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Deaths from any cause were identified and confirmed for the cohort using multiple concurrent approaches including follow-up interviews, written annual questionnaires or telephone contacts with study participants or next-of-kin, surveillance of local hospital records and community obituaries, and linkage with the Social Security Administration Death Master File. (For more information, see Punjabi et al. 2009 (PubMed ID: 19688045))

This variable is the ratio of forced expiratory volume in one minute (FEV1 ) to forced vital capacity (FVC) expressed as a percentage.