Differential Item Functioning for Accommodated Students with Disabilities: Effect of Differences in Proficiency Distributions

Abstract

ABSTRACTThis study examined the effect of similar vs. dissimilar proficiency distributions on uniform DIF detection on a statewide eighth grade mathematics assessment. Results from the similar- and dissimilar-ability reference groups with an SWD focal group were compared for four models: logistic regression, hierarchical generalized linear model (HGLM), the Wald-1 IRT-based test, and the Mantel-Haenszel procedure. A DIF-free-then-DIF strategy was used. The rate of DIF detection was examined among all accommodated scores and common accommodation subcategories. No items were detected for DIF using the similar ability distribution reference group, regardless of method. With the dissimilar ability reference group, logistic regression and Mantel–Haenszel flagged 8–17%, and the Wald-1 and HGLM test flagged 23–38% of items for DIF. Forming focal groups by accommodation type did not alter the pattern of DIF detection. Creating a reference group to be similar in ability to the focal group may control the rate of erroneous DIF detection for SWD.