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 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.
BEGINNING WITH DATA YEAR 2016, THE NIS CONTAINS A FULL YEAR OF ICD-10-CM/PCS CODES.
Beginning with data year 2016, the NIS includes a full calendar year of data with diagnosis and procedure codes reported using the ICD-10-CM/PCS1 coding system. The file structure is similar to the file structure of the NIS in data years 2014 and prior years.
THE 2015 NIS CONTAINS ICD-9-CM AND ICD-10-CM/PCS CODES.
On October 1, 2015, hospital administrative data 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. The data elements and file structure for the 2015 NIS are different. Trends based on diagnoses or procedures will be affected.
AHRQ SOFTWARE TOOLS FOR ICD-10-CM/PCS CODES
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.
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).
What's New in the 2021 NIS?
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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:
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 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.
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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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:
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.
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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:
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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:
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.
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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:
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 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.
|Internet Citation: HCUP Databases. Healthcare Cost and Utilization Project (HCUP). 02 2024. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/nisoverview.jsp?utm_medium=email&utm_source=transaction.
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|Last modified 02/01/24