Differences between boys and girls, as well as between participants with and without missing values, were tested for their statistical significance. Depending on scale level and distribution of the data, the Mann–Whitney-U test or t-test for continuous data and Fisher’s exact test for categorical data were applied. The significance level was set at α = 0.05 for two-sided tests. These analyses were carried out using the statistical software packages IBM SPSS Release 21.0 for Windows (SPSSInc, Chicago, IL, USA).