Occurrences and sources of Differential Item Functioning (DIF) in patient-reported outcome measures: Description of DIF methods, and review of measures of depression, quality of life and general health.

Examination of the equivalence of measures involves several levels, including conceptual equivalence of meaning, as well as quantitative tests of differential item functioning (DIF). The purpose of this review is to examine DIF in patient-reported outcomes. Reviewed were measures of self-reported depression, quality of life (QoL) and general health. Most measures of depression contained large amounts of DIF, and the impact of DIF at the scale level was typically sizeable. The studies of QoL and health measures identified a moderate amount of DIF; however, many of these studies examined only one type of DIF (uniform). Relative to DIF analyses of depression measures, less analysis of the impact of DIF on QoL and health measures was performed, and the authors of these analyses generally did not recommend remedial action, with one notable exception. While these studies represent good beginning efforts to examine measurement equivalence in patient-reported outcome measures, more cross-validation work is required using other (often larger) samples of different ethnic and language groups, as well as other methods that permit more extensive analyses of the type of DIF, together with magnitude and impact.