摘要：Objectives. We elucidated how US late-life disability prevalence has changed over the past 3 decades. Methods. We examined activities of daily living (ADL) and instrumental activities of daily living (IADL) disability trends by using age–period–cohort (APC) models among older adults aged 70 years or older who responded to the National Health Interview Survey between 1982 and 2009. We fitted logistic regressions for ADL and IADL disabilities and for each of the 3 APC trends with 2 models: unadjusted and fully adjusted for age, period, cohort, and sociodemographic variables. Results. The unadjusted and adjusted period trends showed a substantial decline in IADL disability, and ADL disability remained stable across time. Unadjusted cohort trends for both outcomes also showed continual declines across successive cohorts; however, increasing cohort trends were evident in the adjusted models. Conclusions. More recent cohorts of US older adults are becoming more disabled, net of aging and period effects. The net upward cohort trends in ADL and IADL disabilities remain unexplained. Further studies should explore cohort-specific determinants contributing to the increase of cohort-based disability among US older adults. Knowledge of trends in late-life disability is crucial to health professionals because a high prevalence of disability leads to elevated mortality and health risks for individuals and an increased burden of health care costs for society. Studies of late-life disability trends have generally focused on 2 measures of activity limitations: (1) activities of daily living (ADL), such as bathing and toileting, and (2) instrumental activities of daily living (IADL), such as shopping and cooking. Previous studies found declining disability among older Americans during the last 2 decades of the 20th century. 1,2 Disability trend experts investigated differences in disability trends across 5 data sets (Heath and Retirement Study, Medicare Current Beneficiary Survey, National Health Interview Survey [NHIS], National Long Term Care Survey, and Supplement on Aging) and concluded that ADL disability continually declined in the 1990s at a rate of 1.0% to 2.5% per year. 3 Schoeni et al. 1 analyzed ADL and IADL disability trends among Americans aged 70 years and older using data from the NHIS and reported a declining trend for any disability (ADL or IADL limitations) between 1982 and 2002. Although the prevalence of IADL disability declined from 14.5% in 1982 to 8.1% in 2002, the prevalence of ADL disability remained steady over this period. Schoeni et al. 1 concluded that the substantial reduction in US older adult disability was primarily driven by the decline in IADL limitations over this period. This evidence corroborates the conclusion of an earlier review suggesting that most of the decline in late-life disability in the 1980s and 1990s was the result of the reduction of IADL-only disability prevalence, which decreased at a rate of 0.4% to 2.7% per year. 4 More recent analyses revealed a contrary pattern indicating that late-life disability has been increasing among newer birth cohorts who are now moving into older adulthood. 5,6 Seeman et al. 5 used data from 2 cross-sectional waves (1988–1994 and 1999–2004) of the National Health and Nutrition Examination Survey and found that ADL disability, IADL disability, and impaired mobility increased significantly among respondents aged 60 to 69 years in the more recent survey period. For respondents aged 70 to 79 years, a significant increase in IADL disability was found, whereas respondents aged 80 years and older exhibited a modest decrease in functional limitations. 5 Thus, the authors warned of potentially increasing older-age disability among younger US cohorts who are now moving into older adulthood. Contrarily, European researchers combined 2 longitudinal datasets covering 10 European countries and noted that the rates of need for help with self-care and mobility ADL decreased across birth cohorts who were interviewed in 1988 to 1991, 1993 to 1995, and 1998 to 2000. 7 Despite the sobering finding of an increase in disability among older Americans, the disability prevalence comparison by Seeman et al. 5 was limited to merely 2 aggregated survey periods and lacked the specificity of disability trends in more refined periods. Because the results described earlier showed potentially contradictory findings regarding late-life disability trends, we sought to better elucidate how disability prevalence has changed across time among US adults aged 70 years and older. To do so, we used age–period–cohort (APC) models to analyze late-life disability trends between 1982 and 2009. Demographically speaking, time can be captured by 3 temporal dimensions: age, period, and cohort. Specifically, age is a proxy for biological processes that ultimately lead to disease, disability, or death. Period, or survey year, reflects changes in sociocultural, economic, technological, and environmental factors that may affect the entire population at a given time simultaneously but perhaps not equally. For example, a drought may lead to increased food prices, which largely affect those with lower incomes. Finally, cohort describes a unique set of individuals who both are born into a social system during a similar time period and experience similar social experiences over their life course. 8 Each aspect of APC makes a unique contribution to the study of population health, including disability. Aging has an obvious relationship to population health, and period captures the current burden of morbidity, disability, and mortality in the entire population at a given time. Cohort, however, reflects the health of successive generations and is an important dimension for understanding how population health is changing over time. Overall, failure to isolate APC trends risks substantial bias and provides an incomplete picture of population health trends. 8–10 Schoeni et al. 1 presented a thorough description of cohort-unadjusted and age-adjusted period trends for ADL and IADL disability, which results in a clear snapshot of both the current and the past burden of disability among US older adults. However, their approach did not identify cohort trends. We sought to expand their study by extending their analysis of disability trends through 2009, estimating both period-based and cohort-based trends in disability, and also controlling for age effects. Doing so would allow us to confirm whether the recent increase in disability among more recent birth cohorts of US older adults, as reported by Seeman et al., 5 is found in our larger and more contemporary data set. Thus, the purposes of our study were to (1) replicate the ADL and IADL disability prevalence produced by Schoeni et al. 1 and (2) estimate both unadjusted and adjusted APC trends for ADL and IADL disability among US older adults from 1982 through 2009.
