THE VALUE OF THE STUDIED ITEM IN THE MATCHING CRITERION IN DIFFERENTIAL ITEM FUNCTIONING (DIF) ANALYSIS

Abstract

ABSTRACTThe nature of the matching criterion (usually the total score) in the study of differential item functioning (DIF) has been shown to impact the accuracy of different DIF detection procedures. One of the topics related to the nature of the matching criterion is whether the studied item should be included. Although many studies exist that suggest the studied item should always be included in the criterion, the validity of this statement for models other than the Rasch model has not been studied. This study evaluates the effect of including/excluding the studied item in the matching criterion for situations that mimic real testing situations where the assumptions of the Rasch model are violated. A simulation study was conducted where the effect of including/excluding the studied item in the matching criterion was studied relative to different magnitudes of DIF and different group ability distributions, for data that follow the two‐parameter logistic (2PL) item response theory (IRT) and multidimensional item response theory (MIRT) models. Results from the study show that including the studied item leads to less biased DIF estimates and more appropriate Type I error rate, especially when the group ability distributions are different. Systematic bias positioning DIF estimation in favor of the high ability group was consistently found across all simulated conditions when the studied item was excluded from the matching criterion.