DOI: 10.57761/329g-fs48
title: HCUP National Inpatient Database

description: 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.
description: #Documentation  
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 hospital stays, regardless of expected payer, for the hospital stay. 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 healthcare utilization, access, charges, quality, and outcomes. NIS data are available from 1988 through 2018, which allows analysis of trends over time. 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 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.

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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 (2018) 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.

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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](<https://www.hcup-us.ahrq.gov/reports/methods/2014-04.pdf>).



For more details on the 2017 NIS, see the Introduction to the NIS, 2018 ([PDF](<https://www.hcup-us.ahrq.gov/db/nation/nis/NISIntroduction2018.pdf>) file, 1.1 MB; [HTML](<https://www.hcup-us.ahrq.gov/db/nation/nis/NIS_Introduction_2018.jsp>)).





**NIS File Structure**

Beginning with the data year 2016, the NIS is a calendar year file that includes diagnoses and procedures coded using only ICD-10-CM/PCS. The file structure is similar to the file structure of the NIS in data years 2014 and prior years.



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 website under ICD-10-CM/PCS Resources.





**NIS Data Elements**

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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