60 Chapter 4 Stratified random sampling Stratified random sampling at individual level was performed after completion of the informed consent forms using Castor Electronic Data Capture system (Castor EDC, 2019). It was independently carried out by a team member, SS, using a stratified blocked random sampling scheme (block size 4) and stratified by: a) gender (male versus female) and b) school weight of the school the teacher was employed by. We stratified by gender because female teachers experience more stress than their male colleagues (Li, et al., 2020; Ozamiz-Etxebarria et al., 2020). School weight is a government measure used in the Dutch education system to give weight to characteristics of a pupil associated with a higher risk of educational disadvantage (Statistics Netherlands, 2019, https://www. cbs.nl/en-gb). Each pupil is assigned a specific ‘weight’. For every school, the weighted number of pupils is calculated by multiplying the total number of pupils by their respective individual weights. This results in the overall school weight. Depending on stratified random sampling, teachers who were included in the study were allocated to start MBSR either in September (intervention condition) or in March (wait-list control condition) of a particular school year. As the study involved a psychological intervention that teachers actively engaged in, maintaining participant blinding to their assigned condition was not feasible. Procedure and allocation To recruit participants, information about MBSR and the research study was disseminated to school boards in the study area and they were asked to bring it to the attention of their teachers. Teachers were also recruited by social media and during presentations at schools. Teachers who were interested in participating were asked to contact the study team by e-mail, telephone, or they could use an online login link. A member of the research team (JL) contacted each interested teacher for a screening interview by telephone. Details about the program were explained and teachers were able to ask questions. Next, it was determined whether the teacher met eligibility criteria. Teachers who were willing to participate provided written informed consent, after which they received an online questionnaire asking for their demographic characteristics. Other selfreport questionnaires were sent to all teachers before (pre) and after MBSR or waiting list period (November) and at three months follow-up (March). All questionnaires were digitally sent and processed anonymously by Castor EDC, which tracked and logged any manual changes made to the raw data. After 5 days, participants who had not completed the questionnaires were reminded by email. If no response followed, they were contacted per telephone to motivate them to complete the questionnaires. During each school year, the intervention training was taught ‘live’ three times a week between September and November at a centrally located training facility within the region. When registering, participants were given the option to prioritize their preferred training session time (e.g. afternoon, evening), aiming to encourage research participation. These groups were supplemented by diverse participants from elementary schools (e.g. teachers, principals, staff members, school administrators) from the region who registered for the standard
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