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Fatigue is a common symptom in many neurologic diseases and is particularly prevalent in multiple sclerosis. Several scales have been developed to assess the level of fatigue in general populations and in those with multiple sclerosis. 1 2 For example, the Fatigue Impact Scale (FIS) was created in order to measure the impact of fatigue on the quality of life in chronic illnesses. 1 The scale was clearly intended to provide a global score of fatigue and subscale scores since measures of internal consistency and other statistical tests were given for the 40-item scale. Indeed, in later work, Fisk 3 stated that the high internal consistency of the 40-item scale and the fact that the subscales had originally been determined only on a conceptual basis were sufficient to support the assumption that fatigue impact could be measured as a unitary construct.
The Modified Fatigue Impact Scale (MFIS) was created during the development of the Multiple Sclerosis Quality of Life Inventory (MSQLI) 4 by shortening the 40-item FIS. This resulted in a 21-item scale (MFIS) with nine "physical" items, 10 "cognitive items" and two "psychosocial" items. In addition, the response options were reworded, the reason for which was unclear. This scale has been recommended in the American Multiple Sclerosis Council for clinical practice guidelines, which called for further psychometric evaluation to be performed in order to establish its continued suitability as an outcome measure. 5
The power of psychometrics to evaluate health outcome scales has developed considerably in recent years through the addition of modern psychometric techniques to supplement traditional test theory. Pre-eminent in this development has been the application of the Rasch measurement model, 6 which has special properties consistent with fundamental measurement. It is the only way in which ordinal observations of clinical phenomena can be converted into linear measurement. 7 Consequently, the objective of this current study was to analyse the MFIS, as used in a UK multiple sclerosis population, by application of the Rasch model.
Methods
The MFIS was posted to 753 patients with clinically definite multiple sclerosis 8 from two centres in the UK (Walton Centre for Neurology and Neurosurgery, Liverpool and Imperial College Healthcare Trust, London) identified from research databases. The study was approved by the local ethics committee (Sefton...