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 as 2 years before age at Wave 5, the earliest age at which initiation could have occurred. Age at first cannabis use was queried in all waves of data collection. (Reliability of self-reported age at first use could not be estimated in our sample, but moderate test–retest reliability for initiation of alcohol, cigarette and illicit drug use has been reported in large-scale surveys [Johnson and Mott, 2001].)
