The Veterans Health Administration (VHA) is organized into 21 administrative regions called Veterans Integrated Service Networks (VISNs). VISN 16, or the South Central Veterans Affairs (VA) Health Care Network, provides health-care treatment to veterans in an eight-state region that includes Florida, Alabama, Mississippi, Louisiana, Arkansas, Missouri, Oklahoma, and Texas. The network, an integrated health-care system, includes 10 medical centers, 33 community-based outpatient clinics, 7 nursing homes, and 2 domiciliaries. It is one of the largest of the 21 VISNs of the VHA. Geographically, the network spans approximately 170,000 square miles and includes > 1.9 million veterans. The Veterans Health Information Systems and Technology Architecture (Vista)14is the integrated electronic medical record system for the VHA that contains, among other information, records of inpatient stays and outpatient visits, diagnostic codes, current procedural terminology codes, pharmacy records, and laboratory test results.15 All this information, except the content of narrative progress notes, can readily be tabulated by means of automated queries. Prior to 1995 the various facilities maintained their Vista databases separately. At that time, VISN 16 initiated a VISN-wide information system consolidation by standardizing the data from all the facilities in VISN 16. It contained the data since October 1, 1996. The VISN 16 data warehouse is an enriched administrative database and contains prospectively collected data of the patients treated in VISN 16. The computerized records contain both clinical and administrative factors including all patient encounters, pharmacy, laboratory, vital signs, financial, and patient demographic data. In addition, several health-care factors such as smoking and alcohol intake are also recorded. All the diagnoses are coded according to International Classification of Diseases (ninth version, clinical modification). There are 750 million rows of data arranged in relational tables that can be accessed by the Microsoft SQL server (Microsoft; Redmond, WA). The data from each facility are extracted monthly from the Vista, and tables are populated after standardization of the data from all the facilities in the VISN. Access to this data is monitored and controlled. The study protocol, including access and use of the computerized patient clinical information, was authorized by the VISN 16 multisite institutional review board. The informed consent requirement was waived for the study.