2.2. Participants
The study participants were first-grade children with a mean age of 7.7 ± 0.4 years at the beginning of the study and 9.8 ± 0.4 years after the intervention. Parents of all children provided the required information by completing an online questionnaire about their children’s date of birth and sex (with the option to select “male” or “female”). Analyses were conducted using the per-protocol approach. Before the intervention, 583 children (85.6% of the total sample) completed the fruit and vegetable preferences questionnaire, and 233 children (34.1% of the sample) did so after the intervention. Finally, 193 children (28.3% of the total sample; 85 in the control group and 108 in the intervention group) who completed the questionnaires on both occasions were included in this study. To compare the pre- and post-intervention changes between the control group and the intervention group with a power of 80% (α = 0.05), at least 128 children had to be included in the study (version 3.1.9.2; Heinrich Heine University Dusseldorf, Dusseldorf, Germany). The hypothesis that positive change in the fruit and vegetable preferences is associated with an increase in fruit and vegetable intake was tested on 154 children (22.6% of the total sample; 70 in the control and 84 in the intervention group) who completed both fruit and vegetable preferences questionnaire and food frequency questionnaire before and after the intervention. At least 55 children were required for these analyses (80% power; α = 0.05) (version 3.1.9.2; Heinrich Heine University Dusseldorf, Dusseldorf, Germany).