Partial Credit Model

Abstract

The partial credit model (PCM) is a model for constructing measures using items with two or more ordered response categories; that is, items scored polytomously. The model is a simple application of Rasch’s model for dichotomies (Volume One,Chapter 3). Specifically, if responses to itemi are recorded in categories a = 0, 1, …Ai , then the probability of a person’s response to itemi being in categoryx rather than in categoryx − 1 is modeled by the PCM, as being governed by Rasch’s model for dichotomies.