The measurement level and trait-specific reliability of 4 scales of shoulder functioning: An empiric investigation

Objective: To evaluate 4 scales of shoulder function with respect to (1) their precision at different levels of shoulder function and (2) the measurement level of their raw scores (interval vs ordinal). Design: Partial credit model calibration. Setting: Office of private practice orthopedic surgeon with practice limited to the shoulder. Participants: One-hundred ninety-two shoulder patients. Interventions: Participants completed the American Shoulder and Elbow Surgeons Patient Self-Evaluation Form (function subscale, modified), the disability subscale of the Shoulder Pain and Disability Index, the Simple Shoulder Test, and the function subscale of the University of Pennsylvania Shoulder Scale. Main Outcome Measures: The patients’ responses were calibrated by using a partial credit model. We calculated standard errors of measurement and plotted the 95% confidence interval for different levels of shoulder functioning. We compared scales’ raw scores with their equal interval measures obtained in the Rasch calibration. Results: The scales did not measure all levels of shoulder functioning with equal precision, suggesting that commonly used reliability estimates misrepresent scale precision in certain subpopulations. Conclusions: The scales’ raw scores were found to be not of equal interval, calling into question the scoring systems recommended by the developers of these scales and the use of the scores in some statistical procedures.