Sample size
The relevant parts of the original sample size calculation as published in the protocol were as follows.Originally it had been the intention to randomise 300 villages, because this gave over 90% statistical power to detect a difference of 0.25 standard deviations in mean standardised test scores in STRIPES 2. However, incorporating the buffer zones described in the village selection procedure above meant that only 204 villages could be selected. These 204 villages have a mean population of 1487 (minimum 558, maximum 2490) and a standard deviation of 505 (equating to a coefficient of variation of 0.34). Estimating the number of children in each school year from the number under the age of six years old (divided by 6), the mean number of children in each school year is 38.3 (minimum 20, maximum 71) with a standard deviation of 13.3 (a coefficient of variation of 0.35). Assuming that 25% of the children will not satisfy the eligibility criteria, this gives an estimated mean number of eligible children per village of 28.7 with a minimum of 15.We estimated that the 204 villages will include an average of 28.7 eligible students. In the STRIPES trial the estimated effect was a 0.75 SD increase in mean score: however, effects of smaller magnitude than this would still be important to detect. Conservatively assuming that 60% of the eligible children will take the test at the end of the trial, and an intra-cluster correlation coefficient of 0.23 (as seen in the STRIPES trial [As described above, in the sample size calculation we anticipated that 194 of the 204 villages would be randomised. In fact, 196 were randomised, as 6 villages were removed since they were found to be too close to urban areas to be considered rural, and 2 removed because insufficient eligible children were found. Over 7000 children were enumerated in the randomised villages, with over 6000 children taking the test at the end of follow-up.