Chapter 10: Considering Alternative Metrics of Time: Does Anybody Really Know What “Time” Is?

Longitudinal studies (i.e., in which each person is observed at multiple occasions) are a cornerstone of research in psychology and human development and have become increasingly common across fields, such as education and business. Although many developmental questions have initially been addressed using cross-sectional studies, such between-person comparisons of people of different ages at a single point in time are often subject to well-known biases, including cohort effects, self-selection effects, mortality effects, and other problems (for more extended discussion, see Baltes, Cornelius, & Nesselroade, 1979; Baltes & Nesselroade, 1979; Hofer & Sliwinski, 2006; Schaie, 1965, 2008). Longitudinal studies can offer significant advantages over cross-sectional studies, in that not only can they provide cross-sectional, between-person information about interindividual variation (i.e., when the longitudinal study begins as a cross-sectional study of persons at different ages), but because they also provide within-person information about intraindividual change or variation over time (and between-person differences in those within-person changes).