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Concerns with using the Rasch model to estimate DIF when there are large group differences in ability (impact) and the data follow a 3PL model are discussed. This demonstration showed that, with large group ability differences, difficult non-DIF items appeared to favor the focal group and, to a smaller degree, easy non-DIF items appeared to favor the reference group. Correspondingly, the effect sizes for DIF items were biased. With equal ability distributions for the reference and focal groups, DIF effect sizes were unbiased for non-DIF items; effect sizes were somewhat overestimated in absolute values for difficult items and somewhat underestimated for easy items. These effects were explained by showing how the item response function was distorted differentially depending on the ability distribution. The practical implication is that measurement practitioners should not trust the DIF estimates from the Rasch model when there is large impact and examinees are potentially able to answer items correctly by guessing.

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This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 4.0 License.