Application of item response theory for development of a global functioning measure of dementia with linear measurement properties

An ideal measure of global functioning for patients with dementia would dis criminate at very high and very low levels of functioning and would have li near measurement properties such that a given change in score corresponds t o the same amount of change in underlying ability at any part of the abilit y continuum. Using item response theory methods, linearity of test measurem ent can be directly assessed and items can be selected to construct a test with desired measurement characteristics. The purpose of this study was to apply item response theory methods to evaluating and developing global func tioning scales. Subjects were 1207 patients who had received comprehensive dementia evaluations. Items were selected from two measures of cognitive fu nctioning (Mini Mental State Examination, MMS; Blessed Information Memory C oncentration Test, BIMCT) and one measure of independent functioning (Bless ed-Roth Dementia Rating Scale, BRDRS). The MMS and BIMCT showed significant non-linearity of measurement, especially at low and high ability levels. A brief composite measure was created by selecting from the three instrument s 25 items that fit a uniform distribution of item difficulty across the en tire range of ability measured by the three instruments. This composite mea sure and the BRDRS showed better linearity of measurement than the other tw o instruments. Results have implications for development of a psychometrica lly sophisticated, brief measure of global functioning for clinical and res earch use in dementia. Copyright (C) 2000 John Wiley & Sons, Ltd.