1. Association AL. How serious is COPD. http://www.lung.org/lung-health-and-diseases/lung-disease-lookup/copd/learn-about-copd/how-serious-is-copd.html?referrer=https://www.google.co.uk/ [Accessed February 2018].
2. The state of US health, 1990-2010: burden of diseases, injuries, and risk factors;Murray;Jama,2013
3. The clinical and economic burden of chronic obstructive pulmonary disease in the USA;Guarascio;Clin. Outcomes Res.: CEOR,2013
4. From the Global Strategy for the Diagnosis, Management and Prevention of COPD, Global Initiative for Chronic Obstructive Lung Disease (GOLD);GOLD,2018
5. Once-daily single-inhaler triple versus dual therapy in patients with COPD;Lipson;N. Engl. J. Med.,2018