Patient age was grouped by quartiles. Patient race was defined as white, black, Hispanic, Asian, American Indian, or other. Patient income was characterized in this data set by the median income within the patient's residence ZIP code area (<$25 000, $25 001-$30 000, $30 001-$35 000, and >$35 000). Patient discharge disposition was categorized as routine (discharged home); or requiring short-term hospitalization, skilled nursing, intermediate health care facility, another type of health care facility (hospice, rehabilitation center, psychiatric facility), or home health care; discharged against medical advice; or death. Categories for patient insurance included Medicare, Medicaid, private insurance including health maintenance organizations (HMOs), self-pay, no charge, or other (Title V, worker's compensation, and other government sources). Admission source was defined as routine (referrals from physicians, clinics, and HMOs), from the emergency department, from a long-term care facility, transfer from another hospital, or secondary to a court order. Procedure codes 967, 967.1, and 967.2 were used to identify patients having received mechanical ventilation during their hospitalization.