Impact of a financial incentive on case and control participation in a telephone interview

The authors investigated the effect of a 5.00 dollars incentive on participation in a telephone interview among cases and controls in an ongoing study of colorectal cancer. Cases and matched community controls were sent a letter introducing the study. One week later, a nurse called to invite the person to participate in a 30-minute telephone interview. After 1 year of data collection (which began in June 2001), the authors began enclosing a 5.00 dollars bill in the initial letter as an incentive. Incentives were mailed to all potential controls. The authors randomized 50% of a subset of cases to receive the incentive. In the year prior to institution of the incentive, 44.2% of 851 controls participated in the interview, as compared with 56.2% of 1,043 controls in the year after the incentive was instituted (p < 0.001). Among cases randomized to receive the incentive (n = 199), 63.8% participated as compared with 68.4% in the nonincentive group (n = 193) (p > 0.05). Among cases aged 60-69 years, the response rate in the incentive group was reduced by 17% (p = 0.03). Thus, among controls, a small monetary incentive appears to promote a feeling of goodwill toward the research. It does not seem to have an equivalent effect among cases, and in the worst case it may insult or annoy some cases who may otherwise have participated.