Sample size: planned number of clusters
We pre-register the sample of eighth-graders in the academic year of 2018/19. This sample contains 34 schools and 40 classes. The sample size in the analytic sample is not yet known.
We pre-register to repeat the same informational campaign in the following academic year (2019/2020) for those who are now in seventh grade and will be in the eighth year next year. This will be done in September 2019. We know, however, that in our database we have 42 seventh-grade classes in 34 schools. We will not do the informational campaign next year if
• more than 6 schools (20%) do not cooperate with us next year for whatever reasons,
• more than 40% of students in at least 6 schools do not have parental consent.
In these cases, the new eighth-grade cohort will not substantially increase the statistical power, therefore there is no point in including them in the analysis. We will also not extend the trial for another year if political or academic circumstances prevent the extension. We will make the final decision about doing the fieldwork in the next academic year in late September 2019.