All smoking data were self-reported. The assessments of smoking status, number of cigarettes
per day, and years since cessation are described
in Section 1 in the Supplementary Appendix. The
responses in serial follow-up questionnaires were
used to model smoking as a time-dependent variable. In the case of former smokers and persons
who had never smoked, those who did not
complete follow-up questionnaires retained their
smoking status, and the data were not censored.
In the case of current smokers, we censored data
on the date of the second missed questionnaire
to avoid misclassifying those who may have quit
smoking.