An Introduction to Item Response Theory Using the Need for Cognition Scale

This paper provides an introduction to two commonly used item response theory (IRT) models (the two-parameter logistic model and the graded response model). Throughout the paper, the Need for Cognition Scale (NCS) is used to help illustrate different features of the IRT model. After introducing the IRT models, I explore the assumptions these models make as well as ways to assess the extent to which those assumptions are plausible. Next, I describe how adopting an IRT approach to measurement can change how one thinks about scoring, score precision, and scale construction. I briefly introduce the advanced topics of differential item functioning and computerized adaptive testing before concluding with a summary of what was learned about IRT generally, and the NCS specifically.