Differential item functioning (DIF) is formally defined as a lack of equality between two group’s [Page 517]conditional probability functions that relate a trait of measurement to an item’s response data. DIF indicates that examinees who are equal on the trait of measurement, but differ according to some external variable, show differential performance on a test item. For example, DIF is said to be present if groups of examinees who are matched in quantitative aptitude (trait of measurement), but vary in country of origin (external variable), show differential performance on a test item that is intended to measure quantitative aptitude. The presence of DIF is viewed as problematic because it implies that some factor external to the trait being measured is influencing responses to test items ...