Checklist for Working with the 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).

HCUP databases, including the NASS, are consolidated sources of information that can be used for many types of research. Researchers, peer manuscript reviewers, and journal editors need to understand the NASS database design, its strengths and limitations, and how it has changed over time to ensure its appropriate use and to interpret study results. This document provides a checklist of key elements to consider and connects you to NASS informational resources, organized into four sections:
  1. HCUP Data Use Agreement for Nationwide Databases and Acknowledgements
  2. Research Design
  3. Data Analysis
  4. International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) Guidance
The NASS Database Documentation page is the main resource for all information regarding the NASS, including the Introduction to the Nationwide Ambulatory Surgery Sample (NASS), which is recommended as a starting resource for new users.

NASS Data Use and Acknowledgments
  Checklist Item Description Checklist Resource
Obtain and adhere to the HCUP Nationwide Database Data Use Agreement (DUA).a The HCUP DUA governs the disclosure and use of the data, including affirmations to protect individuals, establishments, and the database itself. For general information, review the Responsibilities of the Data Purchaser and the HCUP Nationwide Database Data Use Agreement (DUA).

To access the NASS, you must complete the HCUP Data Use Agreement Training.
Verify privacy protections for individuals and hospitals. Individuals or hospital-owned facilities cannot be identified directly or indirectly.

Reporting cell sizes <10 increases the risk of re-identification and is discouraged, as specified in the Data Use Agreement.
For general information, review the Requirements for Publishing with HCUP Data page on the HCUP User Support (HCUP-US) website.
Cite HCUP, the NASS, and other HCUP tools. HCUP, the NASS, and other supporting tools must be correctly cited in the abstract and manuscript. For more information, review the Suggested Citations for HCUP Databases and Tools page on HCUP-US.
Acknowledge HCUP Partners. Participating HCUP Partners should be listed in the manuscript by name or acknowledged by a hyperlink to the HCUP-US website. For more information, review the List of HCUP Data Partners for Reference in Publications page on HCUP-US.
Return to top
Research Design
Understand inclusion criteria (in-scope major ambulatory surgeries). Unlike inpatient stays and ED visits, ambulatory surgeries are not setting-specific. The NASS is limited to encounters with at least one in-scope major ambulatory surgery on the record, performed at hospital-owned facilities.

Major surgeries are procedures flagged as "narrow" in the HCUP Surgery Flag Software for Services and Procedures. These include major therapeutic procedures involving incision, excision, manipulation, or suturing of tissue that (1) require the use of an operating room, (2) penetrate or break the skin, and (3) involve regional anesthesia, general anesthesia, or sedation to control pain. In addition, the following three procedures, which are primarily performed for a diagnostic purpose, are assigned a narrow surgery flag based on the degree of their invasiveness: (1) biopsies if the procedure is within an internal organ (e.g., brain, deep cervical node, stomach), (2) thoracotomy with or without biopsy, and (3) exploratory laparotomy with or without biopsy.

For data year 2019, in-scope surgeries include Clinical Classifications Software (CCS) for Services and Procedures categories with the following criteria: (1) relatively high major ambulatory surgery volume or aggregate charge total and (2) evidence of reliable reporting from SASD hospital-owned facilities.

For data years 2016-2018, in-scope surgeries include CCS for Services and Procedures categories with the following criteria: (1) relatively high major ambulatory surgery volume, (2) substantial share of major ambulatory surgeries performed in hospital-owned facilities, and (3) evidence of reliable reporting from SASD hospital-owned facilities.
 
Types of records included in the sample may change over time. Prior to data year 2019, the NASS sample was limited to SASD encounters that involved surgeries defined as "narrow" by the HCUP Surgery Flag Software for Services and Procedures. Subsequent analyses revealed additional encounters involving "narrow"or major surgeries that were started in the emergency department and appeared in the State Emergency Department Databases (SEDD) but not in the SASD. As a result, these surgeries are undercounted in the 2016-2018 NASS. The procedures most impacted by this issue include appendectomy and removal of ectopic pregnancy (each undercounted by more than 50%) and cholecystectomy (undercounted by approximately 10%). In subsequent data years, these emergent in-scope surgeries are captured in the NASS. Specifically, the 2019 NASS was based on data selected from both the SASD and SEDD in order to capture both planned and emergent major ambulatory surgeries.  
In-scope CCS procedure groups may change over time. Because the in-scope selection criteria described above are applied each year using the most recent data available and the current HCUP Surgery Flag Software for Services and Procedures, CCS procedure groups may be added to and dropped from the NASS sample from year to year. For example, beginning in 2018, major surgeries assigned to CCS 14, Glaucoma procedures, are now in scope for the NASS sample while CCS 45, Percutaneous Transluminal Coronary Angioplasty (PTCA) became out of scope. For more information on in-scope selection criteria, see the Selection of Major Ambulatory Surgeries (in particular Figure 2) section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US. A complete list of current in-scope CCS procedure groups and a list of procedure group additions and deletions by data year are included in Appendix B of the Introduction to the NASS.
The hospital-owned facility universe changed beginning with data year 2019. Beginning in data year 2019, the hospital-owned facility universe for the NASS was expanded to include more than general acute care and children’s service types of facilities. Consistent with all other HCUP nationwide databases, the 2019 NASS now includes specialty hospitals such as surgical, cancer, heart, and orthopedic facilities owned by community hospitals that performed in-scope major ambulatory surgeries.

