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.