Rasch analysis of the UK functional assessment measure in a sample of patients with traumatic brain injury from the UK national Clinical Database

Objective: To determine whether the UK Functional Assessment Measure (UK FIM+FAM) fits the Rasch model in patients with complex disability following traumatic brain injury. Design: Psychometric evaluation including preliminary exploratory and confirmatory factor analyses followed by Rasch analysis. Participants: A multicentre UK national cohort of 1,956 patients admitted for specialist rehabilitation following traumatic brain injury. Results: The suitability of the Partial Credit Model was confirmed by the likelihood-ratio test (χ2 (df86)=7,325.0, p<0.001). Exploratory and confirmatory factor analyses supported 3 factors (Motor, Communication, Psychosocial). Rasch analysis of the full scale incorporating the 3 factors as super-items resulted in an acceptable overall model fit (χ2 (df24)=36.72, p=0.05) and strict uni-dimensionality when tested on a sub-sample of n=320. These results were replicated in a full sample (n=1,956) showing uni-dimensionality and good reliability with Person Separation Index=0.81, but item trait interaction was significant due to the large sample size. No significant differential item functioning was observed for any personal factors. Neither uniform rescoring of items nor exclusion of participants with extreme scores improved the model fit. Conclusion: The UK FIM+FAM scale satisfies the Rasch model reasonably in traumatic brain injury. A conversion table was produced, but its usefulness in clinical practice requires further exploration and clinical translation.