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 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 . Another area of item performance that is often used for item retention decisions is item bias commonly referred to as differential item functioning dif dif studies are generally implemented to see how the performances of two groups compare on a single item in an assessment though studies can be done with more than two groups. 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. Differential item functioning differential item functioning dif occurs when examinees with the same ability level but from two different groups have different probabilities of endorsing an item clauser mazor 1998. Differential item functioning and differential test functioning dif dtf do test items function in different ways for different groups of test takers item functioning is intended to be invariant with respect to irrelevant aspects of the test takers such as gender ethnicity and socio economic status
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