Variation of height and BMI within school classes in 14-year-old children

The aim of the study was to analyze the variation of height and weight of 14-year-old children within classes of primary schools in Wroclaw. All children were attending the 7th grade of primary schools in Wroclaw, and underwent a medical examination as part of a project (the Health Card of the Child) of the city of Wroclaw. The data were collected between January and December of 1997. The present study included height and weight of 1810 children (917 boys and 893 girls). For analyses, individual height and BMI [kg m^-2] were standardized based on mean and SD of the whole population separately for boys and girls. The SDs of height and BMI in each class were tested by Student-t test for one sample to check the difference in comparison to population with SD equal 1. Additionally, the nested analysis of variance was used, where height and BMI were dependent variables, and classes were nested into schools. SDs of height and BMI within classes are significantly lower than for the whole population in boys and girls, whereas the means did not differ significantly from the whole population. Nested analysis of variance showed a significant effect of class for girls (for height and BMI) but not for boys. Also variation of height significantly differed between schools in girls but not in boys. It is hypothesized that decrease of variation within classes might be caused by the community effect.