The Agency for Healthcare Research and Quality’s (AHRQ) 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 hospital database designed to produce U.S. regional and national estimates of inpatient utilization, access, cost, quality, and outcomes. Unweighted, it contains discharge or billing data from approximately 7 million hospital stays each year. Weighted, it estimates over 33 million hospitalizations nationally. Developed through a Federal-State-Industry partnership sponsored by AHRQ, HCUP data inform decision making at the national, State, and community levels.

This page provides an overview of the AHRQ HCUP NIS. For more details, see NIS Database Documentation and the Introduction to the NIS, 2023 (PDF file, 643 kB) and prior years.

Contents:

• What's New in the 2023 NIS?
• About the AHRQ HCUP NIS
• AHRQ HCUP NIS File Structure
• AHRQ HCUP NIS Data Elements
• NIS Areas of Research and HCUP Publications
• Multi-Year or Trend Analyses
• Purchase the AHRQ HCUP NIS
• NIS Hardware and Software Requirements

• Because of a change in the states available to participate in the 2023 NIS and a need to produce accurate national estimates, the following modifications have been made to the data elements included in the NIS:
  • Remove information identifying the Census region and division of the hospital:
    • Remove the data elements that identified Census region (HOSP_REGION) and Census division (HOSP_DIVISION).
    • Revise the coding of the data elements for the stratum (NIS_STRATUM), hospital identifier (HOSP_NIS), and record identifier (KEY_NIS) to remove the information identifying Census division.
  • Remove collapsed categories for hospital characteristics:
    • In prior years of the NIS, categories for hospital location and teaching status (HOSP_LOCTEACH), ownership (H_CONTRL), and hospital bedsize (HOSP_BEDSIZE) are collapsed for some hospitals. Removing geographic information for hospitals in 2023 obviates the need for these collapsed categories.
  • Limit the information released on patient characteristics:
    • Remove the data element identifying the patient’s race and ethnicity (RACE).
    • Replace the data element identifying the detailed metro status designation of the county of the patient’s residence (PL_NCHS) with a new consolidated data element that distinguishes only two categories: metropolitan and non-metropolitan (PL_NCHS2).
  • The data element for total hospital charge (TOTCHG) is replaced by the data element (TOTCHG_2023) that includes an adjusted total hospital charge that accounts for the hospital charges for discharges in the missing states.
• Changes to the Diagnosis and Procedure Groups file:
  • Add data elements derived from the Chronic Condition Indicator Refined (CCIR) for ICD-10-CM v2025.1.
  • Use v2025.1 for all HCUP software tools included in the Diagnosis and Procedures Group file.
• For more information on NIS data elements, see the NIS Description of Data Elements.
• Please note that these modifications to the 2023 NIS data elements may make comparisons of some estimates across years more difficult. See the Introduction to the NIS, for 2023 (PDF) for more details.

Researchers and policymakers use AHRQ’s NIS to make national estimates of hospital utilization, cost, quality, and outcomes. The NIS contains information on all hospital stays, regardless of expected payer for the hospital stay. NIS data are available from 1988 through 2023, which allows analysis of trends over time. The NIS is sampled from the State Inpatient Databases (SID), including all inpatient data that are currently contributed to HCUP. The number of States participating in the NIS has grown from 8 in the first year to 45, plus the District of Columbia, in 2023.

Redesign of the NIS in 2012

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. Community hospitals are non-federal, acute care hospitals. 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 four major types of changes:

• Revisions to the sample design - the NIS is 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 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 eliminates State and hospital identifiers and other data elements that are not uniformly available across States.
• Revisions to the sampling strategy – the 2012-2022 NIS uses the nine Census Divisions rather than the four Census Regions to stratify, which allows more refined analyses of geographic variations in U.S. hospitalizations and produces more precise estimates by reducing sampling error. For the 2023 NIS, Census Region is used for the sampling strategy because of the change in the availability of state data.

The updated sampling strategy starting in data year 2012 produces more precise estimates than the previous NIS design by reducing sampling error. For many estimates, confidence intervals under the revised design are about half the length of confidence intervals under the previous design.

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 2023 NIS, see the Introduction to the NIS, 2023 (PDF file, 643 kB).

Information on previous years of the NIS may be found in prior years of the Introduction to the NIS at https://hcup-us.ahrq.gov/db/nation/nis/nisarchive.jsp.

