Rasch analysis of the UK Functional Assessment Measure in a sample of patients with traumatic brain injury from the UK national clinical database
Preview of fully accepted paper, still not published in any volume
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.
The UK Functional Assessment Measure (UK FIM+FAM) is a scale used to evaluate the outcome of rehabilitation after acquired brain injury as part of the UK Rehabilitation Outcomes Collaborative (UKROC). The FIM+FAM is administered by clinicians and measures how well -people are recovering their independence. It has 30 separate questions concerning a person’s physical (e.g. eating, toileting, bathing) and cognitive (e.g. expression, memory, safety awareness) independence. Data from 1,956 UK patients who had survived a traumatic brain injury were analysed. The main purpose of this study was to establish that the FIM+FAM meets some of the stringent technical standards for clinical and scientific measurement. A statistical method known as Rasch analysis was used to demonstrate that the FIM+FAM produces accurate and reliable scores. The results showed that the FIM+FAM is a practical and useful scale for measuring change in people in rehabilitation after a traumatic brain injury.
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