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NASS Overview
The Nationwide Ambulatory Surgery Sample (NASS) is a set of ambulatory surgery databases included in the HCUP family. These databases are created by AHRQ through a Federal-State-Industry partnership.
 
Overview of the Nationwide Ambulatory Surgery Sample (NASS)

The Nationwide Ambulatory Surgery Sample (NASS) is part of a family of databases and software tools developed for the Healthcare Cost and Utilization Project (HCUP).

The NASS is the largest all-payer ambulatory surgery database in the United States, yielding national estimates of major ambulatory surgery encounters performed in hospital-owned facilities. Unweighted, it contains approximately 7.3 million ambulatory surgery encounters each year and approximately 9.4 million ambulatory surgery procedures. Weighted, it estimates approximately 10.5 million ambulatory surgery encounters and 13.6 million ambulatory surgery procedures. Major ambulatory surgeries are defined as selected invasive, therapeutic surgical procedures that typically require the use of an operating room and require regional anesthesia, general anesthesia, or sedation (i.e., surgeries flagged as "narrow" in the HCUP Surgery Flag Software). Procedures intended primarily for diagnostic purposes are excluded. Developed through a Federal-State-Industry partnership sponsored by the Agency for Healthcare Research and Quality, HCUP data inform decisionmaking at the national, State, and community levels.

This page provides an overview of the NASS. For more details, see NASS Database Documentation and the Introduction to the NASS, 2016 (PDF file, 481 KB; HTML).

Contents:

Sampled from the State Ambulatory Surgery and Services Databases (SASD), HCUP's NASS can be used to create national estimates of major ambulatory surgery encounters performed in hospital-owned facilities. Major ambulatory surgeries are defined as selected invasive, therapeutic surgical procedures that typically require the use of an operating room and require regional anesthesia, general anesthesia, or sedation (i.e., surgeries flagged as "narrow" in the HCUP Surgery Flag Software).

Key features of the 2016 NASS include:
  • Data from 2,718 hospital-owned facilities located in 33 States and the District of Columbia, approximating a 59-percent stratified sample of U.S. hospital-owned facilities performing selected ambulatory surgeries
  • Data on clinical procedures and diagnoses, disposition of the patient, expected source of payment, and total charges, as well as geographic, hospital-owned facility, and patient characteristics
  • A focus on encounters with at least one "in-scope" ambulatory surgery: an invasive, therapeutic procedure with relatively high procedure volume, a substantial share of procedures in the hospital outpatient setting, and reliable reporting from hospital-owned facilities
  • A supplemental file that provides information on out-of-scope procedures performed during these encounters

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The NASS is delivered as a set of related files. The Hospital File lists provides hospital-level attributes (e.g., teaching status, bed size) for hospital-owned facilities in the NASS as well as the encounter weight and sample stratum information. This hospital-level file contains one observation for each hospital included in the NASS (2,718 hospital-specific records in 2016). The Encounter File links to the Hospital File and contains information on the ambulatory surgery encounter, including patient age, expected source of payment, diagnoses, in-scope and other procedures, charges, and disposition of the patient. This encounter-level file contains 100 percent of ambulatory surgery encounters containing a major ambulatory surgery from the sample of hospitals (7.3 million encounters in 2016). A related Supplemental File contains entries for out-of-scope procedures (approximately 6.0 million records in 2016) that were performed during encounters recorded in the Encounter File, with a key linking to the Encounter File.
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The NASS contains clinical and resource-use information that is included in a typical hospital-owned facility record abstract, including patient characteristics, clinical diagnostic and surgical procedure codes, total charges and expected source of payment, and facility characteristics. Safeguards are applied to protect the privacy of individual patients, physicians, and hospitals (as required by data sources). The NASS is composed of more than 100 clinical and nonclinical variables for each encounter. These include:
  • Current Procedural Terminology (CPT®) procedure codes for encounters with at least one major ambulatory surgery
  • International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnosis codes
  • Patient demographic 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)
  • Total charges
  • Disposition of the patient
  • Hospital characteristics (e.g., region, ownership, teaching status)
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As a uniform, national database, the NASS promotes wide-ranging studies of health care services and supports health care policy and research on a variety of topics, including:
  • Trends in ambulatory surgery
  • Charges for ambulatory surgery services
  • Quality of ambulatory surgery care
  • Impact of health policy changes
  • Access to ambulatory surgery care
  • Utilization of ambulatory surgery services by special populations
The NASS may be used in a variety of publications: Return to Contents
 

The NASS release for data year 2016 is 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 NASS 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 NASS 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 estimates. NASS documentation and tools, including programs for loading the CSV file into SAS, SPSS, or Stata, are also available on the NASS Database Documentation page.

Please note the following based on the software you plan to use:
  • In total, the CSV version of the NASS is about 2.9 GB.
  • The NASS files loaded into SAS are about 2.4 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 NASS files would require approximately 8 GB of available workspace to perform a sort.
  • The NASS files loaded into SPSS are under 2 GB.
With a file this size and without careful planning, space could easily become a problem in a multi-step program with the NASS. 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: NASS Overview. Healthcare Cost and Utilization Project (HCUP). September 2019. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/nassoverview.jsp.
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Last modified 9/26/19