Rasch calibration of physical activity self-efficacy and social support scale for persons with intellectual disabilities.

The purpose of this study was to investigate the construct validity of the Self-Efficacy/Social Support for Activity for persons with Intellectual Disability (SE/SS-AID) scales developed by Peterson, Peterson, Lowe, & Nothwehr (2009). A total of 146 participants with intellectual disabilities completed 6 self-efficacy (SE) items and 18 social support (SS) items. After applying the Rasch rating model, all SE items and 17 SS items fit the model and measured a single-construct. Thus, it was able to determine the item difficulty and person's level of SE and SS for physical activity by calculated logit scores. No items showed evidence for differential functioning by the level of intellectual disability. Model fit of SS subscales (e.g., staff, family, and peer) showed good-fit as well. In conclusion, SE and SS scales for physical activity can be measured more accurately for persons with intellectual disabilities by using the modified scales validated in this study.