Statistical analysis and sample size calculation The study is powered to detect a change considered to be both significant clinically and plausible on the primary outcome. A sample size of N = 720 families will allow measuring a reduction of 15.2% in parent-reported use of violent discipline at home from the initial estimated prevalence of 30% (SKTA 2013) with an α level of 0.05 and 90% power, assuming 0.02 intra-class correlation and 80% participation rate. While each family will likely have more than one child, the outcome concordance across children is likely to be very high (>0.8) and so the sample size calculation considers the number of families, recognizing that the unit of analysis will be the child.