Flow diagram of the progress of schools and children through the trial.

<p>¹Includes all children in school years 1 to 6 (generally aged from 5 to 11 y). The number given here is the average roll over the period of the trial. ²Follow-up children were a randomly selected sample of all children attending the participating schools, whose caregivers were followed up for detailed information about their illness absences. The primary outcome, absence episodes due to illness, is measured only in this group of children. ³We may not have been informed about all children who left the schools. ⁴All follow-up children were included in the analysis. The period of time that each child was in the trial (the exposure period) was adjusted for through the statistical model. ⁵All children who had an absence (for any reason) were included in the analysis, even if they were lost to follow-up at some point (e.g., moved schools). The exposure period was calculated as the average of the school roll over the period of the trial (multiplied by 100; the number of school days that were encompassed by the trial period). Figure adapted from <a href="http://www.plosmedicine.org/article/info:doi/10.1371/journal.pmed.1001700#pmed.1001700-Campbell1" target="_blank">[29]</a>.