HEALTHCARE COST AND UTLIZATION PROJECT – HCUP
A FEDERAL-STATE-INDUSTRY PARTNERSHIP IN HEALTH DATA

Sponsored by the Agency for Healthcare Research and Quality

 

 

INTRODUCTION TO

THE HCUP NATIONWIDE AMBULATORY SURGERY SAMPLE (NASS)

2019

 

 



These pages provide only an introduction to the 2019 NASS.

For full documentation and notification of changes, visit the HCUP User Support (HCUP-US) website at www.hcup-us.ahrq.gov.


 

Issued October 2021

 

Agency for Healthcare Research and Quality
Healthcare Cost and Utilization Project (HCUP)
5600 Fishers Lane
Mail Stop 7W25B
Rockville, MD 20857

 

NASS Data and Documentation Distributed through the HCUP Central Distributor:
Website: www.hcup-us.ahrq.gov
Phone: (866) 290-4287 (toll-free)
Fax: (805) 979-3787
Email: hcup@ahrq.gov



Table of Contents

Return to Introduction

HCUP DATA USE AGREEMENT REQUIREMENTS

All Healthcare Cost and Utilization Project (HCUP) data users, including data purchasers and collaborators, must complete the online HCUP Data Use Agreement (DUA) Training Course and read and sign the HCUP DUA. Proof of training completion and signed DUAs must be submitted to the HCUP Central Distributor.

Data purchasers will be required to provide their DUA training completion code and will execute their DUAs electronically as a part of the online ordering process. The DUAs and training certificates for collaborators and others with access to HCUP data should be submitted directly to the HCUP Central Distributor using the contact information below.

The online DUA Training course is available at www.hcup-us.ahrq.gov/tech_assist/dua.jsp.

The HCUP Nationwide DUA is available on the HCUP User Support (HCUP-US) website at www.hcup-us.ahrq.gov

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HCUP CONTACT INFORMATION

HCUP Central Distributor and HCUP User Support

Information about the content of the HCUP databases is available on the HCUP User Support (HCUP-US) website (www.hcup-us.ahrq.gov).

If you have questions, please review the HCUP Frequently Asked Questions located at www.hcup-us.ahrq.gov/tech_assist/faq.jsp.

If you need further technical assistance, please contact the HCUP Central Distributor and User Support team at:

Phone: 866-290-HCUP (4287) (toll free in the United States)
Email: hcup@ahrq.gov
Fax: (866) 792-5313 (toll free in the United States)

Mailing address:
HCUP Central Distributor
IBM Watson Health
5425 Hollister Ave, Suite 140
Santa Barbara, CA 93111

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WHAT'S NEW IN THE 2019 NATIONWIDE AMBULATORY SURGERY SAMPLE (NASS)?

 

  • Several refinements were made to the NASS sampling design in 2019:

    • The 2019 NASS is based on data selected from both the HCUP State Ambulatory Surgery and Services Databases (SASD) and the State Emergency Department Databases (SEDD) in order to capture both planned and emergent major ambulatory surgeries. See Section 1.1 for more information about this change.

    • The hospital-owned facility universe for the 2019 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.

    • Consistent with all other HCUP nationwide databases, the hospital-owned facility universe for the 2019 NASS 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.

    • The list of in-scope procedures was expanded to include heart value procedures, destruction of lesion or retina and choroid, excision of skin lesion, suture of skin and subcutaneous tissue, and other operations on ovary. Two in-scope procedures from the 2018 NASS were excluded: varicose vein stripping and other diagnostic procedures of female organs. See Appendix B for the list of all in-scope procedures across data years.

    • These changes will cause some discontinuity in estimates from before and after data year 2019. See Section 4.8 for more information about how this affects trending estimates over time.

  • The 2019 NASS includes several new data elements:

    • Patient race and ethnicity

    • Comorbidity measures identified by the AHRQ Elixhauser Comorbidity Software Refined for ICD-10-CM diagnosis codes.

    • Indicators that one or multiple injury-related International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnoses are reported on an encounter record were added to the Encounter File.

Return to Introduction

 

WHAT IS THE NATIONWIDE AMBULATORY SURGERY SAMPLE (NASS)

 

  • The Nationwide Ambulatory Surgery Sample (NASS) is a calendar-year, encounter-level database constructed from the Healthcare Cost and Utilization Project (HCUP) State Ambulatory Surgery and Services Databases (SASD) and State Emergency Department Databases (SEDD).1

  • The NASS is the largest all-payer ambulatory surgery database that has been constructed in the United States, yielding national estimates of major ambulatory surgery encounters performed in hospital-owned facilities. The NASS contains clinical and resource-use information that is included in a typical hospital-owned facility record, including patient characteristics, clinical diagnostic and surgical procedure codes, disposition of patients, total charges, expected source of payment, and facility characteristics.

