Connecting Theory to Methods in Longitudinal Research

Advances in methods for longitudinal data collection and analysis have prompted a surge of research on psychological processes. However, decisions about how to time assessments are often not explicitly tethered to theories about psychological processes but are instead justified on methodological (e.g., power) or practical (e.g., feasibility) grounds. In many cases, methodological decisions are not explicitly justified at all. The disconnect between theories about processes and the timing of assessments in longitudinal research has contributed to misspecified models, interpretive errors, mixed findings, and nonspecific conclusions. In this article, we argue that higher demands should be placed on researchers to connect theories to methods in longitudinal research. We review instances of this disconnect and offer potential solutions as they pertain to four general questions for longitudinal researchers: how time should be scaled, how many assessments are needed, how frequently assessments should occur, and when assessments should happen.