ABSTRACT
BACKGROUND: Fatigue is a common daily experience and a symptom of various disorders. While scholars have discussed the use of the Fatigue Severity Scale (FSS) using item response theory (IRT), the characteristics of the Japanese version are not yet examined. This study evaluated the psychometric properties of the FSS using IRT and assessed its reliability and concurrent validity with a general sample in Japan. METHODS AND MEASURES: A total of 1,007 Japanese individuals participated in an online survey, with 692 of them providing valid data. Of these, 125 participants partook in a re-test after approximately 18 days and had their longitudinal data analyzed. In addition, the graded response model (GRM) was used to assess the FSS items' characteristics. RESULTS: The GRM's results recommended using seven items and a 6-point scale. The FSS's reliability was acceptable. Furthermore, the validity was adequate from the results of correlation and regression analyses. The synchronous effects models demonstrated that the Multidimensional Fatigue Inventory (MFI) enhanced depression, and depression enhanced FSS. CONCLUSION: This study suggested that the Japanese version of the FSS should be a 7-item scale with a 6-point response scale. Further investigations may reveal the different aspects of fatigue assessed by the analyzed fatigue measures.