We investigated several pre-specified subgroup analyses, including stratification by gender and race. Additionally, in separate models we considered the association between TTFC and chronic bronchitis, as well as TTFC and emphysema. We also evaluated the effect of adjusting models of emphysema for chronic bronchitis, and adjusting models of chronic bronchitis for emphysema. Lastly, since emphysema was reported at both PLCO baseline and PLCO follow-up, we further examined the concordance of these reports; we also conducted a sensitivity analyses by comparing the results using emphysema reported at baseline and at follow-up.
