Randomized trial of use of a monetary incentive and a reminder card to increase the response rate to a mailed health survey.

Insufficient response rates are a frequent problem in mailed epidemiologic health surveys. As part of a health survey of 1,235 young adults conducted from November 1992 through January 1993 in Geneva, Switzerland, a randomized factorial trial was carried out to assess the contributions of two tactics to increase response rates. The first incentive was the promise of sending 10 Swiss francs (7 US dollars) to respondents; the second was a red postcard, mailed 2 days after the questionnaire, which reminded potential participants to complete the questionnaire. The most effective strategy according to interim analysis was to be selected for follow-up mailings. Two weeks after the first mailing, response rates were 65% for those who had received both incentives, 57% for those who had been offered the money reward only, 54% for recipients of the reminder card, and 48% for those who had received neither incentive (p based on chi 2 (3 df) test < 0.001). The relative hazards of responding were 1.34 (95% confidence interval 1.15-1.55) for the money reward and 1.22 (95% confidence interval 1.05-1.41) for the reminder card. Follow-up mailings to all nonrespondents included both incentives. At the end of data collection, 1,007 persons (82%) had returned the questionnaire. The final response rates were 83, 84, 82, and 78% in the four groups, respectively (p = 0.29). Both the promise of a monetary reward and a reminder postcard considerably improved early response rates. The use of both incentives in follow-up mailings brought final response rates above 80%, except for those who had received no initial incentive. Testing various combinations of incentives in the early stages of a mailed health survey may be a generally useful strategy.