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NIS Overview
The National (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.

Overview of the National (Nationwide) Inpatient Sample (NIS)

The National (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 publicly available all-payer inpatient health care database in the United States, yielding national estimates of hospital inpatient stays. Unweighted, it contains data from more than 7 million hospital stays each year. Weighted, it estimates more than 35 million hospitalizations nationally. 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.

The 2016 NIS CONTAINS ICD-10-CM/PCS CODES
The 2016 NIS includes a full calendar year of data with diagnosis and procedure codes reported using the ICD-10-CM/PCS coding system. Data elements derived from AHRQ software tools (e.g., Clinical Classification Software (CCS) and the Elixhauser Comorbidity Software) are not available in the 2016 NIS.

THE 2015 NIS CONTAINS ICD-9-CM AND ICD-10-CM/PCS CODES
On October 1, 2015, hospital administrative data in the United States began using ICD-10-CM/PCS, so the first nine months of 2015 contain ICD-9-CM codes and the last three months contain ICD-10-CM/PCS codes. Trends based on diagnoses or procedures will be affected. Data elements and data structure for the 2015 NIS were changed.

THE NIS WAS REDESIGNED BEGINNING WITH 2012
Starting in data year 2012, the NIS is a sample of discharges from all hospitals participating in HCUP. For prior years, the NIS was a sample of hospitals. For details, see the 2012 NIS Redesign Report.



  • The 2016 NIS includes a full calendar year of data with diagnosis and procedure codes are reported using the ICD-10-CM/PCS1 coding system (in place of the ICD-9-CM coding system)
  • Data elements derived from AHRQ software tools (e.g., Clinical Classification Software (CCS) and the Elixhauser Comorbidity Software) are not available in the 2016 NIS because the ICD-10-CM/PCS versions are still under development. For users interested in applying the AHRQ software tools to the ICD-10-CM/PCS data in the 2016 NIS, beta versions of the AHRQ software tools are available for download on the HCUP Tools & Software section of the HCUP-US Web site. A tutorial is available to users interested in applying the AHRQ software tools to the 2016 NIS at www.hcup-us.ahrq.gov/tech_assist/tutorials.jsp.
  • The Inpatient Core File is a single file containing commonly used data elements (e.g. age, expected primary payer, ICD-10-CM/PCS codes, total charges). The difference in 2016 is that it does not contain data elements derived from ICD-10-CM/PCS codes such as the Clinical Classifications Software (CCS) for diagnoses and procedures.
  • The Disease Severity Measures File is a single file containing additional data elements to aid in identifying the severity of the condition for a specific discharge. The difference in 2016 is that it does not contain the Elixhauser Comorbidity Software indicators.
  • The Diagnosis and Procedure Groups File which historically contains data elements derived from the AHRQ software tools is not available with the 2016 NIS.
The HCUP-US Web site has a section on ICD-10-CM/PCS Resources that summarizes key issues for researchers using HCUP and other administrative databases that include ICD-9-CM and ICD-10-CM/PCS coding. The Web page provides general guidance and forewarning to users analyzing outcomes that may be affected by the transition to the ICD-10-CM/PCS coding system and lists other related Web resources.

1 ICD-9-CM: International Classification of Diseases, Ninth Revision, Clinical Modification; ICD-10-CM/PCS: International Classification of Diseases, Tenth Revision, Clinical Modification/Procedure Coding System
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Beginning with the 2012 data year, the NIS approximates a 20-percent stratified sample of all discharges from U.S. community hospitals, excluding rehabilitation and long-term acute care hospitals. The NIS contains information on all patients, regardless of payer, including individuals covered by Medicare, Medicaid, or private insurance, uninsured. The NIS is sampled from the State Inpatient Databases (SID), which include all inpatient data that are currently contributed to HCUP.

Researchers and policymakers use the NIS to make national estimates of health care utilization, access, charges, quality, and outcomes. NIS data are available from 1988 through 2016, which allows analysis of trends over time. The number of States participating in the NIS has grown from 8 in the first year to 46, plus the District of Columbia, at present.

Beginning with 2012 data, the NIS was redesigned to improve national estimates. To highlight the design change, beginning with 2012 data, AHRQ renamed the NIS from the "Nationwide Inpatient Sample" to the "National Inpatient Sample." The redesign incorporates three major types of changes:

  • Revisions to the sample design - the NIS is now a sample of discharge records from all HCUP-participating hospitals, rather than a sample of hospitals from which all discharges were retained.
  • Revisions to how hospitals are defined - the NIS now uses the definitions of hospitals and discharges supplied by the statewide data organizations that contribute to HCUP, rather than the definitions used by the AHA Annual Survey.
  • Revisions to enhance confidentiality - the NIS now eliminates State and hospital identifiers and other data elements that are not uniformly available across States.
The new sampling strategy produces more precise estimates than the previous NIS design by reducing sampling error. For many estimates, confidence intervals under the new design are about half the length of confidence intervals under the previous design.

