2.7. Statistical Analyses 
OSC
EVENTS
A total of 904 students had no baseline data either because they did not complete the baseline questionnaire (N = 624) or were invited for follow-up only (N = 280). They were excluded from this study. Descriptive baseline data were used to (1) characterize the study population, (2) compare high schools and students from the intervention and control groups, and (3) explore if the characteristics of students lost to follow-up differed between the randomized groups (attrition bias). To account for the nested design, multi-level generalized linear models were used to estimate the association between the intervention and perceived stress. The analysis was performed as a three-level model with students (level 1) nested within classes (level 2) and classes nested within high schools (level 3), allowing for a correlation between students from the same class and high school. The model included the independent variable and added baseline measures of PSS-10, and three covariates highly associated with the outcome: gender, OSC, and major life events. The covariates were chosen a priori and were included to increase the precision of the effect estimates [The statistical analyses were carried out using SAS, version 9.4 (SAS Institute Inc, Copenhagen, Denmark). A significance level of 0.05 was chosen a prior. The analyses were pre-specified in a statistical analysis plan, and all co-authors agreed to the plan before the analyses were carried out. 