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 discriminate at very high and very low levels of functioning and would have linear measurement properties such that a given change in score corresponds to the same amount of change in underlying ability at any part of the ability continuum. Using item response theory methods, linearity of test measurement 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 functioning scales. Subjects were 1207 patients who had received comprehensive dementia evaluations. Items were selected from two measures of cognitive functioning (Mini Mental State Examination, MMS; Blessed Information Memory Concentration Test, BIMCT) and one measure of independent functioning (Blessed-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 instruments 25 items that fit a uniform distribution of item difficulty across the entire range of ability measured by the three instruments. This composite measure and the BRDRS showed better linearity of measurement than the other two instruments. Results have implications for development of a psychometrically sophisticated, brief measure of global functioning for clinical and research use in dementia.