Understanding DIF and DTF: Description, Methods, and Implications for Social Work Research

Objective: This article introduces the reader to item response theory (IRT), differential item functioning (DIF), and differential test functioning (DTF), and it demonstrates how a DIF and DTF analysis can be conducted using IRT analysis methods. DIF concerns the possibility that items on a scale work differently for different groups, such as females and males. DIF can accumulate across items on a scale to create DTF, which means the overall scale will have different levels of validity for the different groups. IRT is useful for conceptualizing DIF and DTF, and for conducting DIF analyses. Method: A DIF analysis of scores on Hudson’s Multi-Problem Screening Inventory depression subscale illustrates the concepts of DIF and DTF and demonstrates an IRT analytical approach to detect possible DIF and DTF. Results: DIF was found in 3 of the items on the Multi-Problem Screening Inventory depression subscale, and evidence of possible DTF also was found. The magnitude of the DIF in one item was very large, but the potential DTF was small in magnitude. Conclusions: IRT is useful for conceptualizing DIF and DTF, and it has advantages for conducting DIF and DTF studies. Disadvantages include the need for large sample sizes. The risk of flawed inferences as a consequence of DIF and/or DTF in social work research is discussed.