Sample size
Sample size calculations indicated that a sample size of 10 schools with an average of 100 students (e.g., five schools in the intervention group and five schools in the control group) was necessary to detect a 25% reduction in the number of cigarettes smoked daily at follow-up, with a power of 80% and an alpha of 5%. The cluster design was taken into account with an assumed intra-cluster correlation coefficient for schools (ICC) of 0.025. The ICC was estimated based on the Danish National Youth Study 2014 data [