Consensus Among Differential Item Functioning Effect Size Measures: A Simplified Approach to Reporting Effect Size

Differential item functioning (DIF) is an issue of a measure that can lead to differences in item scores between subpopulations (e.g., sex, race, age) while controlling for a latent trait or ability level. It is important to report DIF effect size measures to understand the practical significance of DIF results as well. We reviewed 15 different DIF effect size measures in the literature, and no previous study had examined whether the 15 DIF effect size measures agreed with one another, which has implications in the number and type of measures to report. The present study investigated the relationship among DIF effect size measures in 1PL, 2PL, and GRM models and provide clear recommendations to applied researchers for their reporting. Our simulation results showed that, for each model, three effect size measures can be reported to obtain the most unique information about uniform DIF and the least bias from sampling conditions (e.g., sample size and impact): Mantel-Haenszel Delta, McFadden's Pseudo R-squared, and a signed DIF measure. However, the optimal signed DIF measure varies by model, with Signed Area under the curve for the 1PL model, Standardized P-difference for the 2PL, and Signed Item Difference in the Sample/Normal Distribution for the GRM. The recommended approach to reporting DIF effect size improves the detection of DIF through practical significance and provides researchers with more tools to address bias.