Design and Analysis of Longitudinal Studies on Aging

This chapter focuses on ways to achieve an integrative understanding of aging-related changes. Comprehensive, developmental theories are sought that combine both between-person (BP) and within-person (WP) sources of information from results based on appropriate designs and tenable assumptions. These two general sources of variability cannot be expected to correspond. There is good evidence that BP differences are important modifiers of WP processes and that WP processes are important components of subsequent BP differences. It is also essential for continued understanding of change that the interpretations of results and theoretical developments be sensitive to different temporal sampling spans, as these will likely reflect different WP processes and influences. The study of WP change emphasizes the interplay among cognitive, emotional, and physiological characteristics in the development and changing contexts that occur over time, and thus is sensitive to the temporal sampling of the design. This requires many different levels of analysis that are each complementary and necessary perspectives on the dynamics of aging. It is only within the context of longitudinal studies on aging that the critical comparison of both BP differences and WP change processes can be evaluated in the same sample of aging individuals.