Testing for Differential Item Functioning Under the D -Scoring Method

Abstract

This study offers an approach to testing for differential item functioning (DIF) in a recently developed measurement framework, referred to as D-scoring method (DSM). Under the proposed approach, called P–Z method of testing for DIF, the item response functions of two groups (reference and focal) are compared by transforming their probabilities of correct item response, estimated under the DSM, into Z-scale normal deviates. Using the liner relationship between such Z-deviates, the testing for DIF is reduced to testing two basic statistical hypotheses about equal variances and equal means of the Z-deviates for the reference and focal groups. The results from a simulation study support the efficiency (low Type error and high power) of the proposed P–Z method. Furthermore, it is shown that the P–Z method is directly applicable in testing for differential test functioning. Recommendations for practical use and future research, including possible applications of the P–Z method in IRT context, are also provided.