woman as a former smoker and the last questionnaire that identified her as a smoker. Prior to 1982, current or former smokers were asked to write in the average number of cigarettes they smoked per day; subsequent questionnaires captured information about smoking intensity in categories (1–4, 5–14, 15–24, 25–34, 35–44, and over 44 cigarettes per day). Pack years was estimated as the sum of the products of smoking intensity (categories were assigned midpoint values, e.g. 5–14 was coded as 10 cigarettes per day, or 0.5 packs per day) at questionnaire k times the interval between questionnaire k and questionnaire k+1 (or half that interval for women who were smokers at questionnaire k and non-smokers at questionnaire k+1). Smoking duration was calculated as the sum of all intervals where a woman was a smoker. Average cigarettes per day (the CPD variable used in this GWAS) was calculated as pack-years divided by smoking duration.
