Design
The evaluation of the Guide Cymru used a two-armed cluster randomised controlled trial. A cluster-design was necessary as The Guide Cymru programme was delivered to the pupils by their teachers. Therefore, the school was the unit of clustering. Further details can be found in the study protocol [Schools were stratified by geographic location to facilitate the researchers visiting the schools. For each location (typically consisting of 6 -12 schools) each school was randomised into either the active or control arm of the study using a computer-generated random sampling procedure to ensure unbiased allocation to each group prior to data collection: (