Sample size and power
A sample size calculation using G-power 3.1 was conducted for the primary outcome measure, defined as the mean difference in the quality of life between two groups at three time points (baseline, 12 weeks intervention, 26 weeks follow-up). We used historical data on the Kidscreen-27 (a measure of QoL) from our own population to estimate the effect size. To detect a median effect size (Cohen’s f = 0.39) with a two-sided significance level of 5% and power of 80% with equal allocation to two arms would require 66 patients in the trial. To allow for 20% drop-out, 80 children are recruited in total.