Improvement in cumulative response rates following implementation of a financial incentive.

Risk estimates arising from case-control studies can be unreliable if the level of response to mailed questionnaires is inadequate. Several studies have reported improved early response rates to mailed questionnaires following the implementation of financial incentives. Improvements in cumulative response rates at the completion of the follow-up period, however, have not been as pronounced. A financial incentive of $5.00 was implemented among control subjects in a large population-based case-control study of Ontario, Canada, women. Required follow-up time and effort were decreased for the controls who received the incentive compared with those who did not. More importantly, cumulative response rates after more than 20 weeks were 20 percent higher among controls who received the incentive.