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

To investigate the impact on subject response of an information brochure and cash incentives included with mailed questionnaires in case-control studies.

A randomized trial was carried out within a case-control study investigating cancer in the Province of Ontario. Brochures were included with half of the mailed questionnaires sent to 7487 cases and 2561 controls. Controls were also sent cash incentives of $2, $5, or no money.

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, residence, sex, and cancer site/status, the change was 0.2% [95% confidence interval (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 confined to urban areas (for $2 and $5, respectively: 5.5% and 14.2% in Toronto, 15.3% 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 included, but was markedly reduced for both the $2 and $5 incentives.

Cash incentives can improve subject response in epidemiologic studies, whereas information brochures do not appear to have an effect.