Psychometric properties of the Fatigue Severity Scale—Rasch analyses of individual responses in a Norwegian stroke cohort

BackgroundThe psychometric properties of the Fatigue Severity Scale (FSS) have not been tested in people with stroke, despite its being the most frequently used instrument measuring fatigue in this population.ObjectiveThe objective of our study was to assess the psychometric properties of the FSS using both traditional and modern test theory approaches in people with stroke.Design and settingA total of 119 patients admitted to two hospitals in Norway with their first stroke were recruited to a longitudinal study of poststroke fatigue. Data on socio-demographics, fatigue and other clinical variables were collected by face-to-face interviews using standardized questionnaires or by mailed questionnaires at four time points six months apart. A total of 428 responses on the FSS were analysed. The internal scale validity, person response validity, unidimensionality, and uniform differential item functioning were assessed by applications of a Rasch model and by Cronbach's alpha. Additionally, concurrent validity was assessed using bivariate correlation analysis.ResultsItems #1 and #2 in the original FSS demonstrated unacceptable high mean-square values. When these two items were removed, the psychometric properties in FSS-7 demonstrated better evidence of validity. No systematic differential item functioning of the items in FSS-7 was found in relation to any of the analysed variables.ConclusionIn patients with stroke, the FSS-7 showed better psychometric properties and had better potential to detect changes in fatigue over time than the FSS-9 version, suggesting satisfactory grounds for removal of items #1 and #2 for its application.