Development and evaluation of the Norwegian Fatigue Characteristics and Interference Measure (FCIM) for stroke survivors: cognitive interviews and Rasch analysis

PURPOSE: There is need for a comprehensive measure of post-stroke fatigue with sound measurement properties. This study aimed to develop the Norwegian Fatigue Characteristics and Interference Measure (FCIM) and assess its content validity, structural validity, and internal consistency. METHOD: This study consisted of three steps: (1) an expert panel developed version 1.0 of the Norwegian FCIM, (2) its content validity was assessed in cognitive interviews with stroke patients (N = 15), (3) a convenience sample of stroke patients (N = 169) completed an online questionnaire with the FCIM, Fatigue Severity Scale, and sociodemographic information; validity and reliability were assessed using Rasch analysis. RESULTS: FCIM version 1.0 included a 10-item characteristics subscale, a 20-item interference subscale, and two pre-stroke fatigue items. The cognitive interviews revealed content validity issues, resulting in two interference items being removed and five items being flagged but retained for Rasch analysis (version 2.0). Rasch analysis led to removal of four items from the characteristics subscale and six more from the interference subscale. The final six-item characteristics subscale and 12-item interference subscale (version 3.0) both showed adequate fit to the Rasch model with indications of unidimensionality and local independence. The interference subscale had a high person separation index. No significant differential item function (DIF) was found in relation to gender, but one item demonstrated DIF in relation to age. CONCLUSION: The cognitive interviews and Rasch analysis demonstrated that the Norwegian version of the FCIM has high content validity, structural validity, and internal consistency. Future research should assess its construct validity, reliability, and responsiveness. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11136-023-03477-z.