid=4991860&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190009 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4991860 Thu, 30 Jun 2011 23:00:00 +0100 4991860 http://www.medworm.com/index.php?rid=4991859&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190008 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4991859 Thu, 30 Jun 2011 23:00:00 +0100 4991859 http://www.medworm.com/index.php?rid=4991858&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190007 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4991858 Thu, 30 Jun 2011 23:00:00 +0100 4991858 http://www.medworm.com/index.php?rid=4920122&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100105 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4920122 Thu, 09 Jun 2011 23:00:00 +0100 4920122 http://www.medworm.com/index.php?rid=4847270&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100090 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4847270 Thu, 19 May 2011 23:00:00 +0100 4847270 http://www.medworm.com/index.php?rid=4944276&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000018 AbstractIn allometry, bivariate techniques related to principal component analysis are often used in place of linear regression, and primary interest is in making inferences about the slope. We demonstrate that the current inferential methods are not robust to bivariate contamination, and consider four robust alternatives to the current methods – a novel sandwich estimator approach, using robust covariance matrices derived via an influence function approach, Huber's M‐estimator and the fast‐and‐robust bootstrap. Simulations demonstrate that Huber's M‐estimators are highly efficient and robust against bivariate contamination, and when combined with the fast‐and‐robust bootstrap, we can make accurate inferences even from small samples. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4944276 Sat, 30 Apr 2011 23:00:00 +0100 4944276 http://www.medworm.com/index.php?rid=4932443&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000250 AbstractProcess models specified by non‐linear dynamic differential equations contain many parameters, which often must be inferred from a limited amount of data. We discuss a hierarchical Bayesian approach combining data from multiple related experiments in a meaningful way, which permits more powerful inference than treating each experiment as independent. The approach is illustrated with a simulation study and example data from experiments replicating the aspects of the human gut microbial ecosystem. A predictive model is obtained that contains prediction uncertainty caused by uncertainty in the parameters, and we extend the model to capture situations of interest that cannot easily be studied experimentally. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4932443 Sat, 30 Apr 2011 23:00:00 +0100 4932443 http://www.medworm.com/index.php?rid=4920121&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100106 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4920121 Sat, 30 Apr 2011 23:00:00 +0100 4920121 http://www.medworm.com/index.php?rid=4910481&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000235 AbstractIn clinical trials examining the incidence of pneumonia it is a common practice to measure infection via both invasive and non‐invasive procedures. In the context of a recently completed randomized trial comparing two treatments the invasive procedure was only utilized in certain scenarios due to the added risk involved, and given that the level of the non‐invasive procedure surpassed a given threshold. Hence, what was observed was bivariate data with a pattern of missingness in the invasive variable dependent upon the value of the observed non‐invasive observation within a given pair. In order to compare two treatments with bivariate observed data exhibiting this pattern of missingness we developed a semi‐parametric methodology utilizing the density‐based empirical likel... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4910481 Sat, 30 Apr 2011 23:00:00 +0100 4910481 http://www.medworm.com/index.php?rid=4882773&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000189 AbstractExtreme values in predictors often strongly affect the results of statistical analyses in high‐dimensional settings. Although they frequently occur with most high‐throughput techniques, the problem is often ignored in the literature. We suggest to use a very simple transformation, proposed before in a different context by Royston and Sauerbrei, as an intermediary step between array preprocessing and high‐level statistical analysis. This straightforward univariate transformation identifies extreme values in continuous features and can thus be used as a diagnostic tool for outliers. The use of the transformation and its effects is demonstrated for diverse univariate and multivariate statistical analyses using nine publicly available microarray data sets. (Source: Biometrical Jo... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4882773 Sat, 30 Apr 2011 23:00:00 +0100 4882773 http://www.medworm.com/index.php?rid=4861334&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000074 AbstractThis paper investigates homogeneity test of rate ratios in stratified matched‐pair studies on the basis of asymptotic and bootstrap‐resampling methods. Based on the efficient score approach, we develop a simple and computationally tractable score test statistic. Several other homogeneity test statistics are also proposed on the basis of the weighted least‐squares estimate and logarithmic transformation. Sample size formulae are derived to guarantee a pre‐specified power for the proposed tests at the pre‐given significance level. Empirical results confirm that (i) the modified score statistic based on the bootstrap‐resampling method performs better in the sense that its empirical type I error rate is much closer to the pre‐specified nominal level than those of other te... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4861334 Sat, 30 Apr 2011 23:00:00 +0100 4861334 http://www.medworm.com/index.php?rid=4852338&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000098 AbstractHuman tumor xenograft models are often used in preclinical study to evaluate the therapeutic efficacy of a certain compound or a combination of certain compounds. In a typical human tumor xenograft model, human carcinoma cells are implanted to subjects such as severe combined immunodeficient (SCID) mice. Treatment with test compounds is initiated after tumor nodule has appeared, and continued for a certain time period. Tumor volumes are measured over the duration of the experiment. It is well known that untreated tumor growth may follow certain patterns, which can be described by certain mathematical models. However, the growth patterns of the treated tumors with multiple treatment episodes are quite complex, and the usage of parametric models is limited. We propose using cubic smo... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4852338 Sat, 30 Apr 2011 23:00:00 +0100 4852338 http://www.medworm.com/index.php?rid=4847269&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000076 AbstractWe propose a likelihood‐based model for correlated count data that display under‐ or overdispersion within units (e.g. subjects). The model is capable of handling correlation due to clustering and/or serial correlation, in the presence of unbalanced, missing or unequally spaced data. A family of distributions based on birth‐event processes is used to model within‐subject underdispersion. A computational approach is given to overcome a parameterization difficulty with this family, and this allows use of common Markov Chain Monte Carlo software (e.g. WinBUGS) for estimation. Application of the model to daily counts of asthma inhaler use by children shows substantial within‐subject underdispersion, between‐subject heterogeneity and correlation due to both clustering of mea... