Randomization
EVENTS
Randomization of classes (cluster allocation) was realized using the RANDARRAY function in excel, which generated random numbers between 0 and 1, with 15 decimal places. For each of the two schools, a random number array was generated for the participating classes within each school (n = 8, n = 12). Because we used stratification for both school and educational track, with the additional restriction of even frequencies within each educational track, we could make duo’s of two classes within the same educational track of the same school. The class with the highest random number was allocated to the growth mindset intervention, while the class with lowest random number was allocated to the control intervention. All participants within each class were invited to participate, see Participants.The following order of events was implemented in this RCT: enrollment of schools, enrollment of classes, consent of individual students, random allocation, start interventions. Allocation was concealed throughout the RCT for participating students, parents and class mentors, but not the researchers. Although we did not explicitly mention the allocation, it was possible for participants to identify the allocated intervention, based on the participant information they received during enrollment. However, this information only described the interventions as ‘