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Methods
Data
Our data came from a series of cross-sectional, retrospective analyses of the National Inpatient Sample (NIS) between 2005 and 2009. The NIS is the largest all-payer database for hospitalizations, accounting for over 8 million hospital discharges. Each year the NIS approximates a 20% stratified sample of short-term, nonfederal and nonrehabilitation hospitals, which represents approximately 1050 hospitals located in 37 (in 2005) to 44 (in 2009) states, and accounts for 95% of discharges. The NIS is part of the Healthcare Cost and Utilization Project (HCUP) and supported by the Agency for Healthcare Research and Quality (AHRQ).
We also used the National Hospital Discharge Survey (NHDS) for comparative analysis. The NHDS data cover an 8% sample of short hospital stays (hospitals with an average length of stay for patients of less than 30 days), which is approximately 270 000 inpatients per year throughout the 50 states.
Diagnoses and procedures were identified by the International Classification of Diseases (ICD-9) published by the World Health Organization (WHO). Diagnoses during hospital stays (from 1 to 15) were categorized into two levels: (i) the diagnosis chiefly responsible for the patient's admission to the hospital and established retrospectively and (ii) any listed conditions that coexisted at the time of admission or that developed during the stay.
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