Testing for Differential Item Functioning Under the D-Scoring Method.

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.