Distinguishing Differential Testlet Functioning From Differential Bundle Functioning Using the Multilevel Measurement Model

Abstract

This study extends the multilevel measurement model to handle testlet-based dependencies. A flexible two-level testlet response model (the MMMT-2 model) for dichotomous items is introduced that permits assessment of differential testlet functioning (DTLF). A distinction is made between this study’s conceptualization of DTLF and that of differential bundle functioning (DBF) with an emphasis on the practical utility of decomposing differential functioning into item- and testlet-specific components. A small-scale simulation study is also conducted to assess estimation of the MMMT-2 model and its measures of DIF and DTLF when compared with SIBTEST’s estimation of DIF and DBF. Results support use of the MMMT-2 model for assessing impact, DIF, and DTLF for tests that include testlet-based dichotomous items with equal item discriminations.