The cross-sectional study has several study groups based on the exposure (smoking behavior) status. Combinations of paired associations are as follows: smokers vs. never-smokers; smokers vs. ex-smokers; never-smokers vs. ex-smokers. Therefore, it bears some feature of a retrospective cohort study, specifically; it provides an opportunity to make comparisons between the groups. While calculating the required sample size, the investigators considered this opportunity as the most important one. In addition to calculating prevalence ratio of COPD for exposure statuses, the cross-sectional study will allow for assessment of prevalence of these conditions and other characteristics in different study groups.