time-varying covariates coded as negative in each year prior to the age at first occurrence and positive for that year and each subsequent year, so that only behaviors that occurred prior to or at the same time as the outcomes counted towards the transition. Analyses were conducted in Stata (StataCorp, 2007) using Huber-White robust standard errors to adjust for the non-independence of observations in twins. Violations of the PH assumption were resolved by estimating piecewise hazard ratios (HRs) for each age range of ≤14, 15–16, and ≥17. AUD symptom onset and AUD diagnosis analyses also included early alcohol use status as a dummy variable as a covariate.
