Effects of an increased financial incentive on follow-up in an online, automated smoking cessation trial: A randomised controlled Study Within a Trial (SWAT).

Poor retention in clinical trials can impact on statistical power, reliability, validity and generalisability of findings and is a particular challenge in smoking cessation studies. In online trials with automated follow up mechanisms, poor response also increases 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). A study within a trial (SWAT), embedded within a host randomised controlled trial. Host trial participants were randomised 1:1 to receive either a £10 or £20 voucher incentive, for completing the 6-month questionnaire. Stratification for randomisation to the SWAT was by minimisation 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. 204 participants were randomised to the SWAT. The £20 and £10 incentives did not differ in completion rate at 6 months (79% versus 74%; p=0.362) but did reduce the proportion of participants requiring manual follow up (46% versus 62%; p=0.018) and the median completion time (7 days versus 15 days; p=0.008). Measure response completeness rates were higher among £20 incentive participants, though differences were small for the host trial's primary smoking outcome. 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. 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 generalisability, and improve statistical power in smoking cessation trials.

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