Using a single question to assess physical activity in older adults: a reliability and validity study

BACKGROUND Single-item physical activity questions provide a quick approximation of physical activity levels. While recall questionnaires provide a more detailed picture of an individual's level of physical activity, single-item questions may be more appropriate in certain situations. The aim of this study was to evaluate two single-item physical activity questions (one absolute question and one relative question) for test-retest reliability, convergent validity, and discriminant validity, in a sample of older adults. METHODS Data was obtained from the Project to Prevent Falls in Veterans, a fall risk-factor screening and modification trial. One question measured absolute physical activity (seldom, moderately, vigorously active) and one measured relative physical activity (more, about as, less active than peers). Test-retest reliability was examined using weighted Kappa statistics (κ) in a sample of 43 subjects. Validity was assessed using correlation coefficients (r) in participants who received clinical assessments (n = 159). RESULTS The absolute physical activity question was more reliable than the relative physical activity question (κ = 0.75 vs. κ = 0.56). Convergent validity, however, was stronger for the relative physical activity question (r = 0.28 to 0.57 vs. r = 0.10 to 0.33). Discriminant validity was similar for both questions. For the relative physical activity question, there was moderate agreement when this question was re-administered seven days later, fair to moderate/good associations when compared with indicators of physical function, and little to no associations when compared with measures hypothesized to be theoretically not related to physical activity. CONCLUSIONS The relative physical activity question had the best combination of test-retest reliability, convergent validity and discriminant validity. In studies requiring a measure of physical activity, where physical activity is not the primary focus and more detailed measures are not feasible, a single question may be an acceptable alternative.