Investigating the Effects of Differential Item Functioning on Proficiency Classification.

This study provides a comprehensive evaluation of the effects of differential item functioning (DIF) on proficiency classification. Using Monte Carlo simulation, item- and test-level DIF magnitudes were varied systematically to investigate their impact on proficiency classification at multiple decision points. Findings from this study clearly show that the presence of DIF affects proficiency classification not by lowering the overall correct classification rates but by affecting classification error rates differently for reference and focal group members. The study also reveals that multiple items with low levels of DIF can be particularly problematic. They can do similar damage to proficiency classification as high-level DIF items with the same cumulative magnitudes but are much harder to detect with current DIF and differential bundle functioning (DBF) techniques. Finally, how DIF affects proficiency classification errors at multiple cut scores is fully described and discussed.