Keywords
depression
Ayudhaya et al. examined the effect of Behavioral Activation on daily step count and heart rate variability among older adults with depression in a study labeled a cluster randomized controlled trial (cRCT). However, only one cluster was assigned to either of the study conditions. Such a design would have zero degrees of freedom for inferential testing, because the variation due to cluster membership cannot be estimated apart from the variation due to treatment assignment. Thus, the intervention effect is completely confounded with the cluster effect. The study should be labeled a quasi-experimental study, not a cRCT. Accordingly, the numerical results should be interpreted as associations but not evidence for causal relationships.Ayudhaya et al. [In cRCTs, there can be and typically are correlated data within each cluster (i.e., unit of randomization) so “errors” (loosely speaking, model residuals) cannot be assumed to be independent across study participants within clusters. Moreover, when clusters (rather than individuals) are assigned to study conditions, the study has a hierarchical structure (individuals nested in clusters) [We commend Ayudhaya et al. for addressing an important question about depression among the older population, and for their collegiality in providing clarification about their study design. The authors’ clarification confirmed our understanding from reading the published methods.Because in Ayudhaya et al.’s study [