Data Source

The national estimates presented in this section of Fast Stats are from the HCUP National (Nationwide) Inpatient Sample (NIS). The NIS is based on data from community hospitals, which are defined as short-term, non-Federal, general, and other hospitals, excluding hospital units of other institutions (e.g., prisons). The NIS includes obstetrics and gynecology, otolaryngology, orthopedic, cancer, pediatric, public, and academic medical hospitals. Excluded are community hospitals that are also long-term care facilities such as rehabilitation, psychiatric, and alcoholism and chemical dependency hospitals. Beginning in 2012, long-term acute care hospitals (LTACs) are also excluded from the sampling frame. However, if a patient received long-term care, rehabilitation, or treatment for psychiatric or chemical dependency conditions in a community hospital, the discharge record for that stay will be included in the NIS.

The NIS is sampled from the HCUP State Inpatient Databases (SID). Beginning with the 2012 data year, the NIS is a 20 percent sample of discharges from all community hospitals participating in HCUP in that data year. For data years 1988 through 2011, the NIS was a 20 percent sample of community hospitals and included all discharges within sampled hospitals. The national estimates presented in this section of Fast Stats were developed using the NIS Trend Weight Files for consistent estimates across all data years (e.g., LTACs were removed from analysis using trend weights).

Inpatient Stays

The unit of analysis in the NIS is the hospital discharge (i.e., the inpatient stay), not a person or patient. This means that a person who is admitted to the hospital multiple times in one year will be counted each time as a separate "discharge" from the hospital. Counts are summarized by discharge year. There were no exclusions applied to the data (e.g., transfers to another acute care hospital are included as separate hospital stays).

Age

Age refers to the age of the patient at admission. Discharges missing age are excluded from results reported by age.

Sex

All nonmale, nonfemale responses are set to missing. Discharges with missing values for sex are excluded from results reported by sex.

Expected Payer

The "expected payer" data element in HCUP databases provides information on the type of payer that the hospital expects to be the source of payment for the hospital bill. Information is reported by the following expected primary payers: Medicare, Medicaid, private insurance, and the uninsured. Uninsured discharges include records in which the expected primary payer was self-pay, charity, and no charge. Discharges for other types of payers (e.g., Workers' compensation, Indian Health Service, State and local programs) are not reported. More information on expected payer coding in HCUP data is available in HCUP Methods Series Report #2014-03, "An Examination of Expected Payer Coding in the HCUP Databases" (multiple documents). Discharges missing expected payer are excluded from results reported by expected payer.

Community-Level Income

Community-level income is based on the median household income of the patient's ZIP Code of residence, with quartiles defined using the U.S. population. Over time, the data element in the NIS for community-level income has changed definitions. Starting in data year 2003, the cut-offs for the quartile designation are determined annually using ZIP Code demographic data obtained from the Nielsen Company, a vendor that compiles and adds value to data from the U.S. Bureau of Census. Nielsen uses intercensal methods to estimate annual household and demographic statistics for geographic areas. The value ranges for the national income quartiles vary by year. Information by community-level income is only reported from 2003 forward because of inconsistent definitions over time in the income-related data elements in the NIS. Income quartile is missing if the patient is homeless or foreign. Discharges missing the income quartile are excluded from results reported by community-level income.

Hospitalization Type

Coding criteria for the six hospitalization types are based on International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis codes, Clinical Classifications Software (CCS) categories, and diagnosis-related groups (DRGs). There are approximately 14,000 ICD-9-CM diagnosis codes. The Clinical Classifications Software (CCS) categorizes ICD-9-CM diagnosis codes into a manageable number of clinically meaningful categories. This clinical grouper makes it easier to quickly understand patterns of diagnoses. DRGs group patients according to diagnosis, type of treatment (procedure), age, and other relevant criteria. Each hospital stay has one assigned DRG.

Each discharge was assigned to a single hospitalization type hierarchically, based on the following order: maternal, neonatal, mental health, injury, surgical, and medical. All discharges are categorized in one of the six mutually exclusive types of service lines.

Actual Cost per Stay

The NIS includes information on total hospital charges for an inpatient stay. Charges represent the amount a hospital billed for the entire hospital stay, excluding professional (physician) fees. Total hospital charges are converted to costs using HCUP Cost-to-Charge Ratios (CCRs) based on hospital accounting reports from the Centers for Medicare & Medicaid Services (CMS). Costs reflect the actual expenses incurred in the production of hospital services, such as wages, supplies, and utility costs. For each hospital in the NIS, a hospital-wide cost-to-charge ratio is used. The average cost per stay is calculated using discharges with nonmissing total costs. Costs are not imputed if total charges are not reported on the discharge record. Costs are only reported from 2000 forward because HCUP Cost-to-Charge Ratios are unavailable prior to 2000.

Inflation-Adjusted Cost per Stay

The actual average cost per stay is inflation adjusted using price indexes for the Gross Domestic Product (GDP) from the U.S. Department of Commerce Bureau of Economic Analysis (BEA) National Income and Product Accounts (NIPA) (Section 1, Table 1.1.4. Price Indexes for Gross Domestic Product). Annual values starting in 1994 for the price indexes were obtained on June 23, 2015. The adjustment used 2010 as the index base so that updates to the trends could retain a consistent base.

Length of Stay

The length of stay (LOS) is the number of days that the patient stayed in the hospital. It is calculated by subtracting the admission date from the discharge date. Same-day stays are therefore coded with a length of stay of 0. The average LOS is calculated using discharges with nonmissing LOS.

In-Hospital Mortality

In-hospital mortality is determined by the discharge disposition of the patient from the hospital. The numerator of the mortality rate is the number of patients within a reporting category (e.g., within a specific diagnosis category) who died in the hospital. The denominator is based on the total number of discharges in the reporting category. Discharges missing discharge disposition are excluded from the numerator and denominator of the in-hospital mortality rate.