ABSTRACT
Research designs and analysis approaches for understanding and explaining aging-related change have developed significantly over the last few decades. A primary focus of these efforts has been in the area of cognitive abilities, particularly in terms of change patterns across the adult life span and short-term intraindividual variability as a potentially more sensitive index of subsequent within-person change. As age itself is a major risk factor for cognitive impairments, the number of people potentially suffering from cognitive changes and related problems will sharply increase in the following decades as a greater proportion of the population reach advanced ages. That cognitive performance declines, on average, throughout the adult life span is in itself neither new nor informative. However, the substantial variability across persons in onset and pattern of age-related cognitive change is significant. While decline in some individuals is relatively modest, in individuals with dementia, progressive and accelerating declines in functioning occur. Individual differences in rates of change offer more and qualitatively better information about development than average trajectories and provide the opportunity for detection of changes in functioning relative to an individual’s own baseline level and the evaluation of other cognitive characteristics, such as intraindividual variation, as a sensitive early marker of future change. As such, the investigation of between-person (i.e., interindividual) individual differences in within-person (i.e., intraindividual) change and variation is essential for understanding development and aging.
