Rasch analysis of The Shoulder Pain and Disability Index (SPADI) in a postrepair rotator cuff sample

STUDY DESIGN: Clinical measurement study: Level of evidence (N/A) INTRODUCTION: The Shoulder Pain and Disability Index (SPADI) is a self-reported outcome measure of pain and disability related to shoulder pathology. In comparison to Classical Test Theory (CTT), Rasch analysis offers a more rigorous examination of the measurement properties of a scale. PURPOSE OF THE STUDY: This study utilizes Rasch analysis to evaluate the psychometric properties of the SPADI to propose potential modifications and avenues for future investigation. METHODS: SPADI scores (n = 212) from participants one-year post rotator cuff repair were collected from an outpatient specialty clinic. Fit to the Rasch model, unidimensionality of the subscales, and areas of bias were evaluated. RESULTS: Both the pain and disability subscales satisfied the requirements of the Rasch model with very minimal modifications and demonstrated unidimensionality. The person separation index was found to be high (P > .80), indicating reliability and internal consistency. Sex and the affected dominant side influenced how people scored on the SPADI (Differential item functioning (DIF)). CONCLUSIONS: The findings suggest some patients in our sample have difficulty discriminating between item responses, particularly within the middle of the scale. Rasch analysis supports the clinical measurement properties of consistency and reliability, previously determined by CTT methods.