Psychometric properties of the Fatigue Severity Scale: Rasch analyses of responses in a Norwegian and a Swedish MS cohort

Background: Rigorous testing of the original Fatigue Severity Scale (FSS—9) with modern psychometric methods is warranted.

Objective: To determine the psychometric properties of the FSS—9 in multiple sclerosis (MS): internal scale validity; person response validity; unidimensionality; uniform differential item functioning; temporal stability of response patterns; and ability to separate people into distinct groups of fatigue.

Methods: Rasch analyses were conducted on data from a Norwegian and a Swedish MS cohort followed for two years.

Results: Item estimations in the FSS—9 did not differ between sex or levels of education but between the cohorts with regard to disability, disease course and time for evaluation, however, items 1 and 2 demonstrated unacceptable high outfit mean-square values in both cohorts. In an FSS—7 item version, items 3 and 4 in the Norwegian and 4 in the Swedish cohort demonstrated unacceptable goodness of fit but high separation indexes. In the FSS—7, the first unidimensional factor explained 87.5% (Norwegian cohort) and 86.4% (Swedish cohort) of the total variation.

Conclusions: In MS, the FSS—7 demonstrates better psychometric properties than the FSS—9; items 1 and 2 neither empirically nor conceptually fit with the other seven items.