The SSAGA did not assess smoking related measures, but information on smoking-related phenotypes were available from a Health and Lifestyle Survey mailed questionnaire that the twins responded to in 1989 (Whitfield et al., 2000). Two smoking related measures were coded: (d) Current smoking: a dichotomous item reflecting whether the participant was a current smoker at the time of questionnaire assessment, with never smokers set to missing;(e) Pack years of cigarettes smoked: a quantitative measure calculated as: Pack years = (cigarettes smoked on an average day x total years smoked) / 20; the number of cigarettes smoked on an average day was calculated from a categorical variable ranging from 1 (never smoked) to 6 (smoked 40+ cigarettes per day).
