Studying Within-Person Variation and Within-Person Couplings in Intensive Longitudinal Data: Lessons Learned and to Be Learned.

Intensive longitudinal designs (e.g., experience sampling methods, daily diary studies, or ambulatory assessments) continue to gain importance in psychological aging research. Empirical research using these designs has greatly facilitated our understanding of short-term within-person processes and has started to approach the question how these processes shape long-term development across the life span. The aim of this viewpoint article is to point out four key issues in intensive longitudinal designs that in our opinion require more attention than they are currently given: (a) improvement in measurement reliability, (b) the necessity to investigate inter-individual differences in short-term dynamics, (c) considerations of the time scale across which dynamic effects unfold, and (d) targeting causality by incorporating experimental methods in intensive longitudinal designs. We illustrate these four key issues by referring to a prominent example of within-person dynamics in prior empirical research: the within-person coupling of stressor occurrence and well-being (stress reactivity).