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 over 30 million ED visits each year. Weighted, it estimates roughly 145 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.


Beginning with data year 2016, the NEDS includes a full calendar year of data with diagnosis and inpatient procedure codes reported using the ICD-10-CM/PCS1 coding system. The file structure is similar to the file structure of the NEDS in data years prior to 2015.

On October 1, 2015, hospital administrative data began using ICD-10-CM/PCS, so the first nine months of calendar year 2015 contain ICD-9-CM1 codes and the last three months of calendar year 2015 contain ICD-10-CM/PCS codes. The file structure for the 2015 NEDS is different than other years.

Data elements derived from AHRQ software tools are not available for ICD-10-CM/PCS data on the NEDS.

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 NEDS. For more details, see NEDS Database Documentation and the Introduction to the NEDS, 2017 (PDF file, 806 KB; HTML).


Two data elements have been added to the NEDS that identify whether the ED visit was related to an initial injury encounter based on ICD-10-CM diagnoses:
  • Whether an ICD-10-CM injury diagnosis for an initial encounter was reported on the record (HCUP data element I10_INJURY)
  • Whether more than one ICD-10-CM injury diagnoses for an initial encounter were reported on the record (HCUP data element I10_MULTINJURY)
Separate data elements for external cause diagnosis codes are discontinued (formerly HCUP data elements I10_ECAUSEn). External cause codes are now included at the end of the ICD-10-CM diagnosis array (HCUP data elements I10_DXn). The length of the diagnosis array has been increased from 30 to 35 codes to accommodate this change. Also, the length of the ICD-10-PCS procedure array has increased from 9 to 15 codes, and the length of the CPT procedure array has increased from 15 to 35 codes.
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Sampled from the State Inpatient Databases (SID) and State Emergency Department Databases (SEDD), HCUP's 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 then 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 2017, which allows researchers to analyze trends over time. Key features of the most recent NEDS database year (2017) 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 984 hospitals located in 36 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, geographic area, and the nature of ED visits (e.g., common reasons for ED visits, including injuries)
  • ED charge information for 87 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 Introduction to the NEDS, 2017 (PDF file, 806 KB; HTML).
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Beginning with data year 2016, the NEDS is a calendar year file that includes diagnosis and inpatient procedures coded using only ICD-10-CM/PCS. The file structure is similar to the file structure of the NEDS prior to 2015 with one exception, data elements based on AHRQ software tools are not available for ICD-10-CM/PCS data on the NEDS.

For the 2015 NEDS, data elements related to diagnoses and procedures are included in the supplemental ED and inpatient files and each supplemental file is split into two files because of the transition to ICD-10-CM/PCS on October 1, 2015 (the beginning of fiscal year 2016). 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.
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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, for ICD-9-CM data only, the mechanism, intent, and severity of injury
  • Discharge status
  • Patient demographics characteristics (e.g., sex, age, 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)
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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: Return to Contents

NEDS releases for data years 2006 through 2017 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, 85 KB; HTML).

Questions regarding purchasing databases can be directed to the HCUP Central Distributor:

E-mail: HCUPDistributor@AHRQ.gov
Telephone: (866) 556-4287 (toll free)
Fax: (866) 792-5313 (toll free)

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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 SecureZIP® from PKWARE.

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 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 12 gigabytes (GB).
  • The NEDS files loaded into SAS are about 10 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 work space approximately three times the size of the file. Thus, the NEDS files would require approximately 30 GB of available workspace to perform a sort.
  • The NEDS files loaded into SPSS are about 27 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 program with the NEDS. It is not unusual to have several versions of a file marking different steps while preparing it for analysis and more versions for the actual analyses; therefore, users should be aware that the amount of space required can escalate rapidly.

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Internet Citation: NEDS Overview. Healthcare Cost and Utilization Project (HCUP). April 2020. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/nedsoverview.jsp.
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Last modified 4/27/20