My primary interest is the statistical analysis of test data, and within this context my research can be divided into four themes: (1) finding statistical solutions to practical data problems, such as handling missing data, handling outliers, and item selection; (2) applications of psychological tests; (3) development of nonparametric item response theory models, including Mokken scale analysis; and (4) the development of categorical marginal models and their application to test data. For all themes, software development (in R) is an important aspect. In addition, I have made methodological and statistical contributions to substantive papers from various areas in the social and behavioral sciences.