Late life function and disability instrument: I. Development and evaluation of the disability component

Background. Efforts to evaluate the effectiveness of clinical and community-based interventions designed to impact late-life disability have been hindered significantly by limitations in current instrumentation. More conceptually sound and responsive measures of disability are needed. Methods. Applying Nagi's disablement model, we wrote questionnaire items that assessed disability in terms of frequency and limitation in performance of 25 life tasks. We evaluated their validity and test-retest reliability with 150 ethnically and racially diverse adults aged 60 and older who had a range of functional limitations, using factor analysis and Rasch analytic techniques to examine and refine the instrument. Results. Our analyses resulted in a 16-item disability component with two dimensions, one focused on frequency of performance and the other addressing limitation in performance of life tasks, with two disability domains within each dimension. The frequency dimension consisted of a personal and a social role domain, and the limitation dimension consisted of an instrumental and a management role domain. Expected differences in summary scores of known-functional limitation groups support the validity of this instrument. Test-retest intraclass correlations of the reproducibility of each overall dimension summary score were moderate to high (intraclass correlation coefficients .68-.82). Conclusions. The Late-Life Function and Disability Instrument has potential to assess meaningful concepts of disability across a wide variety of life tasks with relatively few items.