pcIRT: An R Package for Polytomous and Continuous Rasch Models

Abstract

A key task in psychological assessment is the scaling of new psychological tests and questionnaires. Item response theory (IRT) is a well-established framework for this area of research. At present, IRT comprises a great variety of models, but to date, relatively little attention has been paid to the scaling of nominal and continuous data. The R package pcIRT provides functions for estimating IRT models for polytomous (nominal) and continuous data - the multidimensional polytomous Rasch model (Rasch 1961) and the continuous rating scale model (Müller 1987). Both models are extensions of the dichotomous logistic Rasch model (Rasch 1980) and retain its key feature of the separability of structural and nuisance parameters. The multidimensional polytomous Rasch model is suitable for nominal data under the assumption of a multidimensional space for the response categories, and the continuous rating scale model is a direct extension of the rating scale model developed by Andrich (1978) for continuous data.