In the NHS, SMKAGE was measured at BQ; all other behaviors were measured using cumulative information from the BQ (administered in 1976) and subsequent follow-up questionnaires (one every two years). The majority of NHS subjects (2149) had smoking data available up through the 2002 questionnaire. For those few women (133) with no smoking data available from the 2002 follow-up cycle, we used data from the latest available follow-up. Age at initiation was defined as the age when a subject started smoking cigarettes “regularly.” Former smokers were defined as ever-smokers who identified themselves as non-smokers on any questionnaire (and did not identify as a smoker on any subsequent questionnaire). Age at cessation was explicitly asked in NHS BQ. For women who quit smoking after the BQ, age at cessation was inferred as the median age between the questionnaire that defined the 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
