Differential Item Functioning

Differential item functioning (DIF) is a term used in the item response theory (IRT) psychometrics framework to characterize item bias. A test 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 or endorsing the same response option. Thus, differentially functioning items elicit different responses from test takers of the same ability or trait level. Because subgroups of test takers are often defined in terms of demographic membership (e.g., sex, race, culture), items displaying DIF are sometimes considered “biased” against a particular subgroup. Consider, for example, the items on a standardized test of verbal ability. If the content of one item is related to ...