Differential item functioning results may change depending on how an item is scored: An illustration with the Center for Epidemiologic Studies Depression Scale

The present study investigated potentially biased scale items on the Center for Epidemiologic Studies Depression (CES-D). The 20-item CES-D was scored using two binary methods (presence and persistence) and one ordinal method. Gender differential item functioning (DIF) was explored using Zumbo's OLR method with corresponding logistic regression effect size estimator with all three scoring methods. Gender DIF was found with the CES-D item crying for the ordinal and presence methods of scoring. The persistence scoring method identified two DIF items (effort and hopeful); however, this scoring method appears to be of limited use due to low variability on some items. Overall, the results indicate that the scoring method has an effect on DIF; thus, DIF is a property of the item, scoring method, and purpose of the instrument.