Multi-group measurement invariance of the multiple sclerosis walking scale-12?

Objective: One primary assumption underlying the interpretation of composite multiple sclerosis walking scale-12 (MSWS-12) scores across levels of disability status is multi-group measurement invariance. This assumption was tested in the present study between samples that differed in self-reported disability status. Method: Participants (n=867) completed a battery of questionnaires that included the MSWS-12 and patient-determined disease step (PDDS) scale. The multi-group invariance was tested between samples that had PDDS scores of <= 2 (i.e. no mobility limitation; n=470) and PDDS scores >= 3 (onset of mobility limitation; n=397) using Mplus 6.0. Results: The omnibus test of equal covariance matrices indicated that the MSWS-12 was not invariant between the two samples that differed in disability status. The source of non-invariance occurred with the initial equivalence test of the factor structure itself. Conclusion: We provide evidence that questions the unambiguous interpretation of scores from the MSWS-12 as a measure of walking impairment between samples of persons with multiple sclerosis who differ in disability status.