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 subgroupsaverage 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. 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 . 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. 59 the international journal of educational and psychological assessment september 2012 vol 112 an introduction to differential item functioning hossein karami university of tehran iran abstract differential item functioning dif has been increasingly applied in fairness studies in psychometric circles. Item analysis differential item functioning dif posted by austin fossey we have talked about item difficulty and item discrimination in a classical test theory framework and we have discussed how these indices can be used to flag items that may potentially affect the validity of the inferences we make about the assessment results another area of item performance that is often used for
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