“…The univariate mixture of regression model naturally arises from the presence of omitted or unmeasured binary or categorical predictors. To illustrate this and following Samoilenko et al, suppose that birthweight ( Y ) is a function of fetal sex ( S ) and a given genotype ( G , with 0/1 coding) such as Model can alternatively be written as where and . If G is unmeasured, then model is a mixture of two univariate regressions, and assuming that genotype G and fetal sex are independent, the corresponding mixing proportions are given by the prevalence of the levels of G (eg, π 1 = P ( G = 0) and π 2 = P ( G = 1)).…”