the threshold liability model individuals are affected when P >T, where T is the threshold on the normal distribution which truncates the proportion of affected individuals or disease prevalence (i.e., K), T = Φ−1(1-K), Φ(T) = 1-K, where Φ(T) is the cumulative density function of the normal distribution up to values of T, e.g. if K = 0.05, T = 1.645. The threshold liability of risk scale has much nicer properties than the observed disease scale and provides a framework for comparison of scenarios independent of disease prevalence. The relationship between heritability of liability and the directly estimable parameters of K and λS is (1) [16] with and z the height of the standard normal curve and T 1 = Φ−1(1- λS K), i.e. the threshold T 1<T when λS>1, reflecting that the prevalence amongst sibs of affected individuals, KS is greater than the prevalence in the population as a whole (e.g. if K = 0.05 and λS = 2, z = 0.103, T 1 = 1.282, = 0.371).
