Analysis of incomplete data

in SAS System for Clinical Trials II (2007)

Analysis of incomplete data

in 'SAS System for Clinical Trials II (2005)

Introducing the multivariate dale model in population-based genetic association studies

in Biometrical Journal = Biometrische Zeitschrift (2004), 46(2), 187-202

Until recently, the most common parametric approaches to study the combined effects of several genetic polymorphisms located within a gene or in a small genomic region are, at the genotype level, logistic ... [more ▼]

Until recently, the most common parametric approaches to study the combined effects of several genetic polymorphisms located within a gene or in a small genomic region are, at the genotype level, logistic regressions and at the haplotype level, haplotype analyses. An alternative modeling approach, based on the case/control principle, is to regard exposures (e.g., genetic data such as derived from Single Nucleotide Polymorphisms - SNPs) as random and disease status as fixed and to use a marginal multivariate model that accounts for inter-relationships between exposures. One such model is the multivariate Dale model. This model is based on multiple logistic regressions. That is why the model, applied in a case/control setting, leads to straightforward interpretations that are similar to those drawn in a classical logistic modeling framework. [less ▲]

Multivariate and multidimensional analysis

in Wilson (Ed.) Biometrics (2003)

Multicollinearity in prognostic factor analyses using the EORTC QLQ-C30: identification and impact on model selection

in Statistics in Medicine (2002), 21(24), 3865-3884

Clinical and quality of life (QL) variables from an EORTC clinical trial of first line chemotherapy in advanced breast cancer were used in a prognostic factor analysis of survival and response to ... [more ▼]

Clinical and quality of life (QL) variables from an EORTC clinical trial of first line chemotherapy in advanced breast cancer were used in a prognostic factor analysis of survival and response to chemotherapy. For response, different final multivariate models were obtained from forward and backward selection methods, suggesting a disconcerting instability. Quality of life was measured using the EORTC QLQ-C30 questionnaire completed by patients. Subscales on the questionnaire are known to be highly correlated, and therefore it was hypothesized that multicollinearity contributed to model instability. A correlation matrix indicated that global QL was highly correlated with 7 out of 11 variables. In a first attempt to explore multicollinearity, we used global QL as dependent variable in a regression model with other QL subscales as predictors. Afterwards, standard diagnostic tests for multicollinearity were performed. An exploratory principal components analysis and factor analysis of the QL subscales identified at most three important components and indicated that inclusion of global QL made minimal difference to the loadings on each component. suggesting that it is redundant in the model, In a second approach, we advocate a bootstrap technique to assess the stability of the models. Based on these analyses and since global QL exacerbates problems of multicollinearity, we therefore recommend that global QL be excluded from prognostic factor analyses using the QLQ-C30. The prognostic factor analysis was rerun without global QL in the model, and selected the same significant prognostic factors as before. Copyright (C) 2002 John Wiley Sons, Ltd. [less ▲]

Using word frequencies for testing equivalence between two DNA sequences

in Genetic Epidemiology (2002), 23

Analysis of binary data from a psychiatric study: a local influence approach

in Klein; Korsholm (Eds.) Statistical Modelling. Proceedings of the 16th International Workshop on Statistical Modelling. New Trends in Statistical Modelling. (2001)

Introduction of the multivariate Dale model in genetic association studies.

in American Journal of Human Genetics (2001), 69(4), 1289