Also beginning in data year 2019, consistent with all other HCUP nationwide databases, the hospital-owned facility universe was limited to hospitals included in the American Hospital Association (AHA) Annual Survey of Hospitals that reported performing outpatient surgeries. In prior years, the Centers for Medicare and Medicaid Services (CMS) Provider of Services (POS) data were used to augment the information on hospital-owned facilities.
For more information about the NASS universe definition, see the Ambulatory Surgery Setting and Universe Definition section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US.
Learn how to account for the NASS sampling design. The NASS is a database of all-payer major ambulatory surgeries in the United States. The NASS is sampled from the HCUP State Ambulatory Surgery and Services Databases (SASD).

The NASS is designed to estimate the volume of encounters containing major ambulatory surgeries for all hospital-owned facilities performing major ambulatory surgeries. A national list of all hospital-owned facilities performing ambulatory surgeries is created using the SASD, American Hospital Association (AHA), and Centers for Medicare & Medicaid Services (CMS) data sources. An encounter predictive model is applied to hospital-owned facilities outside the NASS sampling frame and then combined with observed data from sampling frame hospital-owned facilities to create national \encounter volume estimates.
For detailed information, review the Sampling Design of the NASS section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US.

To learn more about how to account for sample design when working with the HCUP nationwide databases, view the HCUP Sample Design tutorial on the HCUP Online Tutorial Series page on HCUP-US. The tutorial will be updated in the future to directly address the NASS sampling design.
Ambulatory surgery encounters that were subsequently admitted to the same hospital are excluded. Because the SASD do not include ambulatory surgery encounters that were subsequently admitted to the same hospital for inpatient care, the NASS does not contain encounters admitted from the ambulatory setting to the inpatient setting. Information on patients admitted to the hospital following ambulatory surgery is included in the HCUP State Inpatient Databases (SID). Although comprehensive tracking of admissions from the ambulatory setting is not possible, it is estimated that less than 0.3 percent of SID discharges represent admitted major ambulatory surgery encounters. For detailed information, review the NASS Data Sources, Hospitals, and Encounters section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US.
Revisit analyses cannot be performed. The NASS cannot be used to conduct revisit analyses. There is no synthetic patient identifier that would allow patient-level analysis to identify individuals with more than one in-scope major ambulatory surgery. In contrast, some HCUP State databases may be used for this type of analysis. For more information on conducting revisit analyses at the State level, review the HCUP Supplemental Variables for Revisit Analyses page on HCUP-US.
No State-level analyses are performed. The NASS is designed for national or regional level analyses of ambulatory surgery-related care. State identifiers are not included in the NASS data.

To conduct State-level analysis, you must use the SASD.
To learn more about the SASD, review the Overview of the State Ambulatory Surgery and Services Databases (SASD) page on HCUP-US.
No facility-level analyses are performed. Hospital-owned facility identifiers are not included in the NASS. The sampling design of the NASS does not support healthcare facility-level analyses.

The unique HCUP NASS hospital number links to other NASS files but does not link to other HCUP databases.

You should not attempt to identify individual facilities, as specified in the Data Use Agreement.
For more information, review the Sampling Design of the NASS section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US.
No physician-level analyses are performed. The NASS does not include physician identifiers.

The sampling design of the NASS does not support physician-level analyses.
For more information, review the NASS Description of Data Elements on the NASS Database Documentation page on HCUP-US.

For more information, review the Sampling Design of the NASS section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US.
Administrative (ICD-10-CM and CPT) codes are appropriate for the outcomes of interest. Administrative codes for the diagnoses or procedures of interest (ICD-10-CM, and Current Procedural Terminology [CPT®), respectively) should be used with care, especially over time, as codes and coding rules change annually. For more information, review the ICD-10-CM and ICD-10-CM/PCS Diagnosis and Procedure Codes and CPT Procedure Codes section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US.

Refer to the ICD-10-CM/PCS Resources page on HCUP-US under Data Innovations for a summary of key issues for researchers using HCUP and other administrative databases that include ICD-10-CM/PCS coding.

