Using Rasch Analysis to Evaluate Accuracy of Individual Activities of Daily Living (ADL) and Instrumental Activities of Daily Living (IADL) for Disability Measurement.

Our study objectives were to examine the accuracy of individual activities of daily living (ADLs) and instrumental ADLs (IADLs) for disability measurement, and determine whether dependence or difficulty is more useful for disability measurement. We analyzed data from 499 patients with 2+ ADLs or 3+ IADLs who participated in a home visiting nurse intervention study, and whose function had been assessed at study baseline and 22 months. Rasch analysis was used to evaluate accuracy of 24 individual ADL and IADL items. The individual items differed in the amount of information provided in measuring functional disability along the range of disability, providing much more information in (usually) one part of the range. While nearly all of the Item Information Curves (IICs) for the ADL dependence, IADL difficulty, and IADL dependence items were unimodal with one information peak each, the IICs for ADL difficulty exhibited a bimodal pattern with two peaks. Which of the individual items performed better in disability measurement varied by the extent of functional disability (i.e., by how disabled the patients were). The information peaks of most ADLs and many IADLs rise or drop steeply in a relatively short distance. Thus, whether dependence or difficulty is superior often changes very quickly along the disability continuum. There was considerable heterogeneity in which individual items provided the most and the least information at the three points of interest examined across the disability range (-2 SD units, mean, +2 SD units). While the disability region (low, medium, and high disability) for which each individual item provided the most information remained quite stable between baseline and 22 months for ADL difficulty, IADL difficulty, and IADL dependence, relatively large shifts occurred for ADL dependence items. At the disability mean dependence items offered more information for assessment than difficulty. While ADLs also provided more information at -2 and +2 SD units, there was more heterogeneity at these points for IADLs, with little difference between dependence and difficulty assessment for some IADLs.