Background: Optimizing participant response rates is important for obtaining representative samples and the timely completion of studies. It is a common practice to use participant incentives to boost response rates, but few studies have systematically examined their effectiveness, particularly among minority groups. Methods:We experimentally tested three incentive strategies for their effectiveness in improving response rates among colorectal cancer cases (n = 3,816) and their relatives (n = 2,353). A 2 x 2 x 2 factorial design compared (a) registered versus first class mail, (b) $5 cash with the initial mailing (yes/no), and (c) $20 promise (yes/no) upon completion of the information form (for cases) or $10 promise (yes/no) upon completion of the baseline survey (for relatives). Outcome measures were provision of contact information on first-degree relatives for cases and completion of the baseline survey for relatives. Results: The response rate among cases was low in all ethnic groups (28-37%) and incentive strategies did not have an effect. Among relatives, the overall baseline survey response rate was 71%, ranging from 66% among Asians to 76% among Whites. Modest absolute increases were observed for payment schedules that included a $5 cash enclosure with the initial mailing in the total sample [odds ratio (OR), 1.65 and 1.47] and among Latinos (OR, 1.94 and 1.74) but not among Asians (OR, 1.61 and 1.55) or African Americans (OR, 1.19 and 1.02). Response rates were not influenced by registered versus firstclass mailing. Conclusion: The effects of incentives in this study were modest with some suggestion of differences by ethnic group and type of incentive.
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