Effective strategies are critical to engage and retain participants of long term studies. A recent systematic review identified that the most common retention strategy employed was a cash/voucher incentive (59/95 studies reviewed)1. The Avon Longitudinal Study of Parents and Children has been collecting data annually from participants for over 30 years. Participants (parents and their offspring now aged ~28) are offered a £10 high street shopping voucher to complete our annual questionnaires. Across those generations we would anticipate upwards of 10,000 completed questionnaires. This comes at a substantial financial burden of over £100,000. Anecdotally, we are aware that some participants do not use their voucher once claimed or would be happy to complete regardless. The study has always provided an opt-out option for participants to indicate that they do not wish to receive their voucher, however, we were interested to see whether an opt-in option could introduce cost saving. We therefore randomised participants receiving invitations to complete the last annual questionnaire (late 2019/early 2020) to opt-out of receiving their voucher or to opt-in.