Covariates included age, gender, race, and comorbidity. Comorbidity was coded as individual indicator variables for ICD-9 diagnoses corresponding to AIDS, COPD, dementia, hypertension, diabetes mellitus, liver disease, peripheral vascular disease, rheumatic disease, and peptic ulcer disease. These comorbidities were chosen based on their inclusion in a clinical comorbidity index (Deyo, Cherkin and Ciol 1992). Smoking behavior, educational attainment, and employment status were not available in this database.
