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NEDS Overview

Overview of the Nationwide Emergency Department Sample (NEDS)

The Nationwide Emergency Department Sample (NEDS) is part of a family of databases and software tools developed for the Healthcare Cost and Utilization Project (HCUP). The NEDS is the largest all-payer emergency department (ED) database in the United States, yielding national estimates of hospital-owned ED visits. Unweighted, it contains data from about 30 million ED visits in 2021. Weighted, it estimates roughly 127 million ED visits. Developed through a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality, HCUP data inform decision making at the national, State, and community levels.

This page provides an overview of the NEDS. For more details, see NEDS Database Documentation and the Introduction to the NEDS, 2021 (PDF).


  • The overall number of ED visits for data year 2021 increased by 3 percent from 2020. The largest increase was in the second quarter, which had 28 percent more discharges than the prior year. The percentage of records in which the patient was admitted to the same hospital decreased from 16 percent in 2020 to 15 percent in 2021.
  • In 2021, Current Dental Terminology (CDT) codes were removed from the NEDS Supplemental ED File because of inconsistent reporting of information across States.
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Sampled from the State Inpatient Databases (SID) and State Emergency Department Databases (SEDD), the HCUP NEDS can be used to create national and regional estimates of ED care. The SID contain information on patients initially seen in the ED and subsequently admitted to the same hospital. The SEDD capture information on ED visits that do not result in an admission (i.e., treat-and-release visits and transfers to another hospital).

NEDS data are available from 2006 through 2021, which allow researchers to analyze trends over time. Key features of the 2021 NEDS database year include:

  • A large sample size, which provides sufficient data for analysis across hospital types and the study of relatively uncommon disorders and procedures
  • Discharge data for ED visits from 993 hospitals located in 39 States and the District of Columbia, approximating a 20-percent stratified sample of U.S. hospital-owned EDs
  • Demographic data such as hospital and patient characteristics, (e.g., hospital teaching status, age, and race/ethnicity of the patient), geographic area (e.g., hospital region), and the nature of ED visits (e.g., common reasons for ED visits, including injuries)
  • ED charge information for 95 percent of ED visits, regardless of the expected payer for the visit
  • Children's hospitals with trauma centers, which are classified with adult and pediatric trauma centers in the current versions of the NEDS.

Information on previous years of the NEDS may be found in the the Introduction to the NEDS.
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Generally, the NEDS is a set of annual files including several discharge-level files and one hospital-level file:

Discharge-Level files
  • Core File contains records for all the ED visits in the SID and SEDD—whether resulting in admission or not—from the sample of hospitals in participating States and the District of Columbia.
    • This file is available in all years of the NEDS.
  • Supplemental ED File contains additional information for patients who were treated in the ED and not admitted directly to the hospital (e.g., released home, transferred). This information came from the SEDD.
    • This file is available in all years of the NEDS.
  • Supplemental Inpatient File contains data elements that are specific to the inpatient stay, such as total charges, length of inpatient stay, and procedure codes from the SID record. Procedures reported on the SID records may have been performed in the ED, but currently there is no way to verify this information.
    • This file is available in all years of the NEDS.
  • Diagnosis and Procedure Groups Files contains additional information on the ICD-10-CM/PCS diagnoses and procedures created by the HCUP software tools.
    • This file is available in the NEDS beginning with data year 2018.
Hospital-level file
  • Hospital File contains one observation for each hospital-owned ED sampled for the NEDS, with its weight and variance estimation data elements. The unit of observation is the ED.
    • This file is available in all years of the NEDS.

In October 2015, the United States transitioned from ICD-9-CM1 to the ICD-10-CM1 diagnosis coding system for most inpatient and outpatient medical encounters and the ICD-10-PCS1 procedure coding system for inpatient hospital procedures. This transition had a direct impact on the reporting of medical services in the NEDS:
  • Starting with the 2016 NEDS, diagnoses and inpatient procedures are coded in ICD-10-CM/PCS.
  • For the 2015 NEDS, data elements related to diagnoses and procedures are split into two files by coding system. Nine months of the calendar year 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 the calendar year 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.
  • Prior to data year 2015, diagnoses and procedures are coded in International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM).

