Limitations
RECRUITMENT
The sample size was relatively small, with only 345 participants recruited during the SWAT, and therefore, the study risks type II error. This is a general design challenge for SWATs, where typically no study-specific calculation is done for the sub-study itself and the recruitment is dependent upon the host trial.Using the GP surgeries as the unit of randomisation was best for the practical implementation of the study, especially with consideration to the host trial and not wanting to compromise the main aims of that investigation. It was also an attractive method to use to overcome the known risk of imbalance due to differences between sites, but this, in turn, created its own difficulty. The way that the pairing of these sites worked out meant that there were many pairs with a site or both sites having not been recruited. This led to only being able to use results from only 10 of the 21 pairs recruited within the initial planned analysis. With the benefit of hindsight, the matched pair design was not the optimal choice due to the substantial number of sites that failed to recruit any participants and therefore did not contribute data to the analysis. In retrospect, this design in a primary care setting, where there is normally a relatively high number of sites that do not recruit Analysing data from matched paired cluster trials is surprisingly difficult as Donner et al. show [The level of compliance with voucher issues was very high within this study, though not 100%. There were five participants within the non-conditional group who did not receive their voucher; none of these participants returned their diary. This lack of compliance could therefore have impacted upon the result. However, the analysis was carried out with these five participants included on an intention to treat as is generally recommended. Given the small number affected it seems unlikely to have affected the findings. No imputation for missing data was made which was a consequence of the design in terms of some sites not recruiting anyone.