DOI: 10.57761/gr09-hq95
title: HCUP National Inpatient Database

description: 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.

Its large sample size is ideal for developing national and regional estimates and enables analyses of rare conditions, uncommon treatments, and special populations.
description: <u>DO NOT</u>

 use this data without referring to the [NIS Database Documentation](<https://www.hcup-us.ahrq.gov/db/nation/nis/nisdbdocumentation.jsp>), which includes:

- Description of NIS Database
- Restrictions on Use

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- Data Elements
- Additional Resources for Data Elements
- ICD-10-CM/PCS Data Included in the NIS Starting with 2015 (More details about this transition available [here](<https://hcup-us.ahrq.gov/db/nation/nis/2015HCUPNationalInpatientSample.pdf>).)
- Known Data Issues
- NIS Supplemental Files
- HCUP Tools: Labels and Formats
- Obtaining HCUP Data

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description: #Before Manuscript Submission  
<u><strong>All manuscripts</strong></u>

 (and other items you'd like to publish) <u><strong>must be submitted to</strong></u>

<u><strong>phsdatacore@stanford.edu</strong></u>

 for approval prior to journal submission.

We will check your cell sizes and citations.

For more information about how to cite PHS and PHS datasets, please visit:

[https:/phsdocs.developerhub.io/need-help/citing-phs-data-core](<https:/phsdocs.developerhub.io/need-help/citing-phs-data-core>)



You must also <u><strong>make sure that your work meets all of the AHRQ (data owner) requirements for publishing</strong></u>

 with HCUP data--listed at [https://hcup-us.ahrq.gov/db/nation/nis/nischecklist.jsp](<https://hcup-us.ahrq.gov/db/nation/nis/nischecklist.jsp>)


description: #HCUP Online Tutorials  
For additional assistance, AHRQ has created the HCUP Online Tutorial Series, a series of free, interactive courses which provide training on technical methods for conducting research with HCUP data. Topics include an [HCUP Overview Course](<https://hcup-us.ahrq.gov/tech_assist/tutorials.jsp>) and these tutorials:

• The HCUP Sampling Design tutorial is designed to help users learn how to account for sample design in their work with HCUP national (nationwide) databases. • The Producing National HCUP Estimates tutorial is designed to help users understand how the three national (nationwide) databases – the NIS, Nationwide Emergency Department Sample (NEDS), and Kids' Inpatient Database (KID) – can be used to produce national and regional estimates. HCUP 2020 NIS (8/22/22) 14 Introduction • The Calculating Standard Errors tutorial shows how to accurately determine the precision of the estimates produced from the HCUP nationwide databases. Users will learn two methods for calculating standard errors for estimates produced from the HCUP national (nationwide) databases. • The HCUP Multi-year Analysis tutorial presents solutions that may be necessary when conducting analyses that span multiple years of HCUP data. • The HCUP Software Tools Tutorial provides instructions on how to apply the AHRQ software tools to HCUP or other administrative databases.

New tutorials are added periodically, and existing tutorials are updated when necessary. The Online Tutorial Series is located on the HCUP-US website at [https://hcup-us.ahrq.gov/tech\_assist/tutorials.jsp](<https://hcup-us.ahrq.gov/tech_assist/tutorials.jsp>)


description: #Important notes about the 2015 data  
In 2015, AHRQ restructured the data as described here:

[https://hcup-us.ahrq.gov/db/nation/nis/2015HCUPNationalInpatientSample.pdf](<https://hcup-us.ahrq.gov/db/nation/nis/2015HCUPNationalInpatientSample.pdf>)



Some key points:

- For the 2015 data, all diagnosis and procedure data elements, including any data elements derived from diagnoses and procedures, were moved out of the Core File and into the Diagnosis and Procedure Groups Files.
- Prior to 2015, and for Q1-3 of 2015, the DX1-30 and PR1-15 variables (which use ICD-9 codes) variables were used, but starting in Q4 of 2015, the I10\_DX1-30 and I10\_PR1-I10-15 (which use ICD-10 codes) were used. The best way to identify discharges for quarter 1-3 or quarter 4 is based on the value of the diagnosis version (DXVER); For quarters 1-3, DXVER has a value of 9; while for quarter 4, DXVER has a value of 10.
- Some other variables also transitioned in Q4 of 2015. Please refer to the link above for more details.
- Starting in 2016, the diagnosis and procedure information returned to the Core file. Additional details about the data in 2016 are available here: [https://hcup-us.ahrq.gov/db/nation/nis/NISChangesBeginningDataYr2016.pdf](<https://hcup-us.ahrq.gov/db/nation/nis/NISChangesBeginningDataYr2016.pdf>)

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description: #NIS Areas of Research and HCUP Publications  
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

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

- [HCUPnet](<https://datatools.ahrq.gov/hcupnet>) is a free, on-line query system based on data from HCUP.
- [HCUP Fast Stats](<https://datatools.ahrq.gov/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](<https://hcup-us.ahrq.gov/reports/statbriefs/statbriefs.jsp>) highlight a variety of health topics.
- Use the [HCUP Publications Search Tool](<https://hcup-us.ahrq.gov/reports/pubsearch/pubsearch.jsp>) to find publications using the NIS.
- Review featured publications on the [HCUP Research Spotlights](<https://hcup-us.ahrq.gov/reports/spotlights.jsp>) page.
- Read publications by the winners of the [HCUP Outstanding Article of the Year Awards](<https://hcup-us.ahrq.gov/news/awards_archive.jsp>).

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description: #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
- AHRQ software tools (not available for 2016-2017)

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description: #NIS File Structure  
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).
- **Severity File** is a single file containing additional data elements to aid in identifying the severity of the condition for a specific discharge.
- **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.

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**Hospital-level files**

- **Hospital File** is a single file containing information on hospital characteristics.

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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 section of the HCUP User Support (HCUP-US) website. Additionally, users may wish to review the [HCUP Software Tools Tutorial](<https://hcup-us.ahrq.gov/tech_assist/tutorials.jsp>), 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 to Transition to ICD-10-CM/PCS.


description: #Overview: What is the NIS?  
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 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.

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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.



Also refer to this [Introduction to the NIS](<https://hcup-us.ahrq.gov/db/nation/nis/NISIntroduction2020.pdf>) document.


description: #The 1% sample  
The 1% sample is based on the KEY\_NIS variable because the base unit of this dataset is discharges. The KEY\_NIS variable only exists in the following tables, however:

- NIS Core
- NIS\_DX\_PR\_GRPS
- NIS Severity

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Any 1% samples that show up for the other tables are random samples that are not based on the KEY\_NIS variable.


description: #The NIS and Multi-Year or Trends Analyses  
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 ](<https://hcup-us.ahrq.gov/db/nation/nis/trendwghts.jsp>)from the [NIS Database Documentation ](<https://hcup-us.ahrq.gov/db/nation/nis/nisdbdocumentation.jsp>)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](<https://hcup-us.ahrq.gov/reports/methods/methods.jsp>), 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](<https://hcup-us.ahrq.gov/datainnovations/HCUP_RecomForReportingTrends_070517.pdf>).