A differential item functioning (DIF) detection method for testlet-based data was proposed and
evaluated in this study. The proposed DIF model is an extension of a bifactor multidimensional
item response theory (MIRT) model for testlets. Unlike traditional item response theory (IRT)
DIF models, the proposed model takes testlet effects into account, thus estimating DIF magnitude
appropriately when a test is composed of testlets. A fully Bayesian estimation method was
adopted for parameter estimation. The recovery of parameters was evaluated for the proposed
DIF model. Simulation results revealed that the proposed bifactor MIRT DIF model produced
better estimates of DIF magnitude and higher DIF detection rates than the traditional IRT DIF
model for all simulation conditions. A real data analysis was also conducted by applying the proposed
DIF model to a statewide reading assessment data set