Conclusion
There was no clear evidence of a statistically significant difference in retention rate according to when a participant is given a monetary incentive for participant completed questionnaire return; however, our study may have lacked precision given the small number of clusters and that a substantial proportion did not recruit any participants. Additionally, there was no clear statistical evidence of a difference in the completion of the diary or questionnaire and the speed with which they were returned to the trial team. However, the analysis lacked precision, and sensitivity analyses which accounted for imbalances between sites suggested genuine effects are plausible. The possibility of a genuine and operationally important difference (to trialists) cannot be ruled out. Furthermore, there was, as expected, a large difference between the gross costs of giving a monetary voucher as a non-conditional incentive as opposed to a conditional incentive in simple monetary terms. It costs approximately £23 more for every diary received to give the vouchers as a non-conditional incentive. It may be cost-effective to give the incentives at the point at which a diary or questionnaire is received by the trial staff than when the participant is recruited. Further studies are needed to explore this aspect of clinical trial conduct.