Unidimensional Item Response Theory

Abstract

This chapter presents an overview of unidimensional item response theory (IRT) modeling. Unidimensional IRT models have become important tools to evaluate the quality of psychological and educational measurement instruments. The chapter distinguishes parametric and nonparametric IRT models, and IRT models for dichotomous and polytomous item scores. It describes model assumptions, and discusses model-data fit procedures and model choice. The also chapter focuses on dichotomous items unless stated otherwise because the procedures underlying fit for polytomous items do not fundamentally differ from the fit procedures for dichotomous items. In discussion on assessing the dimensionality of the data, unidimensionality (UD) and local independence (LI) assumptions were considered. IRT analyses on existing scales show that many scales consist of items and subtests that can be improved through a more rigorous analysis of the quality of individual items. The chapter contributes to the more wide-spread use of IRT analyses in test construction and evaluation.