We cannot exclude the possibility of selection bias in our study. Our study included current smokers in PLCO who were alive, free of lung cancer and completed the follow-up questionnaire which included TTFC. Thus, some highly addicted smokers with the shortest TTFC would have been excluded, biasing findings toward the null. Conversely, individuals who persist in smoking following a diagnosis of COPD are likely more addicted compared to individuals who quit post-diagnosis [19, 30]; current smokers who report COPD at the follow-up questionnaire may be enriched with individuals with shorter TTFC, and we may therefore overestimate the association between TTFC and COPD. These two biases are in opposite directions and the overall effect is likely to be of small magnitude; our risk estimates should approximate the true association. Evidence from studies of other smoking-related diseases suggest that former smokers will exhibit a similar pattern of disease risk in relation to TTFC, as we recently reported for lung cancer [9].
