Table 4 Assumptions of the sample size calculation

100 participants per school

2 full intervention, 2 partial intervention and 4 control schools

Participants of cohorts F, G, H, and I will be included (Table 1)

A significance level (alpha) of 0.05

A power of 80 %

Independent-samples t-test on difference in absolute BMI Z-scores

An intraclass correlation coefficient of 0.01, as based on Amorim et al. [64]

Unequal cluster sizes with a relative efficiency of 90 % [65]

A dropout rate of 20 % (including both study drop-out and natural drop-out such as migration)

An average absolute BMI Z of 0.76 and an SD of 0.60 in the population (these values were calculated over all children aged between 4 and 11 living in Parkstad region who visited the Youth Health Services in 2013).