In addition to case-control status, variables used in these analyses included age at first full cigarette (AFC), age at onset of regular smoking (AOS), age at first full drink (AFD) and age at onset of regular drinking (AOD). All of these variables were retrospectively assessed as single items. Regular smoking was defined as having smoked at least once a week for a period of at least two months. Regular drinking was defined as drinking at least once a month for a period of 6 months or more. Individuals who had never initiated regular drinking were excluded from the AOD analysis. To reduce the influence of extreme values on age at onset variables in regression estimates, AFC, AOS, AFD and AOD were subjected to a “Winsorization” procedure (Tukey 1960). Winsorization involves selecting an equal number of observations at both tails of a distribution, and reassigning their values to be identical to the next-highest or next-lowest value in the distribution. For example, subjects reporting AOS <8 (n=7) were assigned an AOS value of 8, those with AOS over 30 (n=13) were assigned
