Strengths and limitations
This study holds several strengths, including that the Focus intervention was evaluated in a longitudinal cluster-randomized controlled trial with a large baseline sample of youth. Moreover, advanced statistical methods were used to account for clustering and missing data. Secondly, the development of the Focus intervention builds on a large and thorough preparation work, including a needs assessment and a clearly defined program theory of how the intervention was expected to impact students’ smoking decisions [Nonetheless, findings from this study should be considered in the context of the following limitations. First, a large proportion of students missed responding at follow-up. To account for potential selection bias, we performed analyses based on multiple imputation including a range of relevant factors. Nevertheless, missing data reduced statistical power with a risk of introducing type II error. Second, the lack of statistically significant results may be influenced by the short follow-up period in which students’ smoking behaviors were investigated. Changing habits and culture in any setting is a difficult task, and it may take a longer time to find common ground on a managerial level on how to address tobacco-related issues and develop new habits. In this connection, a systematic review among schoolchildren found an effect of smoking preventive interventions at more than one-year follow-up, while no effects were detected at one year or less [Finally, recruiting VET schools showed to be a difficult task, and PBE schools were included due to the many similarities among students attending VET and PBE schools – both according to sociodemographic characteristics and smoking prevalence. Nonetheless, as were also reflected in our results, students in PBE schools could constitute a more vulnerable group (i.e., smoking prevalence was markedly higher among students at PBE schools compared with students at VET schools). Therefore, components of the intervention may not have applied equally to students of different school types. Future studies may further investigate the contextual influences on the effects of smoking preventive interventions.Because we had the inclusion criteria that the school management should be willing to implement the Focus intervention, we expect differences in the management group between included schools and schools who rejected our invitation to participate. The schools had different reasons for the rejection including that their focus and resources were not able to meet the requirement of implementing smoke-free school hours. Accordingly, the engagement in implementing the school tobacco policy cannot be generalized to all schools in Denmark. However, we do not have reasons to suspect that the student population is not representative for vocational students in Denmark, i.e., in another Danish report from 2019 among VET schools, the smoking prevalence was comparable with our study (30% smoked daily and 11% smoked occasionally in VET schools in this study vs. 29% and 9%, respectively) [