SAS macros for longitudinal IRT models

IRT models are often applied when observed items are used to measure a unidimensional latent variable. Originally used in educational research, IRT models are now widely used when focus is on physical functioning or psychological well-being. Modern applications often need more general models, typically models for multidimensional latent variables or longitudinal models for repeated measurements. This paper describes a collection of SAS macros that can be used for fitting data to, simulating from, and visualizing longitudinal IRT models. The macros encompass dichotomous as well as polytomous item response formats and are sufficiently flexible to accommodate changes in item parameters across time points and local dependence between responses at different time points.