Differential item functioning

DIF

Differential Item Functioning (DIF) occurs when people from different groups (commonly gender or ethnicity) with the same latent trait (the same ability/skill) have a different probability to give a certain response on a questionnaire or test. With DIF analysis (mostly the Mantel Haenszel procedure is used) is explored what the amount of DIF is for each item of a test. An item doesn’t display DIF if people from different groups have a different probability to give a certain response; it displays DIF if people from different groups in spite of their same underlying true ability have a different probability to give a certain response. More precisely, an item displays DIF when the difficulty level (beta), the discrimination (alpha) or the lower asymptotes (gamma) of an item differs across groups. Thus, when one or more item parameters differ across groups, an item displays DIF.

Reference: Embretson,S.E., Reise,S.P. (2000) Item Response Theory for Psychologists.

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