Age at first cannabis use was defined as the participant’s age the first time she used cannabis. Cannabis use (one or more times over the lifetime) was reported by 58.2% of the sample. Age at first use of a substance was derived from the wave of assessment in which use of that substance was first reported, toward the end of minimizing potential retrospective reporting bias. In Wave 5, respondents were not specifically asked the age at which they first drank alcohol or the age at which they first smoked a cigarette, but they were asked about age at last use. In the small number of cases in which first use of alcohol or cigarettes was reported for the first time at Wave 5 (8.7% and 1.3%, respectively, of those who ever used), age at first use was estimated in the following way: For those individuals who reported in Wave 5 that they had used alcohol or cigarettes in the previous 24 months but explicitly stated in Wave 4 interviews that they had never used that substance, first use was estimated
