Rasch analysis of the gross motor function measure: validating the assumptions of the rasch model to create an interval-level Measure1 1A commercial party with a direct financial interest in the results of the research supporting this article has conferred or will confer a financial benefit upon the authors or one or more of the authors. The authors will receive royalties from the sale of a book published by Mac Keith Press that describes the development of the GMFM-66.

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, 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, 11mo-12y) with varying types and severity of CP. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURE: The GMFM-88. RESULTS: The Rasch 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.