Smoking status was assessed at baseline in each cohort and was updated on subsequent biennial questionnaires. The initial cohort questionnaires also asked for age at which regular smoking began, age at quitting, and the usual number of cigarettes smoked per day. The number of cigarettes smoked per day was reported every 2 years by category (1–4, 5–14, 15–24, 25–34, 35–44, ≥45). For smoking status and CPD, the last value was carried forward to replace missing values. Duration of smoking and years since quitting were derived based on information from the initial and subsequent questionnaires and updated during each follow-up cycle, and, therefore, their accuracy is within 2 years. Duration of smoking was calculated as the difference between age at smoking initiation and current age for current smokers, or between age at onset and cessation for former smokers. Years since cessation were obtained for former smokers by deducting the age of quitting smoking from current age. Ages at smoking initiation and cessation were continuous values in the NHS cohort but were collected categorically in NHS II (<15, 15–19, 20–24, 25–29, 30–35 years) and HPFS (<15, 15–19, 20–29, 30–39, 40–49, 50–59, ≥60 years).
