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Modeling Dif With The Rasch Model: The Unfortunate Combination Of Mean Ability Differences And Guessing, Christine E. Demars, Daniel P. Jurich

Department of Graduate Psychology - Faculty Scholarship

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 …