Impact on current literature
SENSITIVITY
This was the first study to compare the effect of conditional versus non-conditional monetary incentives on return and completion of a participant-completed questionnaire in a RCT. There was no statistically significant difference in the proportion of questionnaires returned between the times at which the incentive is given in line with the current, limited, research. However, the raw difference of a 15% improvement (17% for the subset of site pairs where both recruited one or more) diaries returned in favour of the conditional incentive arm would be a meaningful one to trialists if genuine. This along with the confidence intervals for the site-level analyses being relatively wide suggests that the analysis lacks precision and that the possibility of a genuine and operationally important difference (to trialists) cannot be ruled out. Sensitivity analyses which are weighted by the number of observations that contributed to the site summary suggest that such an effect is quite possible. This needs exploring in further studies and incorporating this data ultimately within an updated systematic review of incentive use for retention. Additionally, the cost difference between the two approaches suggests that a conditional incentive is more economical.