The Effects of Referent Item Parameters on Differential Item Functioning Detection Using the Free Baseline Likelihood Ratio Test

Abstract

The purpose of this simulation study is to investigate the effects of anchor subtest composition on the accuracy of item response theory (IRT) likelihood ratio (LR) differential item functioning (DIF) detection (Thissen, Steinberg, & Wainer, 1988). Here, the IRT LR test was implemented with a free baseline approach wherein a baseline model was formed by freeing all items except a referent or anchor subset and examining the changes in fit with respect to a series of models wherein 1 item at a time was constrained in addition to the referent(s). The results clearly indicated that the composition of the anchor subtest is important for accurate DIF detection. It was found that using a single highly discriminating rather than a low discriminating referent greatly enhanced the power of the procedure. Moreover, in conditions involving small DIF or smaller sample sizes or both, power appeared to improve when a group of highly discriminating referents was used. These findings have implications for applied research involving short scales and small sample sizes.