Studying a within-person process is reliant on between-person differences over time in intensive longitudinal data

Intensive longitudinal data (ILD) are increasingly the focus of psychological studies implementing ecological momentary assessment, ambulatory assessment, daily diary studies and experience sampling methods. The shared aim of these is to transcend the long-standing research tradition of studying studies between-person phenomena and thus emphasizing interindividual differences. The premise on which such ILD-based research builds is that by going beyond cross-sectional data and the study of interindividual differences, we can gain deeper insights into underlying within-person processes. The increasing use of ILD is bolstered by the rediscovery and refinement of statistical methods that seek to differentiate the between-person from the within-person level. In this Critical Commentary, we investigate whether this premise is justified. To this end, we use two example within-person processes – (i) a uniform measurement bias in the context of (ii) a variable that changes over time – and examine how psychological methods study them. We argue that when current psychological approaches claim to study these within-person processes, they do so by relying on between-person differences over time, which ultimately remains between-person differences research. In order for psychological methods to truly address within-person processes, they will have to devise means of separating concurrent within-person processes, or they will continue to be between-person differences approaches, albeit more fine-grained than to date. We name this epistemological phenomenon of studying within-person processes reliant on between-person differences the Between-Not-Within fallacy.