Late Life Function and Disability Instrument: II. Development and Evaluation of the Function Component

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Abstract
Background. Self-reported capability in physical functioning has long been considered an important focus of research for older persons. Current measures have been criticized, however, for conceptual confusion, lack of sensitivity to change, poor reproducibility, and inability to capture a wide range of upper and lower extremity functioning. Methods. Using Nagi's disablement model, we wrote physical functioning questionnaire items that assessed difficulty in 48 common daily tasks. We constructed the instrument using factor analysis and Rasch analytic techniques and evaluated its validity and test-retest reliability with 150 ethnically and racially diverse adults aged 60 years and older who had a range of functional limitations. Results. Our analyses resulted in a 32-item function component with three dimensions—upper extremity, basic lower extremity, and advanced lower extremity functions. Expected differences in summary scores of known-functional limitation groups support its validity. Test-retest stability over a 1- to 3-week period was extremely high (intraclass correlation coefficients = .91 to .98). Conclusions. The Late-Life Function and Disability Instrument has potential to assess activity concepts related to upper and lower extremity functioning across a wide variety of daily physical tasks and individual levels of physical functioning.