Abstract

Background: Developmental researchers use a variety of research designs to examine agingrelated
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 longterm
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 agingrelevant
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.