Methods
Sample and Procedure
Using convenience sampling, we recruited 273 sixth-graders (125 boys and 148 girls) aged 11 to 13 years from 13 classes of public elementary schools located in large urban areas: fi ve schools in Prague, one in Česk้ Budějovice, and one in Liberec. The urban population in the Czech Republic covers a wide range of social classes, but is relatively homogeneous, with the middle class prevailing. The sample composition by gender and classroom is detailed in Table 1.
Table 1. Sample composition by gender and classroom