Lindenauer PK, Grosso LM, Wang C, Wang Y, Krishnan JA, Lee TA, Au DH, Mularski RA, Bernheim SM, Drye EE. Development, validation, and results of a risk-standardized measure of hospital 30-day mortality for patients with exacerbation of chronic obstructive pulmonary disease. J Hosp Med. 2013 Aug; 8(8):428-35.