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.
Lerdal
A
A
Department of Health Sciences, Buskerud University College, Drammen, Norway. anners.lerdal@ldh.no
Johansson
S
S
Kottorp
A
A
von Koch
L
L
D000328:Adult; D005221:Fatigue / Q000150:complications* / Q000503:physiopathology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000150:complications* / Q000503:physiopathology; D009664:Norway; D011594:Psychometrics / Q000379:methods*; D015203:Reproducibility of Results; D012720:Severity of Illness Index; D011795:Surveys and Questionnaires; D013548:Sweden