Timing payments to subjects of mail surveys: cost-effectiveness and bias

Although mailed surveys are an important component of epidemiological research, results from mailed surveys are often suspect because of poor response rates and the potential for nonresponse bias. Previous work has demonstrated that paying subjects to complete questionnaires increases response rates, but this work has not well addressed the impact of the timing of incentives on total cost, cost effectiveness, and response bias. We surveyed 400 university employees about health benefits. By random allocation, half received a check for $5 along with the mailed survey, and the other half received the promise of $5 on return of a completed survey. The response rates for both groups were about the same (64 and 59%, respectively), but prepayment was less expensive in aggregate and less expensive per response. In addition, we found that subjects with lower salaries were more likely to respond when paid in advance. We conclude that prepayment may actually be less expensive and more cost effective than payment on completion, but that the timing of payment may influence the profile of respondents.