ITEM-BUNDLE DIF HYPOTHESIS-TESTING - IDENTIFYING SUSPECT BUNDLES AND ASSESSING THEIR DIFFERENTIAL FUNCTIONING

This article proposes two multidimensional IRT model-based methods of selecting item bundles (clusters of not necessarily adjacent items cho sen according to some organizational principle) suspected of displayin g DIF amplification. The approach embodied in these two methods is ins pired by Shealy and Stout's (1993a, 1993b) multidimensional model for DIF. Each bundle selected by these methods constitutes a DIF amplifica tion hypothesis. When SIBTEST(Shealy & Stout, 1993b) confirms DIF ampl ification in selected bundles, differential bundle functioning (DBF) i s said to occur. Three real data examples illustrate the two methods f or suspect bundle selection. The effectiveness of the methods is argue d on statistical grounds. A distinction between benign and adverse DIF is made. The decision whether flagged DIF items or DBF bundles displa y benign or adverse DIF/DBF must depend in part on nonstatistical cons truct validity arguments. Conducting DBF analyses using these methods should help in the identification of the causes of DIF/DBF.