Abstract
BACKGROUND: Poor response rates to postal questionnaires can introduce bias and reduce the statistical power of a study. To improve response rates in our trial in primary care we tested the effect of introducing an unconditional direct payment of 5 pounds for the completion of postal questionnaires. METHODS: We recruited patients in general practice with knee problems from sites across the United Kingdom. An evidence-based strategy was used to follow-up patients at twelve months with postal questionnaires. This included an unconditional direct payment of 5 pounds to patients for the completion and return of questionnaires. The first 105 patients did not receive the 5 pounds incentive, but the subsequent 442 patients did. We used logistic regression to analyse the effect of introducing a monetary incentive to increase the response to postal questionnaires. RESULTS: The response rate following reminders for the historical controls was 78.1% (82 of 105) compared with 88.0% (389 of 442) for those patients who received the 5 pounds payment (diff = 9.9%, 95% CI 2.3% to 19.1%). Direct payments significantly increased the odds of response (adjusted odds ratio = 2.2, 95% CI 1.2 to 4.0, P = 0.009) with only 12 of 442 patients declining the payment. The incentive did not save costs to the trial--the extra cost per additional respondent was almost 50 pounds. CONCLUSION: The direct payment of 5 pounds significantly increased the completion of postal questionnaires at negligible increase in cost for an adequately powered study.