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Rasch analysis of the Modified Fatigue Impact Scale (MFIS) in multiple sclerosis
R J Mills1, C A Young, J F Pallant
1Walton Centre for Neurology and Neurosurgery, Lower Lane, Fazakerley, Liverpool, UK. rjm@crazydiamond.co.uk
Background:
The 21-item Modified Fatigue Impact Scale (MFIS) has been recommended as an outcome measure for use in multiple sclerosis and is commonly used to generate an overall score of fatigue.
Objective:
To test if the MFIS total score is valid by application of the Rasch measurement model.
Method:
The MFIS was sent by post to patients with clinically definite multiple sclerosis in two centres in the UK. Data were fitted to the Rasch model.
Results:
Analysis was based on 415 records (55% response). The 21-item scale did not fit the Rasch model mainly because of multidimensionality. The scale was found to contain a "physical" dimension and a "cognitive" dimension, consistent with the original subscale structure. Valid physical and cognitive subscales were derived after deletion of some items.
Conclusion:
The MFIS cannot be used to generate a single overall score of fatigue. The conceptual interaction between the two dimensions remains unclear, which poses problems when interpreting change scores in these individual scales. Studies in which a global MFIS score was used as either an outcome measure or selection tool may need to be re-evaluated.
