Item Response Theory Analysis of the Outpatient Physical Therapy Improvement in Movement Assessment Log (OPTIMAL)

Background. The Outpatient Physical Therapy Improvement in Movement Assessment Log (OPTIMAL) instrument was created to assess the perceived ability of patients receiving physical therapy in adult outpatient settings to perform actions or movements. Its properties must be studied to determine whether it accomplishes this goal. Objective. The objective of this study was to investigate the item properties of OPTIMAL with item response theory. Design. This investigation was a retrospective cross-sectional item calibration study. Methods. Data were obtained from the American Physical Therapy Association, which collected information from outpatient physical therapy clinics through electronic charting databases that included OPTIMAL responses: Item response theory analyses were performed on the trunk, lower-extremity, and upper-extremity subscales of the Difficulty Scale of OPTIMAL. Results. In total, 3,138 patients completed the Difficulty Scale of OPTIMAL at the baseline assessment. The subscale analyses met all item response theory assumptions. The items in each subscale showed fair discrimination. In all analyses, the subscales measured a narrow range of ability levels at the low end of the physical functioning spectrum. Limitations. OPTIMAL, was originally intended to be administered as a whole. In the present study, each subscale was analyzed separately, indicating how the subscales perform individually but not as a whole. Another limitation is that only the Difficulty Scale of OPTIMAL was analyzed, without consideration of the Confidence Scale. Conclusions. OPTIMAL best measures low physical functioning at the baseline assessment in adult outpatient physical therapy settings. The addition of categories to each item and the addition of more challenging items are recommended to allow measurements for a broader range of patients.