The effect of timing of incentive payments on response rates for cohort study telephone interviews in primary care setting with cost-minimization analysis, a randomized controlled trial

Background: The effect of timing of incentive payments on the response rate of telephone surveys is unknown. This study examined whether up-front or delayed incentive payments were associated with higher response rates for participation in a telephone interview administered longitudinal cohort study amongst primary care patients with lower urinary tract symptoms, and to compare the costs between the two timing methods. Methods: This study was conducted as part of a naturalistic observation study on the health-related quality of life and health outcomes of Chinese primary care patients with lower urinary tract symptoms. The incentive payment was in the form of a supermarket gift voucher to the value of HD$50 (US$6.50) and could be used in lieu of cash at a major supermarket chain. 720 subjects with lower urinary tract symptoms were randomly assigned into two groups. One group was offered an incentive of supermarket cash voucher at time of recruitment ('up-front' payment). The other group was told that the voucher would be sent to them after the complete of their 1-year follow-up telephone interview ('delayed' payment). Primary outcomes were the baseline and 1-year follow-up telephone survey response rates. Results: There was no statistical difference in response rates at baseline (p-value = 0.938) or at the 1-year follow-up (p-value = 0.751) between groups. Cost per completed subject interviews for the up-front payment method was USD16.64, whilst cost for the delayed payment was USD 13.85. Conclusions: It appears the timing of incentive payments does not affect response rates for telephone interview surveys conducted on primary care patients in Hong Kong at baseline or at 1-year follow-up. Delayed incentive payments can reduce the overall cost per successful case.