65 Mindfulness-Based Stress Reduction for elementary school teachers: a randomized controlled trial large, effects > 0.50 as moderate, and effect sizes > 0.20 as small. As post-hoc tests, analyses of covariance (ANCOVAs) with condition and baseline scores as covariates were conducted to examine the effect of the intervention on primary and secondary outcomes at each timepoint. In line with the previously published protocol (Lensen et al., 2021), we did not account for potential covariates in the primary analyses. However, to determine whether including covariates substantially changed the models, sensitivity analyses were conducted in which all linear mixed models were re-ran with the following potentially confounding baseline variables included: gender, age, school weight, teachers’ years of experience, number of work days per week, and past or present psychological problems (Lensen et al., 2021). To determine the potential impact of COVID-19, we additionally included a categorical confounding variable in the sensitivity analyses indicating whether people participated before or during COVID-19 (0=before COVID, 1=during COVID). To test the robustness of the results, outlier checks were performed for all outcomes and all timepoints. If outliers were identified, we re-ran the analyses with these outliers excluded. Outliers were defined as scoring lower or higher than 3 standard deviations from the sample mean at the respective time point. Finally, the proportion of participants in both groups showing reliable change on the primary outcome was calculated for postmeasurement and follow-up (Jacobson & Truax, 1992). Uncertainty surrounding the proportions was expressed with 95% binomial confidence intervals. Moderator analyses in terms of the primary outcome measure perceived stress were conducted using fixed effects models. Condition and the corresponding moderator were included as fixed effects as well as the interaction of the two variables. A significant interaction would indicate that the moderator influences the effect of the intervention. The following moderators were analyzed: past or present psychological problems, school weight, teachers’ years of experience and age (Lensen et al., 2021). Data availability, transparency and openness All data has been analyzed by an independent statistician and analyses are in accordance with the previously published protocol (Lensen et al., 2021). We report how we determined our sample size, all data exclusions, all manipulations, and all measures in the study. This study was preregistered; see https://clinicaltrialregister.nl/nl/trial/21817. The data that support the findings of this study are available from the corresponding author upon request. Data were analyzed using R (R Core Team, 2020) using the packages esc for effect size calculation (Lüdecke, 2019) and lme4 (Bates et al., 2018) for linear mixed models. RESULTS Sample characteristics Of the 146 elementary school teachers who were randomized, eight dropped out before the pre-treatment assessment because they got another job (N=5) or did no longer want to participate in the training (N=3). Of the remaining 138, the majority was female 4
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