  • Major ambulatory surgeries are identified through Healthcare Common Procedure Coding System (HCPCS) Level I codes, also known as Current Procedural Terminology (CPT®) codes. In what follows, HCPCS Level I codes will be called CPT codes for brevity. These major ambulatory surgeries are selected major therapeutic procedures that require the use of an operating room, penetrate or break the skin, and involve regional anesthesia, general anesthesia, or sedation to control pain. Procedures intended primarily for diagnostic purposes were excluded. In addition, other selection criteria were applied to major ambulatory surgeries included in the NASS and are described below.

  • A total of 35 HCUP Partner organizations contributed to the 2019 NASS: Alaska, California, Colorado, Connecticut, District of Columbia, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland, Michigan, Minnesota, Missouri, Nebraska, Nevada, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, and Wisconsin. These States are geographically dispersed and account for 83 percent of the total U.S. resident population, an estimated 68 percent sample of the universe of hospital-owned facilities, and an estimated 76 percent sample of the universe of ambulatory surgery encounters.

  • Unweighted, the NASS contains approximately 9.0 million major ambulatory surgery encounters in 2019, corresponding to approximately 11.8 million major ambulatory surgeries (some encounters have more than one major ambulatory surgery). Weighted, it estimates approximately 11.9 million major ambulatory surgery encounters and 15.7 million major ambulatory surgeries in the United States.

  • The NASS is a publicly available database that can be purchased through the HCUP Central Distributor. Currently, the NASS is available only for data years 2016-2019.

  • Users must complete the HCUP Data Use Agreement Training Course before receiving the data.

Return to Introduction

 

UNDERSTANDING THE NATIONWIDE AMBULATORY SURGERY SAMPLE (NASS)

 

  • This document, Introduction to the HCUP Nationwide Ambulatory Surgery Sample (NASS) 2019, summarizes the content of the NASS and describes the development of the NASS sample and weights.

  • Important considerations for data analysis are highlighted, and references to further resources are provided.

  • In-depth documentation for the NASS is available on the HCUP User Support (HCUP-US) website www.hcup-us.ahrq.gov. Please refer to detailed documentation before using the data.

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HEALTHCARE COST AND UTILIZATION PROJECT — HCUP
A FEDERAL-STATE-INDUSTRY PARTNERSHIP IN HEALTH DATA

Sponsored by the Agency for Healthcare Research and Quality


HCUP Nationwide Ambulatory Surgery Sample (NASS)

ABSTRACT

The Nationwide Ambulatory Surgery Sample (NASS) is part of the Healthcare Cost and Utilization Project (HCUP), which is sponsored by the Agency for Healthcare Research and Quality (AHRQ).

The NASS was created to enable analyses of selected ambulatory surgery utilization patterns and to support public health professionals, administrators, policymakers, and clinicians in their decision making regarding this critical source of care. 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, disposition of patients, total charges, expected source of payment, and facility characteristics. Therefore, it enables government entities, industry professionals, and researchers to develop research concepts with data-driven applications.

The NASS is the largest all-payer ambulatory surgery database that has been constructed in the United States, yielding national estimates of major ambulatory surgery encounters performed in hospital-owned facilities. It contains information from 9.0 million ambulatory surgery encounters at 2,958 hospital-owned facilities that approximate an estimated 68 percent stratified sample of U.S. hospital-owned facilities performing ambulatory surgeries. Weights are provided to calculate national estimates totaling 11.8 million ambulatory surgery encounters in 2019.

The NASS is drawn from statewide data organizations that provide HCUP with data from ambulatory surgery encounters. Thirty-five HCUP Partner organizations participated in the 2019 NASS. See Appendix A, Table A.1 for a list of HCUP Partner organizations that contributed to the 2019 NASS.

By stratifying on important facility characteristics, the NASS is designed to be representative of U.S. hospital-owned facilities that perform ambulatory surgeries. Stratification is based on the following characteristics:

  • Geographic region (Northeast, Midwest, South, and West)
  • Hospital bed size (small, medium, and large dependent on region, location, and teaching status)
  • Urban-rural location of the hospital (metropolitan and nonmetropolitan)
  • Hospital teaching status
  • Hospital ownership or control (public, for profit, and not for profit)

Access to the NASS is open to users who sign Data Use Agreements. Uses are limited to research and aggregate statistical reporting.

