Detecting differential item functioning with five standardized item-fit indices in the Rasch model

Abstract

This study examined five Rasch-model-based item-fit indices: unweighted and weighted standardized indices (denoted UWz and Wz), standardized likelihood index (denoted Lz), and Extended Caution Indices (denoted ECI2z and ECI4z), in terms of their distributional properties and the power of detecting item bias or Differential Item Functioning (DIF). The results indicated that although these five standardized item-fit indices did not depart significantly from a normal distribution, it appeared that the Type I error rates were not reasonable. For the power of five standardized item-fit indices to detect DIF, the results showed that all indices did perform poorly across various conditions. These findings lead to the conclusion that all indices used in this study are inadequate fit measures for detecting DIF.