Chapter 10 352 in their best interest.31 This feature is scientifically innovative in the context of smoking cessation programmes, and importantly let the treated individuals to choose the scheme they will be enrolled. By focusing on supporting smoking cessation among hospital employees, the PERSIST trial is executed in a highly relevant setting as in light of the National Prevention Agreement all university hospitals have to be smoke free by 2020 and all general hospitals by 2025.32 Finally, the PERSIST trial progresses from previous studies by extending follow-up beyond the last time point when incentives are provided to investigate if participants remain abstinent when incentives are no longer provided. A limitation of the PERSIST trial is that participants from the intervention and control arm attend the same cessation support training sessions due to practical reasons, as slow inclusion rates otherwise would have caused long waiting periods between signing up and starting the training. Participants in the control arm might perceive this negatively and potentially drop out, causing attrition bias. Another consequence could be spill-over effects;33 sharing of vouchers between control and intervention arm, leading to a double underestimation of treatment effectiveness. To avoid this, participants are informed about their status as early as possible to make sure they have time to accept their status. A third arm consisting of non-personalised incentives could have provided additional potentially valuable information on the effectiveness of personalised incentives versus incentives provided according to a fixed scheme. However, this option would have required unfeasibly large sample size. Abbreviations CI: Confidence interval CO: Carbon monoxide Erasmus MC: Erasmus University Medical Center PERSIST: Personalised Incentives for Supporting Tobacco Cessation among Health care Employees, title of the project Ppm: Parts per million RR: Relative Risk
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