Statistical and Economic Analysis

Analyses follow the intention-to-treat principle, with complete-case analysis used as sensitivity analysis. The process evaluation uses mixed methods under the UK MRC framework for complex interventions. A linear mixed-effects model will assess the intervention effect on SDQ total score across three time points. Cost-effectiveness analysis will estimate incremental cost-effectiveness ratios using KIDSCREEN-10 as the primary utility measure. The main intervention parameter is the group by time interaction term. Effect modification analyses will test variation by sex, age, country, socioeconomic status, school type, and baseline mental health status. SROI analysis will convert social, environmental, and economic outcomes into monetary terms to produce a benefit-to-cost ratio.