Testing for Differential Item Functioning With Measures of Partial Association

Abstract

Differential item functioning (DIF) occurs when an item on a test or questionnaire has different measurement properties for one group of people versus another, irrespective of mean differences on the construct. There are many methods available for DIF assessment. The present article is focused on indices of partial association. A family of average conditional ordinal association measures described by Quade is described and empirically compared to the Mantel-Haenszel (MH) test, the partial Pearson correlation pr, and the Spearman rank correlation prs. Because coefficients of linear association are not meaningful for the binary and ordinal variables usually used in DIF applications, practitioners are urged to seek alternatives to pr and prs. Some of the Quade-family measures are viable alternatives and performed as well as, or better than, the established MH test. Computer code for calculating the average conditional ordinal measures using the free R program is given.