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The HCUP National (Nationwide) Inpatient Sample (NIS) is the research meaning of “NIS database”: a US all-payer inpatient hospital discharge database run by the Agency for Healthcare Research and Quality (AHRQ). Researchers use it to estimate national and regional inpatient utilization, access, cost, quality and outcomes. This page explains what it holds, how access works, and what to check before pooling years.
What “NIS database” means
The abbreviation is shared by unrelated systems. This page covers only the first row.
| Meaning | Field | Covered here? |
|---|---|---|
| HCUP National (Nationwide) Inpatient Sample | Health services and health economics research | Yes |
| Network Information Service (Unix/Linux) | System administration; a distributed database of “maps” | No |
| Nautical Information System (ArcGIS) | Maritime GIS data management | No |
What the HCUP NIS is
AHRQ describes the NIS as “the largest publicly available all-payer inpatient hospital database” in the United States. It belongs to the Healthcare Cost and Utilization Project (HCUP) family; see the HCUP overview for the full set of databases. Unweighted, the NIS contains discharge or billing data from approximately 7 million hospital stays each year; weighted, it supports estimates for more than 35 million hospitalizations nationally.
Who runs it and what it covers
- Steward: AHRQ, through HCUP. Data come from state participation; the number of participating states grew from 8 in the first year (1988) to 44 plus the District of Columbia in 2024. Data years 1988 to 2024 are available.
- Sample design: From the 2012 data year, the NIS approximates a 20-percent stratified sample of all discharges from US community hospitals. Rehabilitation and long-term acute care hospitals are excluded. Strata are the nine Census Divisions for 2012-2022 and the Census Region for 2023-2024.
- Weights: Weights are supplied for national and regional estimates. For trends that use 2011 and earlier data, AHRQ provides revised (trend) weights.
- Content: The Core File includes age, expected primary payer, discharge status, ICD-10-CM and ICD-10-PCS codes and total charges. Other files add severity measures (from 2002), diagnosis and procedure groupings (from 2005) and hospital characteristics. Other elements include sex, ZIP code median household income, hospital ownership and length of stay. Not every element exists in every year.
How to get and use it
- Start at the AHRQ NIS Overview page and the NIS Database Documentation page (links under Sources).
- Complete the online Data Use Agreement Training Tool.
- Sign the “AHRQ HCUP Data Use Agreement for Nationwide Databases”.
- Purchase the data through the AHRQ HCUP Central Distributor online system (CDORS). Prices are listed by AHRQ, not here.
- Receive the encrypted files and the password from the Central Distributor. AHRQ states you need at least 77 GB of free disk space, a zip utility and statistical software such as SAS, SPSS or Stata.
- Use the supplied load programs (SAS, SPSS, Stata) and read the Checklist for Working With the NIS before analysis.
Questions about requests go to AHRQ at [email protected].
Where to find what in the NIS documentation
The documentation page groups material by task. This map pairs each task with the section AHRQ lists for it.
| If you need to… | Go to the documentation section |
|---|---|
| Understand the database and its basic rules | Description of NIS Database (NIS Overview, Introduction to the NIS 2024, NIS Related Reports, Checklist for Working With the NIS) |
| Know what you may and may not do with the data | Restrictions on Use (Data Use Agreement Training, DUA for National Databases, Requirements for Publishing with HCUP Data) |
| Load the files | File Specifications and Load Programs (SAS, SPSS, Stata) |
| Check what each variable contains | Data Elements (Description of Data Elements, Summary Statistics, Frequencies by Diagnosis/Procedure Codes) |
| Add severity, coding or hospital context | Additional Resources (APR-DRG severity, Quality Control, Coding Practices, Hospital Identifiers) |
| Compare years across the design change | 2012 NIS Redesign (Redesign Report, Trend Weights for 1993-2011) |
| Check for errata | Known Data Issues |
| Add cost or hospital variables | Supplemental Files (Cost-to-Charge Ratio, Hospital Market Structure, Hospital Ownership) |
| Group diagnoses or procedures | HCUP Tools: Labels and Formats, including CCSR |
Strengths and limits
Strengths: national scope, all payers, and a data history from 1988 to 2024, with weights for national and regional estimates.
Limits to check before pooling years or designing a study:
- 2012 redesign break. The sample design changed in the 2012 data year, and AHRQ notes a roughly 4% one-time drop in discharge counts starting in 2012. Use the trend weights for analyses spanning 2011 and earlier.
- No hospital-level linkage. Hospital identifiers are removed, and the hospital identifier (HOSP_NIS) is reassigned each year, so hospitals cannot be tracked across years.
- Not for state estimates. AHRQ’s documentation warns why the NIS should not be used to make state-level estimates.
- Changing elements. Some data elements changed in 2024 because of changes in state participation, and not all elements appear in every year.
For comorbidity measures often discussed alongside administrative discharge data, see the Elixhauser comorbidity index guide.
Alternatives
| Resource | Use it when |
|---|---|
| HCUP Nationwide Emergency Department Sample (NEDS) | The question concerns emergency department visits rather than inpatient stays; it is a sibling HCUP national database. |
| Other HCUP databases (state and national) | You need state-level estimates, which AHRQ says the NIS should not be used for. |
| MarketScan | You need a different claims-based source; see that page for its scope. |
FAQ
What does NIS stand for in a research database?
In health services research it stands for the HCUP National (Nationwide) Inpatient Sample, maintained by AHRQ. In computing it can mean the Unix Network Information Service.
Is the NIS free?
AHRQ sells the NIS through its Central Distributor and requires completion of Data Use Agreement training and a signed agreement. See the NIS Overview page for current purchase terms.
How many hospital stays does the NIS contain?
About 7 million stays per year unweighted, representing more than 35 million hospitalizations nationally when weighted (AHRQ, read 2026-11-08).
Can I use the NIS for state-level estimates?
No. AHRQ’s documentation page explicitly advises against it.
Can I follow a hospital across years?
No. Identifiers are removed and HOSP_NIS is reassigned each year.
Sources
- AHRQ HCUP, NIS Overview, read 2026-11-08.
- AHRQ HCUP, NIS Database Documentation, read 2026-11-08.
