Past year alcohol use was assessed at each wave of data collection via the Nicotine, Alcohol, and Drug Substance Use measure [31], which queries frequency and usual quantity of consumption over the past year. There were 8 response options for frequency, starting with “I didn’t within the past year,” “less than 5 times in the past year,” and “more than 5 times in the past year but less than once a month” and 7 response options for quantity, starting with “a sip or a taste,” “less than one bottle/glass/shot,” and “1–2 bottles/glasses/shots.” As we were interested in capturing use beyond a few sips, we created a proxy for age at first full drink: defined as the age at which respondents first reported consuming alcohol in any quantity 5 or more times in the past year, or the age at which respondents first reported consuming 1–2 drinks per occasion at any frequency in the past year.
