We coded the month and year of changes in state-wide minimum purchase age laws, including ‘grandfathering’ clauses, from published peer-reviewed articles (DuMouchel 1987, O’Malley and Wagenaar 1991, Wagenaar 1981–82), from unpublished tables provided by the Statewide Availability Data System (Ponicki 2004), and from a Lexis-Nexis search of news sources (Associated Press 1996). These sources did not make distinctions between laws preventing adults from furnishing alcohol to persons under 21, or preventing persons under-21 from possessing, purchasing, or consuming alcohol. We made no attempt to capture ‘dry counties’ or other local ordinances that may have been stricter than the state-wide minimum purchase ages. Discrepancies between sources were resolved by the authors, based on internal evidence from these sources (see Table S2); however, alternate coding decisions did not lead to substantial changes in our results. Exposures based on current state of residence could be estimated in both samples, and exposures based on state of birth could be estimated in NLAES. To maximize agreement across coding alternatives, and for simplicity of exposition, exposure status was summarized in a binary variable coded as ‘1’
