Effects on subject response of information brochures and small cash incentives in a mail-based case-control study

PURPOSE: To investigate the impact on subject response of an information br ochure and cash incentives included with mailed questionnaires in case-cont rol studies. METHODS: A randomized trial was carried out within a case-control study inv estigating cancer in the Province of Ontario. Brochures were included with half of the mailed questionnaires sent to 7487 cases and 2561 controls. Con trols were also sent cash incentives of $2, $5, or no money. RESULTS: With the brochure, response changed from 75.0% to 75.8% in cases, and from 70.3% to 71.1% in controls. Adjusting for differences in age, resi dence, sex, and cancer site/status, the change was 0.2% [95% confidence int erval (CI) = -1.7-2.1] in cases, and 0.6% (95% CI = -3.1-4.3) in controls. The $2 and $5 incentives increased overall response in controls from 61.9% to 72.8% and 77.2%, respectively, i.e., by 10.9% (95% CI = 6.1-15.6) and 15 .1% (95% CI = 10.4-19.7), after adjustment. This effect was largely confine d to urban areas (for $2 and $5, respectively: 5.5% and 14.2% in Toronto, 1 5.3% and 20.4% in other urban areas vs. 2.7% and 1.0% in rural areas; p = 0 .02). Response time showed little or no improvement when the brochure was i ncluded, but was markedly reduced for both the $2 and $5 incentives. CONCLUSIONS: Cash incentives can improve subject response in epidemiologic studies, whereas information brochures do not appear to have an effect. (C) 2000 Elsevier Science Inc. All rights reserved.