Longitudinal measurement invariance of the Multiple Sclerosis Walking Scale-12

Objective: One primary assumption underlying the unambiguous interpretation of change in Multiple Sclerosis Walking Scale-12 (MSWS-12) scores over time is longitudinal measurement invariance (i.e., Is the same construct being measured over time?). Such an assumption was tested in the present study over periods of 6 and 12 months in persons with relapsing-remitting multiple sclerosis (RRMS). Method: Participants completed a battery of questionnaires that included the MSWS-12 at baseline (n = 269) and 6-months (n = 260) and 12-months (n = 252) follow-up. The data were analyzed using confirmatory factor analysis and a series of nested model comparisons in Mplus 3.0. Results: The results indicated that the unidimensional measurement model and all of its parameters (e.g., factor loadings and item intercepts) were invariant over periods of 6 and 12 months. Conclusion: We provide novel evidence that supports the unambiguous interpretation of scores from the MSWS-12 as a measure of change in walking impairment over time in a sample of persons with MS.