Mapel, D. W., and M. Roberts. “Spirometry, the St. George’s Respiratory Questionnaire, and Other Clinical Measures As Predictors of Medical Costs and COPD Exacerbation Events in a Prospective Cohort”. The Southwest Respiratory and Critical Care Chronicles, Vol. 3, no. 10, Apr. 2015, pp. 1-9, https://pulmonarychronicles.com/index.php/pulmonarychronicles/article/view/196.