Application of a Method of Estimating DIF for Polytomous Test Items

In this paper, a method for studying DIF is demonstrated that can used with either dichotomous or polytomous items. The method is shown to be valid for data that follow a partial credit IRT model. It is also shown that logistic regression gives results equivalent to those of the proposed method. In a simulation study, positively biased type 1 error rates of the method are shown to be in accord with results from previous studies; however, the size of the bias in the log odds is moderate. Finally, it is demonstrated how these statistics can be used to study DIF variability with the method of Longford, Holland, & Thayer (1993).