Considerations for Measuring Functioning of the Elderly: IRM Dimensionality and Scaling Analysis

Abstract

Modern measurement methods were applied in this study to examine the properties of a measure of functioning of the elderly. Measures of functioning form an essential element in health services and outcomes research. Several implications for scale development and improved score precision are presented in this case study. This study examined the structure of responses to the Older Americans Resources and Services (OARS) Activities of Daily Living (ADL) and Instrumental Activities of Daily Living (IADL) scales using item response model (IRM) analysis methods. The analysis illustrates the extension of IRM dimensionality and item analysis to health scales in general. Attention is given to the underlying theory and appropriate interpretation of these methods for health measurement. Data were taken from 1364 elderly Canadians participating in the caregiver component of the Canadian Study of Health and Aging (CSHA). The fit of a two-parameter logistic IRM was compared with a one-parameter (Rasch) model for these data. The dimensionality of responses to the scale was evaluated with an approximate χ2 test of residuals after fitting an IRM based on non-linear factor analysis. Results confirm that ADL and IADL item sets differ in the degree of disability they measure and are well represented as separate dimensions using a two-parameter IRM. Implications are drawn concerning the adequacy of the OARS disability measure for health surveys, while more general conclusions cover the precision of IRM based optimal scoring for functional disability measures.