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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., Competing Interests: Declaration of Conflicting Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

This study established an effect size measure for differential functioning for items and tests' noncompensatory differential item functioning (NCDIF). The Mantel-Haenszel parameter served as the benchmark for developing NCDIF's effect size measure for reporting moderate and large differential item functioning in test items. The effect size of NCDIF is influenced by the model, the discrimination parameter, and the difficulty parameter. Therefore, tables of NCDIF's effect size were presented at given levels of a, b , and c parameters. In addition, a general effect size recommendation for moderate and large NCDIF is also established., Competing Interests: Declaration of Conflicting Interests: The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.