4.4. Strengths and Limitations
EVENTS
The strengths of this study include (1) a systematic, transparent planning process guided by the Intervention Mapping protocol and an explicit program theory informed by a thorough needs assessment, theoretical behavior change techniques, and the best available evidence; (2) a multi-component intervention of a long duration targeting both school environmental stressors and student knowledge, attitudes, and skills; (3) a large sample size of schools and students; (4) a successfully randomized controlled trial design; (5) a high response rates among students; (6) the use of a validated outcome measure; and (7) the use of statistical methods to take the clustering structure of the data and the attrition into account.There are some limitations to the study. First, the enrolment rate of high schools was low. Only one third of invited high schools accepted the study invitation. Principals’ main reasons for declining the study invitation was a lack of time due to external ministerial demands such as spending cuts and the implementation of a new comprehensive education reform. In a previous study, we found that students at HHS schools did not differ noticeably from students at non-participating high schools with regard to their gender, well-being, sense of community, PA, sleep, stress, and meal habits [Second, one third of students were lost to follow-up because they no longer attended one of the participating high schools at the time of the follow-up, or because their high schools were unable to find time to conduct the survey among students. Overall, the attrition was similar among intervention and control groups, but non-responders from the intervention group were more likely to be descendants compared to the control group. On the other hand, the students of the control group had experienced more major life events within the past year compared to the intervention group. We handled the attrition by using multiple imputation of missing data. The complete case analysis (sensitivity analysis) showed similar findings, which may reflect that attrition bias was not a major problem [