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4847269 Sat, 30 Apr 2011 23:00:00 +0100 4847269 http://www.medworm.com/index.php?rid=4819746&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000240 AbstractPublication bias, the fact that studies identified for inclusion in a meta analysis do not represent all studies on the topic of interest, is commonly recognized as a threat to the validity of a meta analysis. One way to explicitly model publication bias is via weighted probability distributions. We adopt the non‐parametric approach initially introduced by Dear and Begg (1992) but impose that the weight function w is monotonely non‐increasing as a function of the p‐value. Since in meta analysis one typically only has few studies or “observations,” regularization of the estimation problem seems sensible. In addition, virtually all parametric weight functions proposed so far in the literature are in fact decreasing. We discuss how to estimate a decreasing weight function in... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4819746 Sat, 30 Apr 2011 23:00:00 +0100 4819746 http://www.medworm.com/index.php?rid=4788221&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000162 AbstractLepage's test combines the Wilcoxon rank‐sum and the Ansari–Bradley statistics. We propose to replace the latter statistic by a Wilcoxon rank‐sum calculated after Levene's transformation. We use the medians for this transformation, i.e. absolute deviations from sample medians are calculated. The new location–scale test can be carried out as a permutation test based on permutations of the original observations, the Levene transformation has to be applied for each permutation in an intermediate step to calculate the test statistic. Simulations indicate that the new test can be more powerful than an O'Brien‐type test and Lepage's test, the latter is the standard nonparametric location–scale test. The new test is illustrated using real data about colony sizes of yellow‐ey... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788221 Sat, 30 Apr 2011 23:00:00 +0100 4788221 http://www.medworm.com/index.php?rid=4788220&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000200 In conclusion, the ANCOVA with equal slopes can be asymptotically justified under random allocation. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788220 Sat, 30 Apr 2011 23:00:00 +0100 4788220 http://www.medworm.com/index.php?rid=4788219&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000147 AbstractTill now, multivariate reference regions have played only a marginal role in the practice of clinical chemistry and laboratory medicine. The major reason for this fact is that such regions are traditionally determined by means of concentration ellipsoids of multidimensional Gaussian distributions yielding reference limits which do not allow statements about possible outlyingness of measurements taken in specific diagnostic tests from a given patient or subject. As a promising way around this difficulty we propose to construct multivariate reference regions as p‐dimensional rectangles or (in the one‐sided case) rectangular half‐spaces whose edges determine univariate percentile ranges of the same probability content in each marginal distribution. In a first step, the correspon... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788219 Sat, 30 Apr 2011 23:00:00 +0100 4788219 http://www.medworm.com/index.php?rid=4788218&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000083 AbstractThe receiver operating characteristic (ROC) curve is a tool commonly used to evaluate biomarker utility in clinical diagnosis of disease. Often, multiple biomarkers are developed to evaluate the discrimination for the same outcome. Levels of multiple biomarkers can be combined via best linear combination (BLC) such that their overall discriminatory ability is greater than any of them individually. Biomarker measurements frequently have undetectable levels below a detection limit sometimes denoted as limit of detection (LOD). Ignoring observations below the LOD or substituting some replacement value as a method of correction has been shown to lead to negatively biased estimates of the area under the ROC curve for some distributions of single biomarkers. In this paper, we develop asy... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788218 Sat, 30 Apr 2011 23:00:00 +0100 4788218 http://www.medworm.com/index.php?rid=4788217&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000095 AbstractLikelihood analysis for regression models with measurement errors in explanatory variables typically involves integrals that do not have a closed‐form solution. In this case, numerical methods such as Gaussian quadrature are generally employed. However, when the dimension of the integral is large, these methods become computationally demanding or even unfeasible. This paper proposes the use of the Laplace approximation to deal with measurement error problems when the likelihood function involves high‐dimensional integrals. The cases considered are generalized linear models with multiple covariates measured with error and generalized linear mixed models with measurement error in the covariates. The asymptotic order of the approximation and the asymptotic properties of the Laplac... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788217 Sat, 30 Apr 2011 23:00:00 +0100 4788217 http://www.medworm.com/index.php?rid=4788216&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190006 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788216 Sat, 30 Apr 2011 23:00:00 +0100 4788216 http://www.medworm.com/index.php?rid=4788215&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190005 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788215 Sat, 30 Apr 2011 23:00:00 +0100 4788215 http://www.medworm.com/index.php?rid=4788214&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190004 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4788214 Sat, 30 Apr 2011 23:00:00 +0100 4788214 http://www.medworm.com/index.php?rid=4718987&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000168 AbstractDimension reduction methods have been proposed for regression analysis with predictors of high dimension, but have not received much attention on the problems with censored data. In this article, we present an iterative imputed spline approach based on principal Hessian directions (PHD) for censored survival data in order to reduce the dimension of predictors without requiring a prespecified parametric model. Our proposal is to replace the right‐censored survival time with its conditional expectation for adjusting the censoring effect by using the Kaplan–Meier estimator and an adaptive polynomial spline regression in the residual imputation. A sparse estimation strategy is incorporated in our approach to enhance the interpretation of variable selection. This approach can be imp... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4718987 Tue, 01 Mar 2011 00:00:00 +0100 4718987 http://www.medworm.com/index.php?rid=4714063&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000122 AbstractIdentifying risk factors for transition rates among normal cognition, mildly cognitive impairment, dementia and death in an Alzheimer's disease study is very important. It is known that transition rates among these states are strongly time dependent. While Markov process models are often used to describe these disease progressions, the literature mainly focuses on time homogeneous processes, and limited tools are available for dealing with non‐homogeneity. Further, patients may choose when they want to visit the clinics, which creates informative observations. In this paper, we develop methods to deal with non‐homogeneous Markov processes through time scale transformation when observation times are pre‐planned with some observations missing. Maximum likelihood estimation via ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4714063 Tue, 01 Mar 2011 00:00:00 +0100 4714063 http://www.medworm.com/index.php?rid=4703656&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100031 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4703656 Tue, 01 Mar 2011 00:00:00 +0100 4703656 http://www.medworm.com/index.php?rid=4614712&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000066 AbstractIntraclass correlation (ICC) is an established tool to assess inter‐rater reliability. In a seminal paper published in 1979, Shrout and Fleiss considered three statistical models for inter‐rater reliability data with a balanced design. In their first two models, an infinite population of raters was considered, whereas in their third model, the raters in the sample were considered to be the whole population of raters. In the present paper, we show that the two distinct estimates of ICC developed for the first two models can both be applied to the third model and we discuss their different interpretations in this context. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4614712 Tue, 01 Mar 2011 00:00:00 +0100 4614712 http://www.medworm.com/index.php?rid=4610239&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201100022 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4610239 Tue, 01 Mar 2011 00:00:00 +0100 4610239 http://www.medworm.com/index.php?rid=4590522&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000070 AbstractThis research is motivated by a pilot colorectal adenoma study, where the outcome of interest is the presence of colorectal adenoma representing risk for colorectal cancer, and the predictors of interest are protein biomarkers that are repeatedly measured with errors along the length of a microscopic structure in the human colon, the colon crypt. Biomarkers of this type are referred to as functional biomarkers. The investigators are interested in identifying features of functional biomarkers that are associated with risk for colorectal cancer. In this paper, we investigate a joint modeling approach, where the binary clinical outcome is modeled using a logistic regression model with the unobserved true functional biomarkers as the predictors. Most existing methods are developed eith... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4590522 Tue, 01 Mar 2011 00:00:00 +0100 4590522 http://www.medworm.com/index.php?rid=4549401&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190003 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4549401 Tue, 01 Mar 2011 00:00:00 +0100 4549401 http://www.medworm.com/index.php?rid=4549400&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190002 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4549400 Tue, 01 Mar 2011 00:00:00 +0100 4549400 http://www.medworm.com/index.php?rid=4488847&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000152 AbstractAnalysis of molecular data promises identification of biomarkers for improving prognostic models, thus potentially enabling better patient management. For identifying such biomarkers, risk prediction models can be employed that link high‐dimensional molecular covariate data to a clinical endpoint. In low‐dimensional settings, a multitude of statistical techniques already exists for building such models, e.g. allowing for variable selection or for quantifying the added value of a new biomarker. We provide an overview of techniques for regularized estimation that transfer this toward high‐dimensional settings, with a focus on models for time‐to‐event endpoints. Techniques for incorporating specific covariate structure are discussed, as well as techniques for dealing with mo... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4488847 Thu, 17 Feb 2011 00:00:00 +0100 4488847 http://www.medworm.com/index.php?rid=4488846&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000155 This article aims to give an overview of such current methods as well as the databases, where this external knowledge can be obtained from. For illustration, two recent methods are compared in detail, a feature selection approach for support vector machines as well as a boosting approach for regression models. As a practical example, data on patients with acute lymphoblastic leukemia are considered, where the binary endpoint “relapse within first year” should be predicted. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4488846 Thu, 17 Feb 2011 00:00:00 +0100 4488846 http://www.medworm.com/index.php?rid=4488845&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000157 AbstractIn medical statistics, many alternative strategies are available for building a prediction model based on training data. Prediction models are routinely compared by means of their prediction performance in independent validation data. If only one data set is available for training and validation, then rival strategies can still be compared based on repeated bootstraps of the same data. Often, however, the overall performance of rival strategies is similar and it is thus difficult to decide for one model. Here, we investigate the variability of the prediction models that results when the same modelling strategy is applied to different training sets. For each modelling strategy we estimate a confidence score based on the same repeated bootstraps. A new decomposition of the expected B... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4488845 Thu, 17 Feb 2011 00:00:00 +0100 4488845 http://www.medworm.com/index.php?rid=4459293&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000048 AbstractSurvival prediction from high‐dimensional genomic data is dependent on a proper regularization method. With an increasing number of such methods proposed in the literature, comparative studies are called for and some have been performed. However, there is currently no consensus on which prediction assessment criterion should be used for time‐to‐event data. Without a firm knowledge about whether the choice of evaluation criterion may affect the conclusions made as to which regularization method performs best, these comparative studies may be of limited value. In this paper, four evaluation criteria are investigated: the log‐rank test for two groups, the area under the time‐dependent ROC curve (AUC), an R2‐measure based on the Cox partial likelihood, and an R2‐measure b... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4459293 Thu, 10 Feb 2011 00:00:00 +0100 4459293 http://www.medworm.com/index.php?rid=4459292&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000154 AbstractIn non‐randomized studies, the assessment of a causal effect of treatment or exposure on outcome is hampered by possible confounding. Applying multiple regression models including the effects of treatment and covariates on outcome is the well‐known classical approach to adjust for confounding. In recent years other approaches have been promoted. One of them is based on the propensity score and considers the effect of possible confounders on treatment as a relevant criterion for adjustment. Another proposal is based on using an instrumental variable. Here inference relies on a factor, the instrument, which affects treatment but is thought to be otherwise unrelated to outcome, so that it mimics randomization. Each of these approaches can basically be interpreted as a simple rewei... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4459292 Thu, 10 Feb 2011 00:00:00 +0100 4459292 http://www.medworm.com/index.php?rid=4459291&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000153 AbstractFinding out biomarkers and building risk scores to predict the occurrence of survival outcomes is a major concern of clinical epidemiology, and so is the evaluation of prognostic models. In this paper, we are concerned with the estimation of the time‐dependent AUC – area under the receiver‐operating curve – which naturally extends standard AUC to the setting of survival outcomes and enables to evaluate the discriminative power of prognostic models. We establish a simple and useful relation between the predictiveness curve and the time‐dependent AUC – AUC(t). This relation confirms that the predictiveness curve is the key concept for evaluating calibration and discrimination of prognostic models. It also highlights that accurate estimates of the conditional absolute risk... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4459291 Thu, 10 Feb 2011 00:00:00 +0100 4459291 http://www.medworm.com/index.php?rid=4501199&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000150 AbstractIn disease screening and prognosis studies, an important task is to determine useful markers for identifying high‐risk subgroups. Once such markers are established, they can be incorporated into public health practice to provide appropriate strategies for treatment or disease monitoring based on each individual's predicted risk. In the recent years, genetic and biological markers have been examined extensively for their potential to signal progression or risk of disease. In addition to these markers, it has often been argued that short‐term outcomes may be helpful in making a better prediction of disease outcomes in clinical practice. In this paper we propose model‐free non‐parametric procedures to incorporate short‐term event information to improve the prediction of a lo... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4501199 Tue, 01 Feb 2011 00:00:00 +0100 4501199 http://www.medworm.com/index.php?rid=4488844&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000159 AbstractThe focus of many medical applications is to model the impact of several factors on time to an event. A standard approach for such analyses is the Cox proportional hazards model. It assumes that the factors act linearly on the log hazard function (linearity assumption) and that their effects are constant over time (proportional hazards (PH) assumption). Variable selection is often required to specify a more parsimonious model aiming to include only variables with an influence on the outcome. As follow‐up increases the effect of a variable often gets weaker, which means that it varies in time. However, spurious time‐varying effects may also be introduced by mismodelling other parts of the multivariable model, such as omission of an important covariate or an incorrect functional ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4488844 Tue, 01 Feb 2011 00:00:00 +0100 4488844 http://www.medworm.com/index.php?rid=4464103&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000257 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4464103 Tue, 01 Feb 2011 00:00:00 +0100 4464103 http://www.medworm.com/index.php?rid=4459290&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000145 We present theoretical results of the conditional prediction error, especially regarding the comparison of different prediction rules and its behavior in the presence of misspecification of the link between longitudinal covariates and survival time. A simulation study investigating the performance of its estimator in finite sample sizes rounds off this paper. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4459290 Tue, 01 Feb 2011 00:00:00 +0100 4459290 http://www.medworm.com/index.php?rid=4432725&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000078 AbstractConcerns have been raised about the use of traditional measures of model fit in evaluating risk prediction models for clinical use, and reclassification tables have been suggested as an alternative means of assessing the clinical utility of a model. Several measures based on the table have been proposed, including the reclassification calibration (RC) statistic, the net reclassification improvement (NRI), and the integrated discrimination improvement (IDI), but the performance of these in practical settings has not been fully examined. We used simulations to estimate the type I error and power for these statistics in a number of scenarios, as well as the impact of the number and type of categories, when adding a new marker to an established or reference model. The type I error was ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4432725 Tue, 01 Feb 2011 00:00:00 +0100 4432725 http://www.medworm.com/index.php?rid=4393660&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000069 AbstractIn clinical studies results are often reported as proportions of responders, i.e. the proportion of subjects who fulfill a certain response criterion is reported, although the underlying variable of interest is continuous. In this paper, we consider the situation where a subject is defined as a responder if the (error‐free) continuous measurements post‐treatment are below a certain fraction of (error‐free) continuous measurements obtained pre‐treatment. Focus is on the one‐sample case, but an extension to the two‐sample case is also presented. The bias of different estimates for the proportion of responders is derived and compared. In addition, an asymptotically unbiased ML‐type estimate for the proportion of responders is presented. The results are illustrated using ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393660 Tue, 25 Jan 2011 10:31:07 +0100 4393660 http://www.medworm.com/index.php?rid=4393657&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000063 AbstractLet (T1, T2) be gap times corresponding to two consecutive events, which are observed subject to random right‐censoring. In this paper, a semiparametric estimator of the bivariate distribution function of (T1, T2) and, more generally, of a functional E [φ(T1,T2)] is proposed. We assume that the probability of censoring for T2 given the (possibly censored) gap times belongs to a parametric family of binary regression curves. We investigate the conditions under which the introduced estimator is consistent. We explore the finite sample behavior of the estimator and of its bootstrap standard error through simulations. The main conclusion of this paper is that the semiparametric estimator may be much more efficient than purely nonparametric methods. Real data illustration is included... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393657 Tue, 25 Jan 2011 10:31:06 +0100 4393657 http://www.medworm.com/index.php?rid=4393656&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000004 AbstractCapture–recapture techniques have been used for considerable time to predict population size. Estimators usually rely on frequency counts for numbers of trappings; however, it may be the case that these are not available for a particular problem, for example if the original data set has been lost and only a summary table is available. Here, we investigate techniques for specific examples; the motivating example is an epidemiology study by Mosley et al., which focussed on a cholera outbreak in East Pakistan. To demonstrate the wider range of the technique, we also look at a study for predicting the long‐term outlook of the AIDS epidemic using information on number of sexual partners. A new estimator is developed here which uses the EM algorithm to impute unobserved values and th... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393656 Tue, 25 Jan 2011 10:31:05 +0100 4393656 http://www.medworm.com/index.php?rid=4393655&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000140 AbstractIn cluster randomized trials (CRTs), identifiable clusters rather than individuals are randomized to study groups. Resulting data often consist of a small number of clusters with correlated observations within a treatment group. Missing data often present a problem in the analysis of such trials, and multiple imputation (MI) has been used to create complete data sets, enabling subsequent analysis with well‐established analysis methods for CRTs. We discuss strategies for accounting for clustering when multiply imputing a missing continuous outcome, focusing on estimation of the variance of group means as used in an adjusted t‐test or ANOVA. These analysis procedures are congenial to (can be derived from) a mixed effects imputation model; however, this imputation procedure is not... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393655 Tue, 25 Jan 2011 10:31:04 +0100 4393655 http://www.medworm.com/index.php?rid=4393654&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000148 AbstractGroup testing, also known as pooled testing, and inverse sampling are both widely used methods of data collection when the goal is to estimate a small proportion. Taking a Bayesian approach, we consider the new problem of estimating disease prevalence from group testing when inverse (negative binomial) sampling is used. Using different distributions to incorporate prior knowledge of disease incidence and different loss functions, we derive closed form expressions for posterior distributions and resulting point and credible interval estimators. We then evaluate our new estimators, on Bayesian and classical grounds, and apply our methods to a West Nile Virus data set. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393654 Tue, 25 Jan 2011 10:31:04 +0100 4393654 http://www.medworm.com/index.php?rid=4393653&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900253 In this study, we propose a two‐stage sequential analysis, in which the optimal sample fraction and the required sample size to achieve a predetermined volume of a joint confidence set are estimated in an interim analysis. Additionally required observations are collected in the second stage according to the estimated optimal sample fraction. At the end of the experiment, data from these two stages are combined and analyzed for statistical inference. Simulation studies are conducted to justify the proposed two‐stage procedure and an example is presented for illustration. It is found that the proposed two‐stage procedure performs adequately in the sense that the resultant joint confidence set has a well‐controlled volume and achieves the required coverage probability. Furthermore, th... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393653 Tue, 25 Jan 2011 10:31:02 +0100 4393653 http://www.medworm.com/index.php?rid=4393652&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190001 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393652 Tue, 25 Jan 2011 10:31:02 +0100 4393652 http://www.medworm.com/index.php?rid=4393651&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201190000 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4393651 Tue, 25 Jan 2011 10:31:01 +0100 4393651 http://www.medworm.com/index.php?rid=4349059&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000038 AbstractPreviously, we showed that in randomised experiments, correction for measurement error in a baseline variable induces bias in the estimated treatment effect, and conversely that ignoring measurement error avoids bias. In observational studies, non‐zero baseline covariate differences between treatment groups may be anticipated. Using a graphical approach, we argue intuitively that if baseline differences are large, failing to correct for measurement error leads to a biased estimate of the treatment effect. In contrast, correction eliminates bias if the true and observed baseline differences are equal. If this equality is not satisfied, the corrected estimator is also biased, but typically less so than the uncorrected estimator. Contrasting these findings, we conclude that there mu... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4349059 Fri, 14 Jan 2011 00:00:00 +0100 4349059 http://www.medworm.com/index.php?rid=4349058&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000146 AbstractSurvival analysis has established itself as a major statistical technique in medical research. Applications in hospital epidemiology, however, are only beginning to emerge. One reason for this delay is that usually complete follow‐up of patients in hospital is feasible. This overview discusses where survival techniques provide additional insight into hospital epidemiology, and where they are, in fact, needed even in the absence of right‐censoring. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4349058 Fri, 14 Jan 2011 00:00:00 +0100 4349058 http://www.medworm.com/index.php?rid=4349057&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000151 AbstractPublication bias and related types of small‐study effects threaten the validity of systematic reviews. The existence of small‐study effects has been demonstrated in empirical studies. Small‐study effects are graphically diagnosed by inspection of the funnel plot. Though observed funnel plot asymmetry cannot be easily linked to a specific reason, tests based on funnel plot asymmetry have been proposed. Beyond a vast range of funnel plot tests, there exist several methods for adjusting treatment effect estimates for these biases. In this article, we consider the trim‐and‐fill method, the Copas selection model, and more recent regression‐based approaches. The methods are exemplified using a meta‐analysis from the literature and compared in a simulation study, based on bi... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4349057 Fri, 14 Jan 2011 00:00:00 +0100 4349057 http://www.medworm.com/index.php?rid=4349056&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000073 AbstractPrognostic models for time‐to‐event data play a prominent role in therapy assignment, risk stratification and inter‐hospital quality assurance. The assessment of their prognostic value is vital not only for responsible resource allocation, but also for their widespread acceptance. The additional presence of competing risks to the event of interest requires proper handling not only on the model building side, but also during assessment. Research into methods for the evaluation of the prognostic potential of models accounting for competing risks is still needed, as most proposed methods measure either their discrimination or calibration, but do not examine both simultaneously. We adapt the prediction error proposal of Graf et al. (Statistics in Medicine 1999, 18, 2529–2545) a... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4349056 Fri, 14 Jan 2011 00:00:00 +0100 4349056 http://www.medworm.com/index.php?rid=4349055&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000234 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4349055 Wed, 01 Dec 2010 00:00:00 +0100 4349055 http://www.medworm.com/index.php?rid=4264431&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000215 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4264431 Wed, 01 Dec 2010 00:00:00 +0100 4264431 http://www.medworm.com/index.php?rid=4252589&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900207 AbstractA two‐stage adaptive design trial is a single trial that combines the learning data from stage 1 (or phase II) and the confirming data in stage 2 (or phase III) for formal statistical testing. We call it a “Learn and Confirm” trial. The studywise type I error rate remains to be at issue in a “Learn and Confirm” trial. For studying multiple doses or multiple enpdoints, a “Learn and Confirm” adaptive design can be more attractive than a fixed design approach. This is because intuitively the learning data in stage 1 should not be subjected to type I error scrutiny if there is no formal interim analysis performed and only an adaptive selection of design parameters is made at stage 1. In this work, we conclude from extensive simulation studies that the intuition is most of... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4252589 Wed, 01 Dec 2010 00:00:00 +0100 4252589 http://www.medworm.com/index.php?rid=4252588&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000028 AbstractAround 1970, the author proposed a general theoretical approach to multiple decision problems (MDPs) of which multiple comparison problems (MCPs) are special cases. Suppose that a sample space is given together with a set of probability distributions defined over . Let a finite partition of the parameter space be given. Based on the observation , an MDP is to decide, which ωa the true parameter θ belongs to. An MD confidence procedure is a mapping ψ from to the class of subsets of A, such that the probability that includes the true parameter θ is not smaller than 1−αθ. Here, 1−αθ is called the level of the confidence procedures and may vary depending on θ∈ωa. The MP confidence procedures are derived from the following proposition. When the ωa's are mutually disjoin... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4252588 Wed, 01 Dec 2010 00:00:00 +0100 4252588 http://www.medworm.com/index.php?rid=4252587&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000195 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4252587 Wed, 01 Dec 2010 00:00:00 +0100 4252587 http://www.medworm.com/index.php?rid=4252586&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201090008 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4252586 Wed, 01 Dec 2010 00:00:00 +0100 4252586 http://www.medworm.com/index.php?rid=4252585&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201090007 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4252585 Wed, 01 Dec 2010 00:00:00 +0100 4252585 http://www.medworm.com/index.php?rid=4189358&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000202 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4189358 Mon, 22 Nov 2010 00:00:00 +0100 4189358 http://www.medworm.com/index.php?rid=4182657&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900299 AbstractThe previous decade can be viewed as a second golden for era Multiple Comparisons research. I argue that much of the success stems from our being able to address real current needs. At the same time, this success generated a plethora of concepts for error rate and power, as well as multiplicity of methods for addressing them. These confuse the users of our methodology and pose a threat. To avoid the threat, it is our responsibility to match our theoretical goals to the goals of the users of statistics. Only then should we match the methods to the theoretical goals. Considerations related to such needs are discussed: simultaneous inference or selective inference, testing or estimation, decision making or scientific reporting. I then further argue that the vitality of our field in th... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4182657 Fri, 19 Nov 2010 00:00:00 +0100 4182657 http://www.medworm.com/index.php?rid=4241233&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000032 AbstractIn inter‐laboratory studies, a fundamental problem of interest is inference concerning the consensus mean, when the measurements are made by several laboratories which may exhibit different within‐laboratory variances, apart from the between laboratory variability. A heteroscedastic one‐way random model is very often used to model this scenario. Under such a model, a modified signed log‐likelihood ratio procedure is developed for the interval estimation of the common mean. Furthermore, simulation results are presented to show the accuracy of the proposed confidence interval, especially for small samples. The results are illustrated using an example on the determination of selenium in non‐fat milk powder by combining the results of four methods. Here, the sample size is sm... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4241233 Fri, 01 Oct 2010 00:00:00 +0100 4241233 http://www.medworm.com/index.php?rid=4200078&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000102 AbstractEarly generation variety trials are very important in plant and tree breeding programs. Typically many entries are tested, often with very little resources available. Unreplicated trials using control plots are popular and it is common to repeat the trials at a number of locations. An alternative is to use p‐rep designs, where a proportion of the test entries are replicated at each location; this can obviate the need for control plots. α‐Designs are commonly used for replicated variety trials and we show how these can be adapted to produce efficient p‐rep designs. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4200078 Fri, 01 Oct 2010 00:00:00 +0100 4200078 http://www.medworm.com/index.php?rid=4189357&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000194 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4189357 Fri, 01 Oct 2010 00:00:00 +0100 4189357 http://www.medworm.com/index.php?rid=4182656&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900134 AbstractTwo‐stage, drop‐the‐losers designs for adaptive treatment selection have been considered by many authors. The distributions of conditional sufficient statistics and the Rao–Blackwell technique were used to obtain an unbiased estimate and to construct an exact confidence interval for the parameter of interest. In this paper, we characterize the selection process from a binomial drop‐the‐losers design using a truncated binomial distribution. We propose a new estimator and show that it is asymptotically consistent with a large sample size in either the first stage or the second stage. Supported by simulation analyses, we recommend the new estimator over the naive estimator and the Rao–Blackwell‐type estimator based on its robustness in the finite‐sample setting. We f... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4182656 Fri, 01 Oct 2010 00:00:00 +0100 4182656 http://www.medworm.com/index.php?rid=4104873&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900294 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104873 Thu, 30 Sep 2010 23:00:00 +0100 4104873 http://www.medworm.com/index.php?rid=4104872&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000030 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104872 Thu, 30 Sep 2010 23:00:00 +0100 4104872 http://www.medworm.com/index.php?rid=4104871&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900273 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104871 Thu, 30 Sep 2010 23:00:00 +0100 4104871 http://www.medworm.com/index.php?rid=4104870&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000035 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104870 Thu, 30 Sep 2010 23:00:00 +0100 4104870 http://www.medworm.com/index.php?rid=4104869&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900266 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104869 Thu, 30 Sep 2010 23:00:00 +0100 4104869 http://www.medworm.com/index.php?rid=4104868&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900277 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104868 Thu, 30 Sep 2010 23:00:00 +0100 4104868 http://www.medworm.com/index.php?rid=4104867&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900222 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104867 Thu, 30 Sep 2010 23:00:00 +0100 4104867 http://www.medworm.com/index.php?rid=4104866&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201090006 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104866 Thu, 30 Sep 2010 23:00:00 +0100 4104866 http://www.medworm.com/index.php?rid=4104865&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201090005 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4104865 Thu, 30 Sep 2010 23:00:00 +0100 4104865 http://www.medworm.com/index.php?rid=3815888&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000054 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3815888 Tue, 03 Aug 2010 23:00:00 +0100 3815888 http://www.medworm.com/index.php?rid=3815890&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900184 We present an efficient alternating expectation-conditional maximization (AECM) algorithm for the computation of maximum likelihood estimates of parameters on the basis of two convenient hierarchical formulations. The techniques for the prediction of random effects and intermittent missing values under this model are also investigated. Our methodologies are illustrated through an application to schizophrenia data. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3815890 Sun, 01 Aug 2010 23:00:00 +0100 3815890 http://www.medworm.com/index.php?rid=3815889&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900310 We consider a regression model where the error term is assumed to follow a type of asymmetric Laplace distribution. We explore its use in the estimation of conditional quantiles of a continuous outcome variable given a set of covariates in the presence of random censoring. Censoring may depend on covariates. Estimation of the regression coefficients is carried out by maximizing a non-differentiable likelihood function. In the scenarios considered in a simulation study, the Laplace estimator showed correct coverage and shorter computation time than the alternative methods considered, some of which occasionally failed to converge. We illustrate the use of Laplace regression with an application to survival time in patients with small cell lung cancer. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3815889 Sun, 01 Aug 2010 23:00:00 +0100 3815889 http://www.medworm.com/index.php?rid=4026887&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900214 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4026887 Sat, 31 Jul 2010 23:00:00 +0100 4026887 http://www.medworm.com/index.php?rid=4017668&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000051 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=4017668 Sat, 31 Jul 2010 23:00:00 +0100 4017668 http://www.medworm.com/index.php?rid=3998078&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000174 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3998078 Sat, 31 Jul 2010 23:00:00 +0100 3998078 http://www.medworm.com/index.php?rid=3983222&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000050 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3983222 Sat, 31 Jul 2010 23:00:00 +0100 3983222 http://www.medworm.com/index.php?rid=3953429&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000137 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3953429 Sat, 31 Jul 2010 23:00:00 +0100 3953429 http://www.medworm.com/index.php?rid=3932732&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900170 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3932732 Sat, 31 Jul 2010 23:00:00 +0100 3932732 http://www.medworm.com/index.php?rid=3920220&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900209 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3920220 Sat, 31 Jul 2010 23:00:00 +0100 3920220 http://www.medworm.com/index.php?rid=3907042&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900203 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3907042 Sat, 31 Jul 2010 23:00:00 +0100 3907042 http://www.medworm.com/index.php?rid=3890352&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000119 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3890352 Sat, 31 Jul 2010 23:00:00 +0100 3890352 http://www.medworm.com/index.php?rid=3890351&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900307 Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3890351 Sat, 31 Jul 2010 23:00:00 +0100 3890351 http://www.medworm.com/index.php?rid=3890350&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201090004 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3890350 Sat, 31 Jul 2010 23:00:00 +0100 3890350 http://www.medworm.com/index.php?rid=3890349&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201090003 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3890349 Sat, 31 Jul 2010 23:00:00 +0100 3890349 http://www.medworm.com/index.php?rid=3801921&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900254 This paper suggests two directional multivariate tests that aim at establishing superiority of a treatment over a control in at least one of several endpoints that are assumed to have a multivariate normal distribution. One of these tests is a one-sided, scale-invariant version of the classical Hotelling T2-test. The other is based on a summary score with weights derived from the data. Both tests overcome an important shortcoming of previous "one-sided" multivariate suggestions, namely that the null hypothesis was restricted to a single point in the multidimensional parameter space. The derivation of the tests is supplemented by simulations investigating their performance and by the application in an osteoporosis trial. (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3801921 Thu, 29 Jul 2010 23:00:00 +0100 3801921 http://www.medworm.com/index.php?rid=3854056&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900051 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3854056 Tue, 27 Jul 2010 23:00:00 +0100 3854056 http://www.medworm.com/index.php?rid=3801922&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900051 In epidemiology, capture-recapture models are commonly used to estimate the size of an unknown population based on several incomplete lists of individuals. The method operates under two main assumptions: independence between the lists (local independence) and homogeneity of capture probabilities of individuals. In practice, these assumptions are rarely satisfied. We introduce a multinomial latent class model that can account for both list dependence and heterogeneity. Parameter estimation is performed by maximizing the conditional likelihood function with the use of the EM algorithm. In addition, a new approach for evaluating the standard errors of the parameter estimates is discussed, which considerably reduces the computational burden associated with the evaluation of the variance of the... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3801922 Tue, 27 Jul 2010 23:00:00 +0100 3801922 http://www.medworm.com/index.php?rid=3794189&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900218 Coherence and consonance are the two important concepts to ensure consistent conclusions drawn from multiple tests. Unlike coherence, consonance of a multiple testing procedure (MTP) has not yet received much attention. Although most of the MTPs used in practice are consonant, cases still exist that dissonant tests have to be used due to their unique advantages, for example, the likelihood ratio tests and sum tests. Consonance adjustments are necessary for such tests to avoid difficulty in interpretation. Moreover, in terms of detecting elementary hypotheses, a consonant test procedure is uniformly more powerful than the corresponding dissonant one. In this paper, several general methods using the partitioning principle to construct (strongly) consonant and strongly coherent MTPs are propo... