Psychometric evaluation of the myasthenia gravis composite using Rasch analysis.

INTRODUCTION The MG Composite (MGC) is a validated outcome measure of the clinical manifestations of myasthenia gravis. METHODS We performed Rasch analyses of the MGC to investigate additional properties, including its unidimensionality and the appropriateness of the weights assigned to the response categories for the MGC items. RESULTS The fit statistics indicated that the 10 items belong together and can be summated for a total score. There was an overall absence of item order distortion between response categories. The Rasch model's expected category response values were compatible with item weights previously assigned. CONCLUSIONS Our analysis suggests that: (1) the score can be summated to estimate an overall disease severity score; (2) the response options of the 10 items are not significantly distorted; and (3) the assigned weights of the response options are appropriate.