Can item response theory reduce patient burden when measuring health status in neurological disorders? Results from Rasch analysis of the SF-36 physical functioning scale (PF-10)

Background-Indices of physical function may have a hierarchy of items. In c ases where this can be demonstrated it may be possible to reduce patient bu rden by asking them to complete only those items which relate directly to t heir own level of ability. Objectives-To determine whether statistical procedures, operationalising wh at is known as item response theory (IRT), can be used to assess the unidim ensionality of the 10 item physical functioning domain of the SF-36 in pati ents with Parkinson's disease and motor neuron disease, and, secondly, to d etermine whether it would be possible to administer subsets of items to cer tain patients, on the basis of their replies to other items in the scale, t hereby reducing patient burden. Methods-Rasch analysis, a form of IRT methodology, of the 10 item physical functioning domain (PF-10) in two neurological patient samples was undertak en and the results compared with results of a Rasch analysis of data gained from a population survey (the third Oxford healthy lifestyles survey). Results-Evidence from the analyses suggests that the PF-10 does not form a perfect hierarchy on a unidimensional scale. However, certain items seem to form a hierarchy, and responses to some of them are contingent on response s to the other items. Conclusions-Rasch analysis of the PF-10 in neurological patients has indica ted that certain items of the scale are hierarchically ordered, and consequ ently not all respondents would need to complete them all: indeed those mos t severely ill would be required to complete less items than those with onl y limited disabilities. The implications of this are discussed.