We defined COPD cases as participants who self-reported a diagnosis of emphysema (on PLCO baseline questionnaire, follow-up questionnaire in 2006, or both), chronic bronchitis (on PLCO baseline questionnaire), or both. The PLCO baseline questionnaire queried specifically about a doctor’s diagnosis of emphysema and chronic bronchitis, whereas the follow-up questionnaire did not query about chronic bronchitis and did not specify whether the diagnosis was by a doctor. References to baseline and follow-up in the manuscript refer to the larger PLCO study design [17, 18]. Covariates were based on data collected at the time of the TTFC query on the follow-up questionnaire.
