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.
