Lash TL, Johansen MB, Christensen S, Baron JA, Rothman KJ, Hansen JG, Sorensen HT. Hospitalization rates and survival associated with COPD: a nationwide Danish cohort study. Lung. 2011 Feb 1;189(1):27-35.
Perez-Gutthann S, Petri M, Hochberg MC. Comparison of two methods of classifying patients with systemic lupus erythematosus. J Rheumatol. 1991 Aug 1;18(8):1176-9.