Figure 1. Prevalence of Chronic obstructive lung disease (COPD) by Global Initiative for Chronic Obstructive Lung Disease (GOLD) criteria (black bars) based on Forced expiratory volume in one second (FEV₁)/Forced vital capacity (FVC)<0.7, and by FEV₁/FEV₆<Lower limit of normal (LNN) (<5th percentile for FEV₁/FEV₆, gray bars) in the three cities during baseline and follow-up.

In addition, mean Forced expiratory time (FET) is plotted (right scale, empty circles) for the post-Bronchodilator tests (post-BD). Prevalence of COPD by FEV₁/FVC criteria (GOLD) varies directly with Forced expiratory time (FET), unlike prevalence based on FEV₁/FEV₆. Prevalences based on a FEV₁/FVC<LLN also depend to a great extent on FET (not shown).