Psychometric properties of the Fatigue Severity Scale in polio survivors

The objective of this study was to evaluate by means of classical test theory and Rasch analysis the scaling characteristics and psychometric properties of the Fatigue Severity Scale (FSS) in polio survivors. A questionnaire,consisting of five general questions (sex, age, age at time of acute polio, sequelae of polio, and new symptoms), the FSS, and three questions from the Visual Analog Scale questions on fatigue was sent to all 196 polio survivors at the Institute for Rehabilitation in Ljubljana. Responses were assessed in terms of Cronbach's a, item-to-total correlation, factor analysis, and Rasch analysis. Of the128 (65.3%) valid questionnaires returned, those presenting no missing values were used for subsequent analyses (n = 117). The FSS showed good internal consistency: Cronbach's a was greater than 0.95,item-to-total correlation ranged from 0.68 to 0.88. A reduction from seven to three rating categories was necessary to comply with criteria for correct category function. Item difficulty estimates spanned from – 0.91to + 1.09 logits. No item bias was found for sex and age.The internal consistency of FSS was high and its item separation reliability good, indicating a satisfactory replicability of item placement in other samples. In conclusion, Rasch analysis enabled us to confirm the validity of FSS (in its 8-item version, without item 1) as a measure of the severity and impact of physical symptoms of fatigue in polio survivors, so providing a useful starting point for further studies aimed at examining additional psychometric aspects and confirming the appropriateness of the simplification of its rating categories.