Key features of the most recent NIS (2016) include:

  • The NIS is drawn from all States participating in HCUP, representing more than 97 percent of the U.S. population.
  • The NIS approximates a 20-percent stratified sample of discharges from U.S. community hospitals, excluding rehabilitation and long-term acute care hospitals.
  • The self-weighting design of the new NIS reduces the margin of error for estimates and delivers more stable and precise estimates than previous versions of the NIS.
  • The NIS protects patient confidentiality because State and hospital identifiers are no longer provided.
  • The new NIS retains a large sample size, which enables analyses of rare conditions, uncommon treatments, and special patient populations.
The NIS was redesigned to improve national estimates. Changes to the NIS may impact some types of analyses. For example, the elimination of hospital identifiers means that hospital linkages can no longer be done with the NIS and the sampling of discharges means that analyses relying on a census of discharges from sampled hospitals (e.g. hospital volume analysis) can no longer be performed. Because inpatient data are available for many individual States through the HCUP Central Distributor, state inpatient data can be used for those analyses that require a census of discharges from individual hospitals, local market areas, and States.

For a detailed description of the 2012 NIS redesign, please see the 2012 NIS Redesign Report.

For more details on the 2016 NIS, see the Introduction to the NIS, 2016 (PDF file, 658 KB; HTML).

Information on previous years of the NIS may be found in prior years of the Introduction to the NIS at https://www.hcup-us.ahrq.gov/db/nation/nis/nisarchive.jsp.
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The 2016 NIS is a calendar year file based on discharge date that includes diagnoses and procedure coded using only ICD-10-CM/PCS. The file structure of the 2016 NIS is similar to the file structure of the NIS prior to 2015 with one exception, data elements derived from AHRQ Software Tools are not available in the 2016 NIS because the ICD-10-CM/PCS versions are still under development. For users interested in applying the AHRQ software tools to the ICD-10-CM/PCS data in the 2016 NIS, beta versions of the AHRQ software tools are available for download on the HCUP Tools & Software section of the HCUP User Support (HCUP-US) Web site. A tutorial is available to users interested in applying the AHRQ software tools to the 2016 NIS as part of the HCUP Online Tutorial Series.

For the 2015 data, because of the transition to ICD-10-CM/PCS on October 1, 2015, data elements related to diagnoses and procedures are included in files that are split into two files. Nine months of the 2015 data with ICD-9-CM codes (discharges from January 1, 2015 - September 30, 2015) are in one set of files labeled Q1-Q3. Three months of 2015 data with ICD-10-CM/PCS codes (discharges from October 1, 2015 - December 31, 2015) are in a separate set of files labeled Q4. More information about the changes to the HCUP databases for ICD-10-CM/PCS and use of data across the two coding systems may be found on the HCUP-US Web site under ICD-10-CM/PCS Resources.
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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 clinical and nonclinical data elements for each hospital stay, including:
  • International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis, procedure, and external cause of injury codes prior to October 1, 2015
  • International Classification of Diseases, Tenth Revision, Clinical Modification/Procedure Coding System (ICD-10-CM/PCS) diagnosis, procedures, and external cause of morbidity codes beginning October 1, 2015
  • Patient demographic characteristics (e.g., sex, age, race, median household income for ZIP Code)
  • Hospital characteristics (e.g., ownership)
  • Expected payment source
  • Total charges
  • Discharge status
  • Length of stay
  • Severity and comorbidity measures
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As a uniform, multi-State database, the NIS promotes 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. Users of the NIS should expect a one-time decrease to historical trends for discharge counts of about 4 percent beginning with data year 2012. Users should also expect smaller one-time disruptions to historical trends for rates and means estimated from the NIS, beginning with data year 2012.

For trends analysis using NIS data 2011 and earlier, revised weights should be used to make estimates comparable to the new design beginning with 2012 data. These new discharge trend weights replace the earlier NIS Trend Weights that were developed for the 1988-1997 NIS following the 1998 NIS redesign. The new trend weights are available for download as ASCII files, along with SAS®, SPSS®, and Stata® load programs, under 1993-2011 NIS Trend Weights Files from the NIS Database Documentation page on the HCUP-US Web site. The report Using the HCUP Nationwide Inpatient Sample to Estimate Trends, available on the HCUP-US Web site under Methods Series, includes recommendations for trends analysis.

For recommendations on reporting trends across the implementation of the new coding system, please see the report HCUP Recommendations for Reporting Trends Using ICD-9-CM and ICD_10-CM/PCS Data.
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NIS releases for data years 1988 through 2016 are available for purchase online through the Online HCUP Central Distributor.

All HCUP data users, including data purchasers and collaborators, must complete the online HCUP Data Use Agreement Training Tool, and must read and sign the Data Use Agreement for Nationwide Databases (PDF file, 86 KB; HTML).

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

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

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The NIS Database is distributed as fixed-width ASCII-formatted data files delivered via secure digital download from the Online HCUP Central Distributor. The files are compressed and encrypted with SecureZIP® from PKWARE. To load and analyze the NIS data on a computer, users will need the following:
  • The password provided by the HCUP Central Distributor
  • A hard drive with at least 15 gigabytes of space available
  • A third-party zip utility such as ZIP Reader, SecureZIP®, or WinZip®, or Stuffit Expander®. (The built-in utilities in Windows® and Macintosh® cannot decompress or decrypt these zip files and will produce an error message warning of incorrect password and/or file or folder errors.)
  • 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 (beginning with 2004), are also available on the NIS Database Documentation page.

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Internet Citation: HCUP Databases. Healthcare Cost and Utilization Project (HCUP). August 2018. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/nisoverview.jsp.
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Last modified 8/13/18