Intensive longitudinal studies typically examine phenomena that vary across time, individuals, contexts, and other boundary conditions. This poses challenges to the conceptualization and identification of replicability and generalizability, which refer to the invariance of research findings across samples and contexts as crucial criteria for trustworthiness. Some of these challenges are specific to intensive longitudinal studies, others are similarly relevant for the work with other complex datasets that contain multilayered sources of variation (individuals nested in different types of activities or organizations, regions, countries, etc.)This article opens with discussing the reasons why research findings may fail to replicate. We then analyze reasons why research findings may falsely appear to be non-replicable when in fact they were as such replicable, but lacked generalizability due to heterogeneity between samples, subgroups, individuals, time points, and contexts. Following that, we propose conceptual and methodological approaches to better disentangle non-replicability from non-generalizability and to better understand the exact causes of either problem. In particular, we apply Lakatos’s proposition to examine not only whether but under what boundary conditions a theory is a useful description of the world, to the question whether and under which conditions a research finding is replicable and generalizable. Not only will that contribute to a more systematic understanding of and research on replicability and generalizability in longitudinal studies and beyond, but it will also be a contribution to what has been called the heterogeneity revolution (Bryan et al., 2021; Moeller, 2021).

Learning in a teaching practicum is highly intraindividual. However, to date, this dynamic process has been investigated mainly through trait-like, interindividual (between-person) statistics, whereas a state-like intraindividual (within-person) perspective, especially regarding motivation, has been rarely applied. This study aimed to disentangle the basic needs of 115 preservice teachers into their trait-like, interindividual and state-like, intraindividual proportions using biweekly measurement occasions. This analytical approach allowed us to relate preservice teachers’ situational basic needs satisfaction to specific learning occasions in the practicum. Applying a random intercept cross-lagged panel model, we found generally few carry-over effects from measurement occasion to measurement occasion. Furthermore, negative cross-lagged interrelations between competence and autonomy as well as competence and relatedness were found at the beginning of the teaching practicum. School mentoring predicted the general satisfaction of all three basic needs. Implications of this new analytical perspective for further research are discussed.

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