Esther Mertens

| 121 Components of School-Based Interventions Cluster Randomized Trials (Higgins et al., 2016) randomization (random vs. quasi- random assignment) and completeness of outcome data (percentage of drop-out) were analyzed as covariates. Additionally, the type of comparison group (passive: No intervention/waitlist vs. active: Care as usual/other intervention) was examined as covariate to examine absolute versus relative effects of the interventions. Characteristics of methodological rigor that predicted the overall effect sizes were included as covariates in further analyses. To analyze which components were associated with stronger or weaker intervention effects, moderation analyses were conducted. Moderation analyses were conducted only if both levels of the moderator (i.e., component present or not) contained at least three effect sizes (Crocetti, 2016). Results Descriptive Characteristics The present meta-analysis included 104 publications reporting on 99 unique interventions. In total, 529 effect sizes were extracted from the publications comparing the intervention with the control condition on the intrapersonal domain ( k = 218) or the interpersonal domain ( k = 311). Four effect sizes were extreme outliers, more than four standard deviations above the mean. All were derived from the same study (Haynes & Avery, 1979) and believed to be unrepresentatively high. These four effect sizes were therefore winsorized. The studies, published between 1979 and 2019 (Median publication year: 2013), were conducted in the USA ( k = 36), Canada ( k = 2), Europe ( k = 45), Australia ( k = 7), Asia ( k = 13), and Africa ( k = 1). In total, the studies comprised 97,884 participants with an average age of 13.70 years ( SD = 1.50) at the start of the intervention and mean sex distribution of 49% boys ( SD = 16.43). Of the studies reporting ethnicity (59%), participants represented mostly ethnic majority in 59% of the studies, mostly ethnic minorities in 28% of the studies, and mixed ethnic majority and minorities in 13% of the studies. Appendix B provides the key characteristics of the included studies. Overall Effect Sizes Interventions had a small positive effect on students’ intrapersonal domain ( d = .19, 95% CI [.13; .25]). More specifically, the positive intervention effects for self-esteem and self-regulationwere somewhat stronger than for internalizing problems andwellbeing. No significant intervention effect was found for resilience (see Table 1). Interventions 6