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3794189 Mon, 26 Jul 2010 23:00:00 +0100 3794189 http://www.medworm.com/index.php?rid=3790208&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900173 This article, motivated by an AIDS clinical study, discusses a hierarchical Bayesian nonlinear mixed-effects modeling approach to dynamic ODE models without a closed-form solution. In this model, we fully integrate viral load, medication adherence, drug resistance, pharmacokinetics, baseline covariates and time-dependent drug efficacy into the data analysis for characterizing long-term virologic responses. Our method is implemented by a data set from an AIDS clinical study. The results suggest that modeling HIV dynamics and virologic responses with consideration of time-varying clinical factors as well as baseline characteristics may be important for HIV/AIDS studies in providing quantitative guidance to better understand the virologic responses to ARV treatment and to help the evaluation ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3790208 Mon, 26 Jul 2010 23:00:00 +0100 3790208 http://www.medworm.com/index.php?rid=3854057&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900168 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3854057 Sun, 25 Jul 2010 23:00:00 +0100 3854057 http://www.medworm.com/index.php?rid=3790209&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900168 In Cox proportional hazard models with censored survival data, estimates of treatment effects with some important covariates omitted will be biased toward zero (Gail et al., Biometrika 71: 431-444). This can be especially problematic in meta-analyses that combine estimates of parameters from studies where different covariate adjustments are made. Presently, few constructive solutions have been provided to address this issue. In this paper, we review the existing meta-analysis methodologies for aggregated patient data (APD) and propose two meta-regression models (meta-ANOVA model and meta-polynomial model) with indicators of different covariates in Cox proportional hazard models to adjust the heterogeneity of treatment effects due to omitted covariates across studies. Both parametric and no... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3790209 Sun, 25 Jul 2010 23:00:00 +0100 3790209 http://www.medworm.com/index.php?rid=3771649&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900188 The three-arm design with a test treatment, an active control and a placebo group is the gold standard design for non-inferiority trials if it is ethically justifiable to expose patients to placebo. In this paper, we first use the closed testing principle to establish the hierarchical testing procedure for the multiple comparisons involved in the three-arm design. For the effect preservation test we derive the explicit formula for the optimal allocation ratios. We propose a group sequential type design, which naturally accommodates the hierarchical testing procedure. Under this proposed design, Monte Carlo simulations are conducted to evaluate the performance of the sequential effect preservation test when the variance of the test statistic is estimated based on the restricted maximum like... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3771649 Tue, 20 Jul 2010 23:00:00 +0100 3771649 http://www.medworm.com/index.php?rid=3854058&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900188 (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3854058 Sun, 18 Jul 2010 23:00:00 +0100 3854058 http://www.medworm.com/index.php?rid=3710064&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900206 Multiple testing problems are complex in evaluating statistical evidence in pivotal clinical trials for regulatory applications. However, a common practice is to employ a general and rather simple multiple comparison procedure to handle the problems. Applying multiple comparison adjustments is to ensure proper control of type I error rates. However, in many practices, the emphasis of the type I error rate control often leads to a choice of a statistically valid multiple test procedure but the common sense is overlooked. The challenges begin with confusions in defining a relevant family of hypotheses for which the type I error rates need to be properly controlled. Multiple testing problems are in a wide variety, ranging from testing multiple doses and endpoints jointly, composite endpoint, ... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3710064 Tue, 29 Jun 2010 23:00:00 +0100 3710064 http://www.medworm.com/index.php?rid=3655333&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900182 Bivariate mixed effects models are often used to jointly infer upon covariance matrices for both random effects (u) and residuals (e) between two different phenotypes in order to investigate the architecture of their relationship. However, these (co)variances themselves may additionally depend upon covariates as well as additional sets of exchangeable random effects that facilitate borrowing of strength across a large number of clusters. We propose a hierarchical Bayesian extension of the classical bivariate mixed effects model by embedding additional levels of mixed effects modeling of reparameterizations of u-level and e-level (co)variances between two traits. These parameters are based upon a recently popularized square-root-free Cholesky decomposition and are readily interpretable, eac... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3655333 Thu, 10 Jun 2010 23:00:00 +0100 3655333 http://www.medworm.com/index.php?rid=3647954&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000091 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3647954 Wed, 09 Jun 2010 23:00:00 +0100 3647954 http://www.medworm.com/index.php?rid=3675997&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000012 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3675997 Tue, 08 Jun 2010 23:00:00 +0100 3675997 http://www.medworm.com/index.php?rid=3647955&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.201000012 No Abstract (Source: Biometrical Journal) Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3647955 Tue, 08 Jun 2010 23:00:00 +0100 3647955 http://www.medworm.com/index.php?rid=3647960&cid=s_33756_70_f&fid=33756&url=http%3A%2F%2Fdx.doi.org%2F10.1002%252Fbimj.200900131 The clinical pulmonary infection score (CPIS) and bronchoalveolar lavage (BAL) are important diagnostic variables of pneumonia for forcefully ventilated patients who are susceptible to nosocomial infection. Because of its invasive nature, BAL is performed for patients only if the CPIS is greater than a certain threshold value. Thus, CPIS and BAL are closely related, yet BAL values are substantially missing. In a randomized clinical trial, the control and oral treatment groups were compared based on the outcomes from these procedures. Because of the relevance of both outcomes with respect to evaluating the efficacy of treatments, we propose and examine a nonparametric test based on these outcomes, which employs the empirical likelihood methodology. While efficient parametric methods are ava... Biometrical Journal journals http://www.medworm.com/rss/comments.php?id=3647960 Mon, 07 Jun 2010 23:00:00 +0100 3647960