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. Differential item functioning dif is the preferred psychometric term for what is otherwise known as item bias an item displays dif when test takers possessing the same amount of an ability or trait but belonging to different subgroups do not share the same likelihood of correctly answering the item. 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. It is called differential item functioning dif that the item characteristics vary across groups the plots excerpted from lee et al 2015 illustrate the item characteristic curves for the young and old groups
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