43 Mindfulness-Based Stress Reduction intervention for elementary school teachers: a mixed method study Mindfulness teacher The MBSR will be given by a mindfulness teacher who meets the criteria of the Association of Mindfulness Based Teachers in the Netherlands and Flanders and the internationally agreed good practice guidelines of the UK network for Mindfulness-Based Teachers (mindfulnessteachersuk.org, 2015). If the trainer notes that a participant might be in need of additional mental support or if a participant indicates this him- or herself, a licenced psychologist is available who can be contacted during, as well as after the program. Sample size calculation In determining the necessary number of participants, we have presumed a significance level (α) of 0.05 and a moderate effect size (δ) of 0.50 (e.g., Verweij et al, 2016). To achieve a power of 80%, we will need to include 64 participants in the intervention as well as the control group. With an estimated 17.5% drop-out, which is comparable to a similar study by Verweij et al, (2016), we will strive to recruit 155 participants divided over three school years. Data analysis All analyses will be performed in R (Core team R, 2020) by an independent statistician who is further not involved in the project. The results will be reported in accordance with the extended CONSORT statement (Schulz et al., 2010). Descriptive statistics will be calculated for all variables of interest. Effectiveness The primary analyses will be based on the intention-to-treat principle and linear mixed models (LMMs) will be used for the primary analyses. LMMs adequately deal with missing data at random and can be used to account for the hierarchical structure of the data (i.e. multiple assessments nested within teachers, teachers nested within groups and schools). Time, condition and time by condition interactions will be specified as fixed factors to examine the effect of the intervention over time on primary and secondary continues outcomes. Baseline scores of the corresponding outcome will additionally be entered in the model to account for baseline imbalances. In accordance with the CONSORT statement, we will not statistically test whether the two groups differ in baseline variables. Therefore, the models that we will primarily report will not be adjusted for additional covariates. However, to examine whether adjusting the model for additional covariates substantially changes conclusions drawn from the analyses, we will conduct sensitivity analyses with the models adjusted for the following potentially confounding baseline variables: sex, age, school weight, teachers’ years of experience, number of work days per week and past or present psychological problems. All models will also be run with completers only as additional sensitivity analysis. Post hoc tests will be conducted for between-group differences at each timepoint using analyses of covariance (ANCOVA), 3
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