Exposure opportunity was assessed using procedures developed for earlier waves of the National Household Survey and discussed by Anthony and colleagues (Wagner & Anthony, 2002; Wilcox et al., 2002). Such questioning would precede questioning on drug use (including licit drugs) and would involve respondents reporting the age (if any) at which they first had access to each drug class (including cannabis, other illicit drugs, and the licit drugs tobacco and alcohol), irrespective of whether they used it at that occasion or not.
