Differential item functioning is a statistical characteristic of an item that shows the extent to which the item might be measuring different abilities for members of separate subgroups average item scores for subgroups having the same overall score on the test are compared to determine whether the item is measuring in essentially the same way for all subgroups the presence of dif requires review and judgment and it does not necessarily indicate the presence of bias dif analysis provides an. Overview in brief differential item functioning dif occurs when groups such as defined by gender ethnicity age or education have different probabilities of endorsing a given item on a multi item scale after controlling for overall scale scores an item is labeled as having dif when people with the same latent ability but from different . In differential item functioning eds holland wainer 1993 angoff explains that dif is occurring when an item displays different statistical properties between groups after those groups have been matched on a measure of proficiency to put it another way we need to first account for differences in the groups abilities and then see if there are still differences in the item performance. Differential item functioning dif investigates the items in a test one at a time for signs of interactions with sample characteristics in the widely used mantel haenszel procedure 1959 wwwraschorg memo39pdf rmt 1989 32 51 53 reference and focal groups are identified which differ in a discernible way. Differential item functioning dif has been increasingly applied in fairness studies in psychometric circles judicious application of this methodology by the researchers however requires an
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