Comparison of the psychometric properties of two fatigue scales in multiple sclerosis

Objective: To compare psychometric functioning of the Fatigue Severity Scale (FSS; Krupp, LaRocca, Muir-Nash, & Steinberg, 1989) and the Modified Fatigue Impact Scale (MFIS; MSCCPG, 1998) in a community sample of persons with multiple sclerosis (MS). Method: A self-report survey including the FSS, MFIS, demographic, and other health measures was completed by 1271 individuals with MS. Analyses evaluated the reliability and validity of the scales, assessed their dimensional structures, and estimated levels of floor and ceiling effects. Item response theory (IRT) was used to evaluate the precision of the MFIS and FSS at different levels of fatigue. Results: Participants had a mean score on the FSS of 5.1 and of 44.2 on the MFIS. Cronbach's alpha values for FSS and MFIS were all 0.93 or greater. Known-groups and discriminant validity of MFIS and FSS scores were supported by the analyses. The MFIS had low floor and ceiling effects, and the FSS had low floor and moderate ceiling effects. Unidimensionality was supported for both scales. IRT analyses indicate that the FSS is less precise in measuring both low and high levels of fatigue, compared with the MFIS. Conclusions: Researchers and clinicians interested in measuring physical aspects of fatigue in samples whose fatigue ranges from mild to moderate can choose either instrument. For those interested in measuring both physical and cognitive aspects of fatigue, and whose sample is expected to have higher levels of fatigue, the MFIS is a better choice even though it is longer. IRT analyses suggest that both scales could be shortened without a significant loss of precision.