2.3. Participants

The COSI study included students in 2nd and 3rd grade (7–8 y and 8–9 y olds respectively), and the HBSC study incorporated students in 5th, 7th and 9th grades (11y, 13y, and 15y olds, respectively). In the COSI study, there were 153 general education schools (urban n = 81, rural n = 72) and in the HBSC study, there were 110 general education schools (urban n = 54, rural n = 56) in the final sample. All present children from the chosen classes were eligible for participation. Exceptional children outside the age groups, absentees, or those without returned parental forms were excluded from the analyses.

Children who studied in the first grade (n = 422) were excluded from the data set of the COSI study, because the coverage of this grade category does not correspond to the scope of our study. In addition, children aged 8 years in the HBSC study (n = 11) were excluded from the sample of children in the HSBC study as outliers from the HBSC age groups of 11y, 13y and 15y olds.