Chapter 10 350 completion of the study. They will be stored securely and adequately protected from damage. Statistical methods The primary analysis compares continuous smoking abstinence rates at twelve months after the group-based training session in the intervention arm against the control arm, following an intention-to-treat analysis. Secondary continuous smoking abstinence at fifteen months is assessed to see if abstinence is sustained after the incentives stopped. All randomised participants will be included in the final analysis, those lost to follow up will be classified as smoking except for an unavoidable loss to follow up (e.g. untraceable contact information or death, following the Russel standard).23 Generalised linear models with a logit link function will be used to assess the difference in the primary outcome between control and intervention arm at t =12 months and to account for clustering between hospitals. Additionally, subgroup analyses will be performed to identify possible subgroupspecific effects of the intervention. Planned subgroup analyses are based on: 1. Readiness-to-quit. Preparation stage versus contemplation stage;10, 20 2. Degree of tobacco dependence. Fagerström score below 5 versus Fagerström score of 5 or higher;19 3. Present bias. Delayed versus present reward preference. Delayed reward preference = more than 50% larger delayed reward is chosen, or present reward preference = less than 50% larger delayed reward is chosen;21, 22 4. Income. Based on gross monthly income in €. Low income = below 1500 versus moderate/high income = above 1500.30 and 5. Educational level. Based on the International Classification of Education (ISCED).28 Lower educated = ISCED 1-4 versus higher educated = ISCED 5-8. Subgroup analysis are not included in the power analysis and therefore expected to be suggestive. For missing data on subgroup information, multiple imputation will be performed.
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