To check for year-to-year variation in administrative codes, consult with a medical coding professional.
Account for year- based differences in data element availability in the NASS. The study design should account for differences in data element availability across States and across data years. For example, the number of diagnosis codes present can vary by year. For more information, review the NASS Description of Data Elements on the NASS Database Documentation page on HCUP-US.
HCPCS Level II codes are excluded from the NASS. The NASS includes only CPT codes (Healthcare Common Procedure Coding System (HCPCS) Level I codes). HCPCS Level II codes are excluded. Procedures that are exclusively or predominantly reported on facility records using HCPCS Level II codes will be underreported in the NASS. For this reason, CCS 45, Percutaneous Transluminal Coronary Angioplasty (PTCA) was removed from the NASS beginning in 2018. A complete list of current in-scope CCS procedure groups and a list of procedure group additions and deletions by data year are included in Appendix B of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US.
Return to top
Data Analysis
Use weights for national estimates. The major, in-scope ambulatory surgery encounter weight (DISCWT) in the NASS Encounter File should be used for producing nationwide, encounter-level statistics where the ambulatory surgery encounter is the unit of analysis. To learn how to apply NASS weights, view the Producing National HCUP Estimates On-line Tutorial on the Tutorial Series page on HCUP-US.
Account for the design of the NASS when calculating standard errors. There are two methods for calculating standard errors for estimates produced from the NASS.

Because the NASS is a stratified cluster sample, proper statistical techniques must be used to calculate standard errors and confidence intervals.
To learn how to calculate standard errors, view the HCUP Calculating Standard Errors On-line Tutorial on the Tutorial Series page on HCUP-US.

Review the HCUP Methods Series Report # 2003-02, Calculating Nationwide Inpatient Sample Variances. Prior to 2012, the NIS used stratified sample design similar to the NASS, so techniques appropriate for the NIS prior to 2012 are also appropriate for the NASS.
Account for clustering or nesting of observations. Discharges in the NASS are clustered, or nested, within hospital-owned facilities. Hierarchical linear modeling (HLM) is one way to account for this design aspect of the NASS. For information on using HLM, review the HCUP Methods Series Report # 2007-01, Hierarchical Modeling Using HCUP Data. Although this report references the NIS, HLM can also be applied to the NASS.
Account for missing values. Several techniques are available to assess and reduce the impact of missing data when using the NASS. One strategy is to use imputation to replace missing values with acceptable values. Another strategy is to use sample weight adjustments to compensate for missing values.

Within the NASS, the total charges data element (TOTCHG) tends to be missing more than other data elements.
For general information, review the Missing Values section of the Introduction to the NASS on the NASS Database Documentation page on HCUP-US. The percentage of encounters with missing information about total charges is reported in the Limitations of the NASS section of the Introduction to the NASS.

For detailed information on missing values, review the HCUP Methods Series Report # 2015-01, Missing Data Methods for the NIS and SID. Although the report uses examples from the NIS and the SID, the methods may also be applied to the NASS.
Calculate rates of in-scope major surgeries per population when you need to control for differences in the underlying populations. There are several sources of population data that can be used with the HCUP databases to calculate rates of in-scope major surgeries per population to improve comparisons between subgroups (e.g., region of the country). More information is available in HCUP Methods Series Reports by Topic page on HCUP-US Population Denominator Data for Use with the HCUP Databases (multiple documents; updated annually).
Estimate incidence or prevalence. The NASS can be used to estimate incidence or prevalence of both common and rare conditions in some, but not all, scenarios. For information on estimating incidence and prevalence, review the HCUP Methods Series Report # 2016-06, Using the HCUP Databases to Study Incidence and Prevalence.
Return to top
ICD-9-CM to ICD-10-CM/PCS Transition
Use current versions of HCUP Tools for ICD-10-CM coded data. ICD-10-CM coding guidance is continuing to evolve. HCUP software tools for ICD-10-CM will be updated and should be reapplied throughout the research process. For this reason, it is important to always use the most current version of these tools.

Beginning with the 2018 NASS, Clinical Classifications Software Refined (CCSR) ICD-10-CM diagnosis categories are available on the DX_PR_GRPS file. For earlier NASS data years, users can download and apply the current version of the CCSR for ICD-10-CM tool.
Consult the HCUP Tools & Software page on HCUP-US regularly for the most current versions of the HCUP software tools, including the CCSR for ICD-10-CM.
Return to top
a HCUP data users acknowledge that violation of the AHRQ confidentiality statute is subject to a civil penalty of up to $14,140 under 42 U.S.C. 299c-3(d), and that deliberately making a false statement about this or any matter within the jurisdiction of any department or agency of the Federal Government violates 18 U.S.C. 1001 and is punishable by a fine, up to five years in prison, or both. Violators of this Agreement may also be subject to penalties under state confidentiality statutes that apply to these data for particular states.

Internet Citation: Checklist for Working with the NASS. Healthcare Cost and Utilization Project (HCUP). October 2021. Agency for Healthcare Research and Quality, Rockville, MD. www.hcup-us.ahrq.gov/db/nation/nass/nasschecklist.jsp.
Are you having problems viewing or printing pages on this website?
If you have comments, suggestions, and/or questions, please contact hcup@ahrq.gov.
Privacy Notice, Viewers & Players
Last modified 10/18/21