The devil's in the details: Variation in estimates of late-life activity limitations across national cohort studies.

Background: Assessing activity limitations is central to aging research. However, assessments of activity limitations vary, and this may have implications for the populations identified. We aim to compare measures of activities of daily living (ADLs) and their resulting prevalence and mortality across three nationally-representative cohort studies: the National Health and Aging Trends Study (NHATS), the Health and Retirement Survey (HRS), and the Medicare Current Beneficiary Survey (MCBS).

Methods: We compared the phrasing and context of questions around help and difficulty with six self-care activities: eating, bathing, toileting, dressing, walking inside, and transferring. We then compared the prevalence and 1-year mortality for difficulty and help with eating and dressing.

Results: NHATS, HRS, and MCBS varied widely in phrasing and framing of questions around activity limitations, impacting the proportion of the population found to experience difficulty or receive help. For example, in NHATS 12.4% [95% confidence interval (CI) 11.5%-13.4%] of the cohort received help with dressing, while in HRS this figure was 6.4% [95% CI 5.7%-7.2%] and MCBS 5.3% [95% CI 4.7%-5.8%]. When combined with variation in sampling frame and survey approach of each survey, such differences resulted in large variation in estimates of the older population of older adults with ADL disability.

Conclusions: In order to take late-life activity limitations seriously, we must clearly define the measures we use. Further, researchers and clinicians seeking to understand the experience of older adults with activity limitations should be careful to interpret findings in light of the framing of the question asked.