Differential item functioning in the Unified Dyskinesia Rating Scale (UDysRS)

Objective: Test if differential item functioning due to gender, age, race/ethnicity, or education impacts Unified Dyskinesia Rating Scale scores. Background: Testing rating scales for differential item functioning is a core validation step. If differential item functioning exists, interpretation of item scores must consider secondary influences on dyskinesia ratings. Methods: Using Unified Dyskinesia Rating Scale translation databases (N = 3,132), we tested uniform and nonuniform differential item functioning. We required confirmation by two independent methods and considered differential item functioning pertinent if McFadden pseudo R 2 magnitude statistics exceeded negligible ratings. Results: No age, race/ethnicity, or education nonuniform differential item functioning was identified. Gender nonuniform differential item functioning occurred for 2 items, both with negligible magnitude. Gender, race, and education uniform differential item functioning was observed for multiple items, all with negligible magnitude. Conclusions: The Unified Dyskinesia Rating Scale items effectively capture dyskinesia severity without pertinent gender, age, race/ ethnicity, or education influence. © 2017 International Parkinson and Movement Disorder Society