Measuring disability: application of the Rasch model to activities of daily living (ADL/IADL).

This paper describes a comparative analysis of (ADL) and (IADL) items administered to two samples, 4,430 persons representative of older Americans, and 605 persons representative of patients with rheumatoid arthrisit (RA). Responses are scored separately using both Likert and Rasch measurement models. While Likert scoring seems to provide information similar to Rasch, the descriptive statistics are often contrary if not contradictory, and estimates of reliability from Likert are inflated. The test characteristic curves derived from Rasch are similar despite differences between the levels of disability with the two samples. Correlations of Rasch item calibrations across three samples were .71, .76, and .80. The fit between the items and the samples, indicating the compatibility between the test and subjects, is seen much more clearly with Rasch with more than half of the general population measuring the extremes. Since research on disability depends on measures with known properties, the superiority of Rasch over Likert is evident.