2.4. Statistical Analyses
sedentary behaviors
SENSITIVITY
Sample size was estimated according to (i) the CONSORT 2010 statement with extension to randomized pilot trials and (ii) Cohen’s recommendations, which define effect-size bounds as small (ES: 0.2), medium (ES: 0.5) and large (ES: 0.8, “grossly perceptible and therefore large”). With 100 participants (33 and 66 children, unbalanced in favor to GT group), an effect size greater than 0.6 could be obtained for a two-sided type I error at 0.05 and 80% statistical power. As this study was a cluster-randomized trial, the sample size needed to be inflated to take into account between- and within-school variability. For an intra-class correlation coefficient at 0.05 and considering around 25 children by school, at least 218 children in 9 classes were necessary.Statistical analyses were performed using Stata software (version 15; StataCorp, College Station, TX, USA). All tests were two-sided, with an alpha level set at 5%. Categorical data were expressed as number of children and associated percentages, and continuous data as mean ± standard deviation.Longitudinal analyses were performed using linear mixed models for quantitative dependent variables (e.g., grip strength) and generalized linear mixed models with logit link functions for binary dependent variables (e.g., sedentary bias). In these models, the following fixed effects were studied: baseline value of the studied variable, group (CON and GT), time of measurement (T0 and T1) and their interaction (time × group). The schools and children were considered random effects. Sensitivity analyses were performed in order to evaluate the impact of missing data on the results, considering for each variable only children with complete data (at both T0 and T1).In the GT group, Spearman’s correlation coefficients (rho) were calculated between baseline levels of physical activity and sedentary behaviors and the change between T0 and T1 in each of the parameters was studied. The changes were calculated in percentage, except for flexibility (because of zero values), for which a simple difference was calculated (division by zero is not possible). Spearman’s correlation coefficients were then interpreted as follows (absolute value): ≥0.70 (strong correlation), 0.50–0.69 (moderate correlation), 0.30–0.49 (low correlation), 0.00–0.29 (no or negligible correlation).