Differential item functioning dif 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. 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. 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. 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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