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Overview of the Nationwide Inpatient Sample (NIS)
The HCUP Nationwide Inpatient Sample (NIS) is a set of longitudinal hospital inpatient databases included in the HCUP family. These databases are created by AHRQ through a Federal-State-Industry partnership.
 
 
Nationwide Inpatient Sample

The Nationwide Inpatient Sample (NIS) is part of a family of databases and software tools developed for the Healthcare Cost and Utilization Project (HCUP). The NIS is the largest all-payer inpatient health care database in the United States, yielding national estimates of hospital inpatient stays. Unweighted, it contains data from approximately 8 million hospital stays each year. Weighted, it estimates roughly 40 million hospitalizations.

Developed through a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality (AHRQ), HCUP data inform decisionmaking at the national, State, and community levels.

This page provides an overview of the NIS. For more details, see NIS Database Documentation and the Introduction to the NIS, 2011 (PDF file, 1.3 MB; HTML).

Contents:

Sampled from the State Inpatient Databases (SID), HCUP's NIS contains all discharge data from more than 1,000 short-term and non-Federal hospitals each year, which approximates a 20 percent stratified sample of U.S. community hospitals. The NIS contains charge information on all patients, including individuals covered by Medicare, Medicaid, or private insurance, as well as those who are uninsured.

The NIS can be used to identify, track, and analyze national trends in health care utilization, access, charges, quality, and outcomes.

NIS data are available from 1988 through 2011, which allows analysis of trends over time. The number of States in the NIS has grown from 8 in the first year to 46 at present.

Key features of the most recent NIS database year (2011) include:
  • A sampling frame of hospitals that comprises approximately 97 percent of all hospital discharges in the United States
  • A large sample size, which enables analyses of rare conditions, such as congenital anomalies; uncommon treatments, such as organ transplantation; and special patient populations, such as the uninsured
  • Hospital identifiers for many States that permit linkages to the American Hospital Association (AHA) Annual Survey Database (Health Forum, LLC © 2012) and county identifiers that permit linkages to the Area Resource File.
Free HCUP Tools & Software are also available to identify preventable hospitalizations, estimate costs, assess quality of care and patient safety, categorize diagnoses and procedures, and identify comorbidities.

Information on previous years of the NIS may be found in the Introduction to the NIS, 2011 (PDF file, 1.3 MB; HTML).
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The NIS contains clinical and resource-use information that is included in a typical discharge abstract, with safeguards to protect the privacy of individual patients, physicians, and hospitals (as required by data sources). It contains more than 100 clinical and nonclinical data elements for each hospital stay, including:
  • Primary and secondary diagnoses and procedures
  • Admission and discharge status
  • Patient demographic characteristics (e.g., sex, age, race, median household income for ZIP Code)
  • Hospital characteristics (e.g., ownership, size, teaching status)
  • Expected payment source
  • Total charges
  • Discharge status
  • Length of stay
  • Severity and comorbidity measures
Elements included in the NIS are not always available for all States, including the American Hospital Association (AHA) hospital identifier. Please see the AHA Hospital Identifier Map for participation.
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As a uniform, multi-State database, the NIS promote comparative studies of health care services and supports health care policy research on a variety of topics, including:
  • Utilization of health services by special populations
  • Hospital stays for rare conditions
  • Variations in medical practice
  • Health care cost inflation
  • Regional and national analyses
  • Quality of care and patient safety
  • Impact of health policy changes
  • Access to care
The NIS is used in a variety of publications:

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Spanning more than 20 years of data, the NIS is ideal for longitudinal analyses. However, the database has undergone changes over time, including the sampling and weighting strategy used. For analyses that include data years before 2003, the NIS Trends Supplemental Files (NIS-Trends) are available for data years 1988 through 2002. These files contain revised trend weights and data elements that are consistently defined across data years. Trend analyses limited to data years 2003 and later do not need the NIS-Trends.
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NIS releases for data years 1988 through 2011 are available for purchase through the HCUP Central Distributor.

Prior to purchasing HCUP data, all individuals are required to take the online HCUP Data Use Agreement Training Course, and users of the NIS must read and sign the Data Use Agreement for Nationwide Databases (PDF file, 50 KB; HTML).

Instructions for purchasing the NIS are available in the Nationwide Inpatient Sample (NIS) Application Kit (PDF file, 131 KB).

Questions about purchasing databases can be directed to the HCUP Central Distributor:

E-mail: HCUPDistributor@AHRQ.gov
Telephone: (866) 556-4287 (toll free)
Fax: (866) 792-5313 (toll free)

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The NIS is distributed as fixed-width ASCII formatted data files compressed with SecureZIP® from PKWARE. To load and analyze NIS data on a computer, you will need the following:
  • A DVD drive
  • A hard drive with 15 gigabytes of space available
  • A third-party zip utility such as ZIP Reader, 7-Zip, or WinZip®
  • SAS®, SPSS®, Stata® or similar analysis software
The data set includes weights for producing national and regional estimates. NIS documentation and tools, including programs for loading the ASCII file into SAS, SPSS, or Stata, are also available on the NIS Database Documentation page.

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Internet Citation: HCUP Databases. Healthcare Cost and Utilization Project (HCUP). December 2013. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov//nisoverview.jsp.
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Last modified 12/11/13