Rasch analysis of the Gross Motor Function Measure: Validating the assumptions of the Rasch model to create an interval-level measure

Objectives: To describe the Rasch analysis of the Gross Motor Function Measure (GMFM-88) and to demonstrate how the assumptions of unidimensionality, sample-free measurement, and test-free measurement were validated to create an interval level measure. Design: Cross-sectional and longitudinal (12-mo) data from a prospective study of motor development in children with cerebral palsy (CP) were used for the analysis. Setting: Motor assessments were completed at 18 children's ambulatory rehabilitation centers in Ontario, I Canada, by pediatric physical therapists trained in the use of the GMFM-88. Participants: The first 537 of 682 children enrolled into a longitudinal study of motor development in children with CP. Children had a mean age of 6.43+/-2.75 years (range, 11 mo-12y) with varying types and severity of CP. Interventions: Not applicable. Main Outcome Measure: The GMFM-88. Results: The Pasch analysis, in conjunction with clinical decisions, identified 66 items from the GMFM-88 that formed a unidimensional measure (GMFM-66). Assumptions of Sample-free and test-free measurement were confirmed, and a user friendly scoring program was developed. Conclusions: The GMFM-66 is an interval-level measure of gross motor function for children with CP; it should improve the scoring, interpretation, and overall clinical and research utility over the original GMFM.