Felix Naughton

Introduction Poor retention in clinical trials can impact on statistical power, reliability, validity, and generalizability of findings and is a particular challenge in smoking cessation studies. In online trials with automated follow-up mechanisms, poor response also increases the resource need for manual follow-up. This study compared two financial incentives on response rates at 6 months follow up, in an online, automated smoking cessation feasibility trial of a cessation smartphone app (Quit Sense). Aims and Methods A study within a trial (SWAT), embedded within a host randomized controlled trial. Host trial participants were randomized 1:1 to receive either a £10 or £20 voucher incentive, for completing the 6-month questionnaire. Stratification for randomization to the SWAT was by minimization to ensure an even split of host trial arm participants and by 6-week response rate. Outcome measures were: Questionnaire completion rate, time to completion, number of completers requiring manual follow-up, and completeness of responses. Results Two hundred and four participants were randomized to the SWAT. The £20 and £10 incentives did not differ in completion rate at 6 months (79% vs. 74%; p = .362) but did reduce the proportion of participants requiring manual follow-up (46% vs. 62%; p = .018) and the median completion time (7 days vs. 15 days; p = .008). Measure response completeness rates were higher among £20 incentive participants, though differences were small for the host trial’s primary smoking outcome. Conclusions Benefits to using relatively modest increases in incentive for online smoking cessation trials include more rapid completion of follow-up questionnaires and reduced manual follow-up. Implications A modest increase in incentive (from £10 to £20) to promote the completion of follow-up questionnaires in online smoking cessation trials may not increase overall response rates but could lead to more rapid data collection, a reduced need for manual follow-up and reduced missing data among those who initiate completing a questionnaire. Such an improvement may help to reduce bias, increase validity and generalizability, and improve statistical power in smoking cessation trials. Trial Registration Host trial ISRCTN12326962, SWAT repository store ID 164.