a pairwise correlation using a second, and more complicated, method that takes into account the age of onset for smoking among sibling pairs as well as censored values for those who may later begin smoking but have not by the time of the survey. This method uses a multivariate survival model with shared frailty among twin pairs by zygosity (Guo and Rodriguez 1992). The frailty variance is similar to a random intercept in a multilevel model and large estimates are indicative of similarity in the timing of smoking among pairs of twins. By comparing the frailty estimates between MZ and DZ twin pairs, we can infer genetic influence on the timing of a particular behavior (Guo and Tong 2006). This model is specified in equation 1.
