All of these behaviors were measured by baseline questionnaire (BQ) in the PLCO (administered from 1994–2001) [55]. Age at initiation was defined as the age when a subject started smoking “regularly for six months or longer.” Former smokers were defined as ever-smokers who did not smoke regularly at BQ and were asked to report the age at which they stopped smoking regularly. Ever smokers were asked to provide information on the number of cigarettes they smoked per day, in categories (1–10, 11–20, 21–30, 31–40, 41–60, 61–80, over 80). For continuous analyses we assigned subjects to the midpoint of their category (or 90 cigarettes per day for over 80). Duration was derived from data on age at smoking initiation and age at cessation. Pack years was derived by converting cigarettes per day to packs per day (CPD/20) and multiplying this figure by duration.
