Intraclass Correlation in Cluster Randomized Controlled Trials: A Meta-Analysis of Studies on Major Depression.

Background: Though cluster randomized trials (CRTs) provide robust evidence for intervention by controlling contamination of interventions, there could be some loss of statistical efficiency. The Consolidated Standards of Reporting Trials (CONSORT) recommends reporting intraclass correlation coefficients (ICC) to understand this phenomenon, though not many studies follow this. This meta-analysis explored the compliance of CRTs in major depression for reporting ICC besides deriving the pooled ICC and pooled mean differences of intervention outcomes.

Methods: Thirty-four articles on CRTs in major depression were identified from PubMed, Cochrane Library, PsychINFO, and Embase, and relevant data were extracted. Only 20 studies were eligible for meta-analysis of intervention, among which 8 reported ICC. We used DerSimonian and Laird's inverse variance method to calculate the pooled estimates.

Results: Only eight (40%) of the CRTs reported using ICC both for designing the study and examining intervention outcomes. The pooled ICC was 0.07 (95% confidence interval [CI]: 0.05, 0.09) with a low heterogeneity (I² = 28%). Among the 20 studies, 65% used different psychosocial methods alone as intervention, with substantial heterogeneity. The pooled standardized mean difference of depression scores (-0.46; 95% CI: -0.79, -0.13) indicated the effectiveness of psychosocial interventions irrespective of combined pharmacotherapy (z = 2.71, p value = 0.01). Further, a subgroup analysis of intervention effects revealed that the results were significant only for the CRTs with ICC conformity.

Conclusions: The ICCs can affect the intervention outcomes. Therefore, as indicated by this meta-analysis, CRTs must adhere to the CONSORT guidelines on reporting ICC. Future CRTs on major depression can utilize the pooled ICC estimate from this study, especially for sample size estimations.