Randomisation
RECRUITMENT
This SWAT used a matched pair cluster randomised parallel group design. The trial sites were the unit of randomisation. The sites were cluster-randomised within matched pairs in order to keep the process as straightforward as possible for the recruiters. Randomisation was performed in two waves (before the start of seasons 2 and 3) using computer-generated random numbers carried by one of the investigators (JAC). We paired the recruiting sites from Season 1 according to previous levels of recruitment and they were then randomised within these pairs prior the start of season 2. Sites were paired with a site with the next nearest level of previous recruitment. New sites starting during seasons 2 and 3 were randomised in matched pairs, but using the practice list size as the indicator of potential recruitment. Randomisation took place before the beginning of the second recruitment season. Sites continuing from season two carried on in their original randomised group. If there were to be an uneven number of sites and so an unpaired site, this site would continue on in the trial with the incentive given at a time more convenient for them, but they would not be included in the analysis of this SWAT. GP practices were not blinded to their allocation due to the necessity of them either distributing the incentives initially or not. Participants were unaware that there was a SWAT taking place as it was thought this might influence whether or not they returned their diary.