Rasch analysis of the UK Functional Assessment Measure in a sample of patients with traumatic brain injury from the UK national clinical database

Abstract

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 re-scoring 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.