Background: Sending a monetary incentive with postal questionnaires has been found to improve\ud the proportion of responders, in research in non-healthcare settings. However, there is little\ud research on use of incentives to improve follow-up rates in clinical trials, and existing studies are\ud inconclusive. We conducted a randomised trial among participants in the Managing Injuries of the\ud Neck Trial (MINT) to investigate the effects on the proportion of questionnaires returned and\ud overall non-response of sending a £5 gift voucher with a follow-up questionnaire.\ud Methods: Participants in MINT were randomised to receive either: (a) a £5 gift voucher (incentive\ud group) or (b) no gift voucher (no incentive group), with their 4 month or 8 month follow-up\ud questionnaire. We recorded, for each group, the number of questionnaires returned, the number\ud returned without any chasing from the study office, the overall number of non-responders (after\ud all chasing efforts by the study office), and the costs of following up each group.\ud Results: 2144 participants were randomised, 1070 to the incentive group and 1074 to the no\ud incentive group. The proportion of questionnaires returned (RR 1.10 (95% CI 1.05, 1.16)) and the\ud proportion returned without chasing (RR 1.14 (95% CI 1.05, 1.24) were higher in the incentive\ud group, and the overall non-response rate was lower (RR 0.68 (95% CI 0.53, 0.87)). Adjustment for\ud injury severity and hospital of recruitment to MINT made no difference to these results, and there\ud were no differences in results between the 4-month and 8-month follow up questionnaires.\ud Analysis of costs suggested a cost of £67.29 per additional questionnaire returned.\ud Conclusion: Monetary incentives may be an effective way to increase the proportion of postal\ud questionnaires returned and minimise loss to follow-up in clinical trials
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