71 4 first stars in the figure can be interpreted as one teacher who used the tool five times between M0 and M3. Figure 4.2 Measurement moments in the experimental group and the control group. 4.3.4 Participants In total, 58 mathematics teachers participated in the study. Mathematics was chosen as the subject for this intervention study because in general, math lessons have a clear structure and a clear period of instruction. Mathematics is an obligatory school subject for all students. It is regarded as one of the most important subjects in secondary schools (in addition to reading, writing and spelling). Regular Dutch secondary schools were contacted by email and phone and teachers were also contacted individually, inviting them to participate in the study. Teachers were informed about the study design, about what was expected from them, and about the Impact! tool. Twenty-eight teachers from 23 schools were assigned to the experimental group and 32 teachers from 25 schools were assigned to the control group. One teacher in the experimental group stopped for health reasons. One other teacher was excluded from the analyses as this teacher did not complete the questionnaires at all despite multiple requests to do so. At both M2 and M3, 24 teachers from the experimental group completed the digital questionnaire (see section 4.3.5 of this chapter). An overview of the background characteristics of the 60 teachers in both groups is presented in Table 4.1. At M1, M2 and M3, respectively, 58%, 58% and 60% were male, and the other characteristics remained stable. This shows that both groups are similar with respect to teachers’ gender, age and nationality. Teachers in the control group had slightly more years of general teaching experience, but this difference was not statistically significant.
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