Randomisation restriction
The 40 administrative areas were randomly allocated in a 1:1 ratio to early or delayed roll-out of the intervention. Randomisation was restricted to ensure that the trial sequences were reasonably balanced with respect to several key covariates that were thought to be associated with the outcome: the population size of young people aged 15–30 years; location in the northern or southern part of the study area; proximity to a major road. The tolerance thresholds for balance were defined through an iterative process in which different thresholds were tried and the number and the validity of the acceptable allocations were examined. With unrestricted randomisation, every pair of clusters has the same probability of being allocated to the same sequence. When evaluating our restricted randomisation scheme, we aimed to obtain a sufficient number of acceptable allocations and a reasonably uniform distribution of joint allocation probabilities.The mean population size of young people in the 40 clusters was 606. Just over half (21, 52.5%) of the clusters were in the northern surveillance area, and 9 clusters (22.5%) were along the major road. We restricted to allocation options that had absolute differences in the mean cluster size between the two sequences of ≤ 100, the proportion of northern clusters in each sequence was 40.5–64.5%, and the proportion along the main road was 12.4–32.6% (mean ± 1.5 standard deviations). Without restrictions, there were 13.8 × 10