Addressing Practice Effects in Population-Based Studies of Trends in Late-Life Dementia and Cognitive Impairment

Practice effects, defined by the American Psychological Association as "any change or improvement that results from practice or repetition of task items or activities," have been a growing focus in longitudinal studies of the cognitive functioning of older adults.A PubMed search yields more than 1 500 results since the term first appeared in 1953.More than 200 articles use the term combined with "Dementia" or "Cognitive Impairment," nearly two-thirds of which have appeared in the last 10 years.A key focus in recent years has been on accounting for biases related to practice effects in cohort studies to isolate the impact of aging or disease progression on cognitive function (1).Several articles have compared approaches for adjusting for practice effects in statistical models, again in the context of intraindividual, age-related change (2-4).An additional topic, relevant for this essay, highlights that practice effects are not simply a statistical nuisance to be avoided but may be a key marker of cognitive function (5-9).The existing literature is thin with respect to addressing practice effects when studying national trends in late-life dementia and cognitive function (10-12), and there is disagreement on how to address practice effects in this context (13,14).This essay offers insights into challenges in addressing practice effects when studying dementia and cognition trends with national panel surveys that periodically replenish their samples.We first summarize 2 articles appearing in this supplemental issue that address practice effects in the context of national dementia or cognition trends.We then provide a review of existing recommendations for addressing practice effects in studies modeling cognitive decline with age and outline challenges with these methods when studying trends with national panel surveys that periodically replenish their samples.We then describe the logic of approaching practice effects as a misclassification issue.Finally, we provide suggestions moving forward for better addressing cognition and dementia trends in a national survey context.