DIFAS

DIFAS: Differential Item Functioning Analysis System. Differential item functioning (DIF) is an important consideration in assessing the validity of test scores (Camilli & Shepard, 1994). A variety of statistical procedures have been developed to assess DIF in tests of dichotomous (Hills, 1989; Millsap & Everson, 1993) and polytomous (Penfield & Lam, 2000; Potenza & Dorans, 1995) items. Some of these procedures, such as the Mantel-Haenszel chi-square and the Mantel-Haenszel common odds ratio, can be conducted using widely available statistical software. However, many DIF detection procedures are specific to the area of educational and psychological measurement and thus require specialized software for their use. In addition, several newly developed DIF detection procedures, such as the Breslow-Day test for trend in odds ratio heterogeneity (Penfield, 2003) and the Liu-Agresti cumulative common log-odds ratio (Penfield & Algina, 2003), are not currently available in any statistical software package. As a result, the Differential Item Functioning Analysis System (DIFAS) was developed to provide a cost-effective and easy-to-use program for conducting many of the common nonparametric DIF detection procedures, as well as several new DIF detection procedures that are not available in other statistical packages.