Objective: To investigate the effect of monetary incentive and the dose-response relationship of participants' response rates in surveys.
Methods: Three databases were searched for randomised controlled trials (RCTs) that investigated the effect of monetary incentives on participants' first and final response rates. First response is defined as the responses after the participant was initially contacted and final response is defined as the responses after several reminders were sent. The potential dose-response relationship of the amount of monetary incentive on the relative response rate (RRR) was established by fitting a restricted cubic spline function based on the robust-error meta-regression model.
Results: 105 RCTs were identified. The first RRR increased by 49% (RRR=1.49; 95% CI 1.29 to 1.72) when monetary incentives were provided. Dose-response analysis revealed that an amount between US$6.25 and US$8 had the maximum effect on increasing the first response rate. On average, the final RRR increased almost by 20% (RRR=1.18; 95% CI 1.11 to 1.25) with monetary incentive compared to no-monetary incentive. An amount between US$10 and US$15 had the maximum effect on the final response rate, with an increase in the final RRR of 34% (RRR=1.34; 95% CI 1.19 to 1.51). There was a significant increase in the response rate when two or more reminders were sent.
Conclusion: Monetary incentives and reminders improve the response rates. Future studies need to consider providing monetary incentives and sending at least two reminders to increase the response rate and reduce the chances of non-response bias.
Keywords: Clinical trials; Infectious diseases; epidemiology; therapeutics.
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