Sample size requirements to detect the effect of preventive intervention. On the y axis is the sample size required to detect an effect of intervention as statistically significant with a power of 80%. On the x axis is the hazard ratio reflecting how much an intervention reduces the risk of severe mental illness. The two plotted lines demonstrate the dependence of sample size requirements on the risk of developing severe mental illness in the absence of an intervention (base rate). In a high-risk population of offspring of parents with severe mental illness (base rate 32%), the sample size required is 2.5 times lower than in the general population (base rate 13%).