Am J Epidemiol. 2023 Aug 11:kwad169. doi: 10.1093/aje/kwad169. Online ahead of print.
ABSTRACT
Cognitive functioning in older age profoundly impacts quality of life and health. Whilst most research in cognition in older age has focussed on mean levels, intraindividual variability (IIV) around this may have risk factors and outcomes independent of the mean. Investigating risk factors associated with IIV has typically involved deriving a summary statistic for each person from residual error around a fitted mean. However, this ignores uncertainty in the estimates, prohibits exploring associations with time-varying factors, and is biased by floor/ceiling effects. To address this, we propose a mixed-effects location scale beta-binomial model to estimate average probability and IIV in a word recall test in the English Longitudinal Study of Aging. After adjusting for mean performance, an analysis of 9,873 individuals across 7 (mean: 3.4) waves (2002-2015), found IIV greater: at older ages; with lower education; in females; with more difficulties with activities of daily living; in later cohorts; and when interviewers recorded issues potentially affecting the tests. Our study introduces a novel method to identify groups with greater IIV in bounded discrete outcomes. Our findings have implications for daily functioning and care, with further work needed to identify the impact for future health outcomes.
PMID:37579319 | DOI:10.1093/aje/kwad169
