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 healthcare database designed to produce U.S. regional and national estimates of inpatient utilization, access, cost, quality, and outcomes. Unweighted, it contains data from around 7 million hospital stays each year. Weighted, it estimates around 35 million hospitalizations nationally. Developed through a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality (AHRQ), HCUP data inform decision making at the national, State, and community levels.

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

This page provides an overview of the NIS. For more details, see NIS Database Documentation and the Introduction to the NIS, 2021 (PDF file, 1.2 MB; HTML).

Contents:

• What's New in the 2021 NIS?
• About the NIS
• NIS Data Elements
• NIS File Structure
• NIS Areas of Research and HCUP Publications
• The NIS and Multi-Year or Trend Analyses
• Purchase the NIS
• NIS Hardware and Software Requirements

• The overall number of discharges for data year 2021 increased by 3 percent from 2020. In the first quarter of 2021, the number of discharges was lower by 7 percent compared to the first quarter of 2020. However, discharges in 2021 increased 16 percent in the second quarter, 3 percent in the third quarter, and less than 1 percent in the fourth quarter. The overall in-hospital mortality rate increased from 2.8 percent in 2020 to 3.1 percent in 2021.

Researchers and policymakers use the NIS to make national estimates of health care 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 2021, 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 47, plus the District of Columbia, at present.

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 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 (2021) include:

• The NIS is drawn from all States participating in HCUP, covering 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 2012 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 2021 NIS, see the Introduction to the NIS, 2021 (PDF file, 1.2 MB; 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.

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

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.
• 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 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-CM are also available in this file.

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)

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. 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 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 2021 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, 260 KB; HTML).

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

Email: HCUP@AHRQ.gov
Telephone: (866) 290-4287 (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 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 25 gigabytes of space available
• A third-party zip utility such as ZIP Reader, 7-Zip^©, SecureZIP^®, WinZip^®, or Stuffit Expander^®.
• 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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