Effect of Purification Procedures on DIF Analysis in IRTPRO.

Purification of the test has been a well-accepted procedure in enhancing the performance of tests for differential item functioning (DIF). As defined by Lord, purification requires reestimation of ability parameters after removing DIF items before conducting the final DIF analysis. IRTPRO 3 is a recently updated program for analyses in item response theory, with built-in DIF tests but not purification procedures. A simulation study was conducted to investigate the effect of two new methods of purification. The results suggested that one of the purification procedures showed significantly improved power and Type I error. The procedure, which can be cumbersome by hand, can be easily applied by practitioners by using the web-based program developed for this study.