Abstract
Objectives To evaluate the effectiveness of a contingent financial incentive (£10 note in addition to a routinely provided £10 voucher) versus no contingent financial incentive, on improving the retention rate in a randomised controlled trial (RCT). Methods A two arm 'Study within a Trial' (SWAT) embedded within a host RCT (SCIMITAR+). Participants were randomised to the SWAT using a 2:1 (intervention:control) allocation ratio. The primary outcome measure was the proportion of participants completing a CO breath measurement at the first SCIMITAR+ follow up time point (6 months). Secondary outcomes were withdrawing from follow-up after contact and time from assessment due date to completion. Analyses were conducted using logistic or Cox Proportional Hazards regression as appropriate. Results A total of 434 participants were randomised into this SWAT. Completion of the CO breath measurement at 6 months was 88.5% (n=247) in the intervention arm of the SWAT and 85.4% (n=123) in the control arm (Difference 3.1%, OR 1.29, 95% CI 0.71-2.33, p=0.41). There was also no evidence of a difference in the proportion of participants withdrawing from follow-up after contact (intervention n=7 (2.5%), control n=5 (3.5%); OR 0.76, 95% CI 0.23-2.44, p=0.64), nor in terms of proximity of 6-month visit completion to due date (HR 1.07, 95% CI 0.86-1.33, p=0.55). Conclusion It is unclear if contingent financial incentives increased rates of face-to-face follow-up completion within the SCIMITAR+ trial population. However, the sample size of this SWAT was constrained by the size of the host trial and power was limited. This SWAT adds to the body of evidence for initiatives to increase response rates in trials.
Highlights
Attrition is a major problem for randomised controlled trials (RCTs) with 25% experiencing more than 10% attrition[1]
In total, 434 participants were randomised into this study within a trial (SWAT) (n=286, 65.9% intervention group; n=148, 34.1% control group)
Due to randomisation occurring at a single time point, eleven participants were excluded from analysis as they withdrew from SCIMITAR+ following randomisation but prior to being contacted for their 6-month visit
Summary
Attrition is a major problem for randomised controlled trials (RCTs) with 25% experiencing more than 10% attrition1.Bower et al (2014) identified financial incentives as an effective retention strategy (RR 1.18; 95% CI 1.09 to 1.28), and effectiveness was increased if this incentive was provided on receipt of a completed questionnaire (RR 1.25; 95% CI 1.14 to 1.38). Bailey et al (2013) identified that varying the incentive level (£20 compared to £10) increased response to postal questionnaires by up to 10%.Sample size The sample size was determined by the number of participants followed-up at 6 months in SCIMITAR+ from the point at which this SWAT was embedded.Randomisation Simple randomisation using random numbers was carried out by an independent statistician at the York Trials Unit using Stata v136. Attrition is a major problem for randomised controlled trials (RCTs) with 25% experiencing more than 10% attrition. Bailey et al (2013) identified that varying the incentive level (£20 compared to £10) increased response to postal questionnaires by up to 10%. Sample size The sample size was determined by the number of participants followed-up at 6 months in SCIMITAR+ from the point at which this SWAT was embedded. Randomisation Simple randomisation using random numbers was carried out by an independent statistician at the York Trials Unit using Stata v136. All potentially participants eligible for inclusion, at the time of randomisation, were allocated with a 2:1 allocation ratio (intervention:control) due to the anticipated effectiveness of financial incentives increasing questionnaire response rates
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