For each wave, participants reported frequency of marijuana use and
cigarette use (number of days they used each substance) in the past
month. Marijuana use frequency was measured using a single item
question with 5 ordinal options coded as: 0 = zero days; 1 = one day
a month or less; 3 = more than one day a month but less than one
day a week; 9 = more than one day a week but less than daily; 27 =
every day. Cigarette use frequency was measured using a single item
question with 9 ordinal options coded as: 0 = zero days; 1 = one day;
3=2–3 days; 5 = 4–5 days; 7 = 6–7 days; 9=8–10 days; 16= 11–20 days; 25 = 21–29 days; 30 = every day. Alcohol use frequency is
included in supplement for comparison purposes (S.1 Figure) and
relationships between alcohol use frequency and cigarette use and
marijuana use frequency are shown in S.4 Table.