Sample size and study power
As for any GAN centre, all students (selected by grade or by age) of 13-14 years within all schools (Ferizaj, Gjakova, Gjilan, Peja, and Prizren) or within a random sample of them (Prishtina) were invited to participate to achieve the minimum required sample size of 1000. Additional details of the sample size and power are described elsewhere12,14 Participation rates in Kosovo (80.0% or more) have been also described in detail previously15.