This NIS is an annual, calendar year file, except for data year 2015, which was split into two parts because of the switch from ICD-9-CM to ICD-10-CM/PCS coding. There are three discharge-level files and one hospital-level file:

NIS Discharge-level files

• Core File is a single file containing commonly used data elements (e.g., age, expected primary payer, discharge status, ICD-10-CM/PCS codes, total charges).
  • This file is available in all years of the NIS.
  • Linkage between the discharge-level files
    
    • Prior to the 2012 NIS, the HCUP unique record identifier (KEY) provided the linkage between the discharge-level files.
    • Beginning with the 2012 NIS, the unique NIS record number (KEY_NIS) provides the linkage between the discharge-level files.
• Severity File is a single file containing additional data elements to aid in identifying the severity of the condition for a specific discharge.
  • This file is available beginning with the 2002 NIS.
• Diagnosis and Procedure Groups File is a single file containing additional information on the ICD-10-CM diagnoses and ICD-10-PCS procedures that is created by the Agency for Healthcare Research and Quality (AHRQ) software tools.
  • This file is available beginning with the 2005 NIS.
  • For data years 2016-2017, this file was not available in the NIS. Data elements derived from the ICD-10-CM/PCS-based AHRQ software tools were not included in the NIS because they were still in development and testing.
  • Beginning with data year 2018, this file is available in the NIS and includes data elements derived from the Clinical Classifications Software Refined (CCSR) for ICD-10-CM diagnoses.
  • Beginning with data year 2019, data elements derived from the Elixhauser Comorbidity Software Refined for ICD-10-CM, the CCSR for ICD-10-PCS procedures, and Procedure Classes Refined for ICD-10-PCS are also available in this file.
  • Beginning with data year 2023, data elements derived from the Chronic Condition Indicator Refined (CCIR) for ICD-10-CM are available on the file.

NIS Hospital-level files

• Hospital File is a single file containing information on hospital characteristics.
  • This file is available in all years of the NIS.
  • Linkage between the Inpatient Core File and the Hospital File
    
    • Prior to the 2012 NIS, the HCUP hospital identifier (HOSPID) provided the linkage between the NIS Inpatient Core File and the Hospital File.
    • Beginning with the 2012 NIS, the NIS hospital number (HOSP_NIS) provides the linkage between the NIS Inpatient Core File and the Hospital File. The HOSP_NIS values are reassigned each year, so they cannot be used to link hospitals across years.

Users interested in applying AHRQ software tools to the NIS for data years including ICD-10-CM/PCS-coded data to produce data elements currently unavailable in the database files may do so by downloading the respective tool(s) from the Research Tools section of the HCUP User Support (HCUP-US) website. Additionally, users may wish to review the HCUP Software Tools Tutorial, which provides instructions on how to apply the AHRQ software tools to HCUP or other administrative databases.

For information on changes to the NIS in data year 2015 due to transition from ICD-9-CM to ICD-10-CM/PCS coding, refer to the document 2015 Healthcare Cost and Utilization Project (HCUP) National Inpatient Sample: Change in Structure and Data Elements Caused by Transition to ICD-10-CM/PCS.

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
• AHRQ software tools (not available for 2016-2017)

For information on modifications to the NIS data elements in data year 2023 due to the changes in the participating states relative to prior years of the NIS, see the Introduction to the NIS, 2023 (PDF) and the NIS Descriptions of Data Elements.

As a uniform, multi-State database, the NIS promotes comparative studies of healthcare services and supports healthcare policy research on a variety of topics, including:

• Utilization of health services by special populations
• Hospital stays for rare conditions
• Variations in medical practice
• Healthcare cost inflation
• Regional and national analyses
• Quality of care and patient safety
• Impact of health policy changes

The NIS is used in a variety of publications and online tools:

• HCUPnet is a free, on-line query system based on data from HCUP.
• HCUP Fast Stats is an online query tool that uses visual displays to compare national or State statistics on a range of healthcare topics.
• HCUP Statistical Briefs highlight a variety of health topics.
• Use the HCUP Publications Search Tool to find publications using the NIS.
• Review featured publications on the HCUP Research Spotlights page.
• Read publications by the winners of the HCUP Outstanding Article of the Year Awards.

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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.

Because of the change in the participating states in the 2023 NIS, changes were made to the NIS data elements. See What's New in the 2023 AHRQ HCUP NIS?

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 discharge trend weights replace the earlier NIS Trend Weights that were developed for the 1988-1997 NIS following the 1998 NIS redesign. The 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 website. The report Using the HCUP National Inpatient Sample to Estimate Trends, available on the HCUP-US website 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.

NIS releases for data years 1988 through 2023 are available for purchase online through the AHRQ HCUP Central Distributor Online Reporting System (CDORS).

All HCUP data users, including data purchasers and collaborators, must complete the online AHRQ HCUP Data Use Agreement Training Tool, and must read and sign the AHRQ HCUP Data Use Agreement for Nationwide Databases (PDF file, 260 KB; HTML).

Questions about purchase or re-use of the AHRQ HCUP databases can be directed to the AHRQ HCUP Central Distributor:

Email:HCUP-RequestData@ahrq.gov Return to Contents

The NIS Database is distributed as fixed-width ASCII-formatted data files delivered via secure digital download from the AHRQ HCUP Central Distributor Online Reporting System (CDORS). The files are compressed and encrypted with 7-Zip^©. 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 77 gigabytes of space available
• A third-party zip utility such as ZIP Reader, 7-Zip^©, SecureZIP®, 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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