All survival analyses were conducted using STATA version 8.2 (StataCorp, 2005), with the Huber-White robust variance estimator to compute standard errors and confidence intervals adjusted for non-independence (i.e., the correlated nature) of data on twin pairs. To examine potential violation of the proportional hazards assumption, such as might be the case if AD effects on reproductive onset differ for earlier versus later age periods, the Grambsch and Therneau test of Schoenfeld residuals (Grambsch & Therneau 1994) was employed. Where appropriate, age-interactions were computed and modeled to correct proportional hazards violations (Cleves, Gould, & Gutierrez, 2004).
