Crystal Smit

Chapter 2 30 condition did not receive any intervention. All children completed the same pre- intervention and post-intervention measures. Power calculations were conducted using the program G*Power 3.1 (Faul, Erdfelder, Buchner, & Lang, 2009). To detect a small to medium effect (Valente et al., 2003) using a MANCOVA: repeated measures within-between interaction ( f = .20) with two groups and two measurements, 199 participants were needed (power = .80, p = .05). In order to take attrition into account, a larger number of students were recruited. Participants The participants were recruited through their primary schools. Twenty-nine urban and suburban primary schools in the Netherlands were invited to participate. Schools were eligible for participation if they were not involved in any water stimulation program. Ten schools expressed interest in participating; however, six of these were unable to participate due to difficulties scheduling the study. All schools participating in the study included more than 95% of children with a Dutch or West-European background. After gaining active consent from the headmasters of the schools, passive consent was obtained from the children’s caretakers (i.e., the caretakers were informed about the study and could withdraw their child from participating). Out of 255 children, 9 (4%) caretakers withdrew their child from the study. At the outset of the study, we obtained informed consent from children. All children who were present at the baseline measurement ( N =243) agreed to participate. As shown in Figure 2.1, the schools allocated to the intervention condition included 144 children versus 111 children in the control schools. Of these, 243 (95%) children provided baseline data ( n = 134 intervention, n = 109 control). Five children (4%) in the intervention schools and five (5%) in the control schools did not complete the post-intervention questionnaire. The reason for attrition was children being absent from school on the day of testing. In addition, an entire class ( n = 23) in the intervention condition was excluded from the analysis, because the teacher undermined the study and discouraged the children to participate