Extended Rasch Modeling: The R Package eRm

This package vignette is an update and extension of the papers published in the Journal of Statistical Software (special issue on Psychometrics, volume 20) and Psychology Science (Mair and Hatzinger 2007b,a). Since the publication of these papers, various extensions and additional features have been incorporated into the package. We start with a methodological introduction to extended Rasch models followed by a general program description and application topics. The package allows for the computation of simple Rasch models, rating scale models, partial credit models and linear extensions thereof. Incorporation of such linear structures allows for modeling the effects of covariates and enables the analysis of repeated categorical measurements. Item parameter estimation is performed using cml, for the person parameters we use joint ml. These estimation routines work for incomplete data matrices as well. Based on these estimators, item-wise and global (parametric and non-parametric) goodness-of-fit statistics are described and various plots are presented.