Rasch Model Analysis Gives New Insights Into the Structural Validity of the QuickDASH in Patients With Musculoskeletal Shoulder Pain

STUDY DESIGN: Cross-sectional secondary analysis of a prospective cohort study. BACKGROUND: The shortened version of the Disabilities of the Arm, Shoulder and Hand questionnaire (QuickDASH) is a widely used outcome measure that has been extensively evaluated using classical test theory. Rasch model analysis can identify strengths and weaknesses of rating scales and goes beyond classical test theory approaches. It uses a mathematical model to test the fit between the observed data and expected responses and converts ordinal-level scores into interval-level measurement. OBJECTIVE: To test the structural validity of the QuickDASH using Rasch analysis. METHODS: A prospective cohort study of 1030 patients with shoulder pain provided baseline data. Rasch analysis was conducted to (1) assess how the QuickDASH fits the Rasch model, (2) identify sources of misfit, and (3) explore potential solutions to these. RESULTS: There was evidence of multidimensionality and significant misfit to the Rasch model (chi(2) = 331.09, P<.001). Two items had disordered threshold responses with strong floor effects. Response bias was detected in most items for age and sex. Rescoring resulted in ordered thresholds; however, the 11-item scale still did not meet the expectations of the Rasch model. CONCLUSION: Rasch model analysis on the QuickDASH has identified a number of problems that cannot be easily detected using traditional analyses. While revisions to the QuickDASH resulted in better fit, a shoulder-specific version is not advocated at present. Caution needs to be exercised when interpreting results of the Quick DASH outcome measure, as it does not meet the criteria for interval-level measurement and shows significant response bias by age and sex.