Psychometric properties of three Fatigue Rating Scales in individuals with late effects of polio

Objective To evaluate the psychometric properties of the Fatigue Severity Scale (FSS), the Fatigue Impact Scale (FIS), and the Multidimensional Fatigue Inventory (MFI-20) in persons with late effects of polio (LEoP). More specifically, we explored the data completeness, scaling assumptions, targeting, reliability, and convergent validity. Methods A postal survey including FSS, FIS, and MFI-20 was administered to 77 persons with LEoP. Responders received a second survey after 3 weeks to enable test-retest reliability analyses. Results Sixty-one persons (mean age, 68 years; 54% women) responded to the survey (response rate 79%). Data quality of the rating scales was high (with 0%-0.5% missing item responses), the corrected item-total correlations exceeded 0.4 and the scales showed very little floor or ceiling effects (0%-6.6%). All scales had an acceptable reliability (Cronbach's α ≥ 0.95) and test-retest reliability (intraclass correlation coefficient, ≥ 0.80). The standard error of measurement and the smallest detectable difference were 7%-10% and 20%-28% of the possible scoring range. All three scales were highly correlated (Spearman's correlation coefficient r_s=0.79-0.80; p < 0.001). Conclusion The FSS, FIS, and MFI-20 exhibit sound psychometric properties in terms of data completeness, scaling assumptions, targeting, reliability, and convergent validity, suggesting that these three rating scales can be used to assess fatigue in persons with LEoP. As FSS has fewer items and therefore is less time consuming it may be the preferred scale. However, the choice of scale depends on the research question and the study design.