Effect of prepaid and promised financial incentive on follow-up survey response in cigarette smokers: A randomized controlled trial

Background: Monetary incentive is often used to increase response rate in smokers' survey, but such effect of prepaid and promised incentives in a follow-up survey is unknown. We compared the effect of different incentive schemes on the consent and retention rates in a follow-up survey of adult cigarette smokers. Methods: This was a randomized controlled trial (RCT) in Hong Kong, China. Smokers who completed a non-incentivized baseline telephone smoking survey were invited to a 3-month follow-up, with randomization into (1) the control group (no incentive), (2) a promised HK$100 (US$12.8) incentive upon completion, (3) a promised HK$200 (US$25.6) incentive upon completion, or (4) a prepaid HK$100 incentive plus another promised HK$100 incentive ("mixed incentive"). Crude risk ratios from log-binomial regression models were used to assess if the 3 incentive schemes predicted higher rates of consent at baseline or retention at 3-month than no incentive. Results: In total, 1246 smokers were enrolled. The overall consent and retention rates were 37.1 and 23.0%, respectively. Both rates generally increased with the incentive amount and offer of prepaid incentive. The mixed incentive scheme marginally increased the retention rate versus no incentive (26.8% vs 20.3%; risk ratio (RR) = 1.32; 95% CI: 1.00-1.76; P = 0.053), but not the consent rate (RR = 1.13; 95% CI: 0.93-1.38; P = 0.22). Among the consented participants, approximately 50% in the mixed incentive group received the mailed prepaid incentive, who achieved a higher retention rate than the group without incentives (82.8% vs 56.1%; RR = 1.48; 95% CI: 1.21-1.80; P < 0.01). Conclusion: The mixed incentive scheme combining the prepaid and promised incentive was effective to increase the follow-up retention rate by 48%. We recommend this mixed incentive scheme to increase the follow-up retention rate. More efficient methods of delivering the incentive are needed to maximize its effects. Trial registration: U.S. Clinical Trials registry (clinicaltrials.gov, retrospectively registered, reference number: NCT03297866).