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.

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The NEDS 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). The NEDS is composed of more than 100 clinical and nonclinical variables for each hospital stay. These include:
  • ICD-10-CM/PCS diagnosis, procedure, and external cause of morbidity codes (starting on October 1, 2015)
  • ICD-9-CM diagnosis, procedures, and external cause of injury codes (prior to October 1, 2015)
  • Current Procedural Terminology, Fourth Edition (CPT®-4) procedure codes on ED visits that do not results in an admission to the same hospital
  • Identification of injury-related ED visits and the mechanism and intent of injury (the availability of these data elements varies over time)
  • Discharge status
  • Patient demographics characteristics (e.g., sex, age, race/ethnicity, urban-rural designation of residence, national quartile of median household income for patient's ZIP Code)
  • Expected payment source (e.g., Medicare, Medicaid, private insurance, self-pay, no charge, and other insurance type)
  • Total ED charges (for ED visits) and total hospital charges (for inpatient stays for ED visits that result in admission)
  • Hospital characteristics (e.g., region, trauma center indicator, urban-rural location, teaching status)
  • Data elements derived from the HCUP software tools beginning with data year 20182

2 Users interested in applying HCUP software tools to the NEDS to produce data elements unavailable for a certain data year may do so by downloading the respective tool(s) from the HCUP Research Tools section of the HCUP-US website. Further, users may wish to review the HCUP Software Tools Tutorial, which provides instructions on how to apply the HCUP software tools to HCUP or other administrative databases.

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As a uniform, multi-State database, the NEDS promotes comparative studies of healthcare services and supports healthcare policy and research on a variety of topics, including:
  • Use of and charges for ED services
  • Medical treatment effectiveness
  • Quality of ED care
  • Impact of health policy changes
  • Access to care
  • Utilization of health services by special populations
The NEDS is used in a variety of publications and online tools: Return to Contents

NEDS releases for data years 2006 through 2021 are available for purchase through the 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 regarding purchasing databases can be directed to the HCUP Central Distributor:

Telephone: (866) 290-4287 (toll free)
Fax: (866) 551-4587

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The NEDS data set is extremely large. The data are distributed as comma-separated value (CSV) 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 NEDS data on a computer, users will need the following:
  • The password provided by the HCUP Central Distributor
  • A hard drive with 50 to 100 (GB) of space available
  • A third-party zip utility such as 7-Zip©, ZIP Reader, Secure ZIP®, WinZip®, or Stuffit Expander®
  • SAS®, SPSS®, Stata® or similar analysis software
The data set includes weights for producing national and regional estimates. NEDS documentation and tools, including programs for loading the CSV file into SAS, SPSS, or Stata, are also available on the NEDS Database Documentation page.

Please note the following based on the software you plan to use:
  • In total, the CSV version of the NEDS is 50 gigabytes (GB).
  • The NEDS files loaded into SAS are about 14 GB. Most SAS data steps will require twice the storage of the file, so that the input and output files can coexist. The largest use of space typically occurs during a sort, which requires workspace approximately three times the size of the file. Thus, the NEDS files would require approximately 40 GB of available workspace to perform a sort.
  • The NEDS files loaded into SPSS are estimated about 30 GB.
  • Because Stata loads the entire file into memory, it may not be possible to load every data element in the NEDS Core file into Stata. Stata users will need to maximize memory and use the "_skip" option to select a subset of variables. More details are provided in the Stata load programs.
With a file this size and without careful planning, space could easily become a problem in a multi-step process. It is common to produce several versions of a file during data preparation, as well as further multiple versions for analysis. Therefore, the amount of space required could escalate rapidly.

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Internet Citation: NEDS Overview. Healthcare Cost and Utilization Project (HCUP). March 2024. Agency for Healthcare Research and Quality, Rockville, MD.
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Last modified 03/19/24