Intraindividual Variability, Change, and Aging: Conceptual and Analytical Issues

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Abstract
Background: Developmental researchers use a variety of research designs to examine aging-related changes. Most longitudinal studies of aging are based on research designs that feature successive, widely spaced, assessments to estimate changes in cognitive performance. Such designs assume that short-term variations in cognitive performance are small relative to long-term changes or have modeled such phenomena as nuisance parameters. Objective: There is now sufficient empirical evidence to establish intraindividual cognitive variability as a systematic source of individual differences and of important predictive value for aging-relevant outcomes. Methods: After an overview of types of change, potential underlying processes, and adequate analytic designs, we discuss consequences for lifespan aging research. Results: We emphasize that interpretations of both cross-sectional and longitudinal results need to consider and specify theoretical assumptions about short-term and long-term changes. Conclusions: Above and beyond the analysis of long-term mean changes, short- term changes are an important aspect of aging-related change, and their analysis may help to explain psychological processes of adaptation.