For more information on the NASS, visit the AHRQ-sponsored HCUP User Support (HCUP-US) website at www.hcup-us.ahrq.gov/db/nation/nass/nassdbdocumentation.jsp.

Return to Introduction

 

INTRODUCTION TO THE NATIONWIDE AMBULATORY SURGERY SAMPLE (NASS)

 

1. Overview of NASS Data

The Healthcare Cost and Utilization Project (HCUP) Nationwide Ambulatory Surgery Sample (NASS) was created to enable analysis of selected ambulatory surgery utilization patterns and to support public health professionals, administrators, policymakers, and clinicians in their decision making regarding this critical source of care. The NASS has many research, policy, and other data-driven applications because it contains clinical and nonclinical information about major ambulatory surgeries and diagnoses as well as geographic, facility, and patient characteristics.

1.1 NASS Data Sources, Hospitals, and Encounters

The 2019 NASS is sampled from the HCUP State Ambulatory Surgery and Services Databases (SASD) and State Emergency Departmet Databases (SEDD). The SASD which include various types of outpatient services, such as observation stays, lithotripsy, radiation therapy, imaging, chemotherapy, and labor and delivery. The specific types of ambulatory surgeries and outpatient services included in each SASD vary by State and data year. All SASD include data on ambulatory surgery encounters from hospital-owned facilities. Some States include data from nonhospital-owned facilities, although these are not included in the NASS.2. The SEDD capture emergency encounters at hospital-owned emergency departments that do not result in hospitalization. The SASD and SEDD do not include ambulatory surgery encounters that were subsequently admitted to the same hospital for inpatient care. As such, the NASS does not contain any encounters admitted to the inpatient setting from the ambulatory setting. Information on patients admitted to the hospital following ambulatory surgery is included in the HCUP State Inpatient Databases (SID).

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 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%).

The number of States, hospital-owned facilities, and ambulatory surgery encounters in the NASS varies by year (Table 1).

Table 1. Number of States, Hospital-Owned Facilities, and Encounters in the NASS by Year



Data Year States in the NASS Number of Hospital-Owned Facilities Number of AS Encounters, Unweighted Number of AS Encounters, Weighted for National Estimates
2019 AK, CA, CO, CT, DC, FL, GA, HI, IA, IL, IN, KS, KY, MD, ME, MI, MN, MO, NC, ND, NE, NJ, NV, NY, OH, OK, OR, PA, SC, SD, TN, TX, UT, VT, WI 2,958 8,994,101 11,880,487
2018 CA, CO, CT, DC, FL, GA, IA, IL, IN, KS, KY, MD, ME, MI, MN, MO, NC, ND, NE, NJ, NV, NY, OH, OK, OR, PA, SC, SD, TN, TX, VT, WI (HI and UT data were not available) 2,699 7,693,084 10,696,131
2017 CA, CO, CT, DC, FL, GA, IA, IL, IN, KS, KY, MD, ME, MI, MN, MO, NC, ND, NE, NJ, NV, NY, OH, OK, OR, PA, SC, SD, TN, TX, UT, VT, WI (HI data were not available) 2,737 7,647,636 10,570,649
2016 CA, CO, CT, DC, FL, GA, HI IA, IL, IN, KS, KY, MD, ME, MI, MN, MO, NC, ND, NE, NJ, NV, NY, OH, OK, OR, PA, SC, SD, TN, TX, UT, VT, WI 2,751 7,608,879 10,623,113
Abbreviations: AS, Ambulatory Surgery; NASS, Nationwide Ambulatory Surgery Sample.


The 2019 NASS sample comprises data from 35 HCUP Partner organizations (34 States and the District of Columbia). Appendix A, Figure A.1 represents the geographic distribution of the HCUP Partner organizations that contributed to the 2019 NASS. The HCUP NASS States with the District of Columbia account for 83 percent of the U.S. population in 2019, an estimated 68 percent of hospital-owned facilities performing ambulatory surgeries, and an estimated 76 percent of ambulatory surgery encounters. Details on the percentage of population, encounters, and facilities by region are provided in Appendix A, Table A.4 and Appendix A, Table A.5.

The NASS is limited to encounters with at least one in-scope major ambulatory surgery on the record, performed at hospital-owned facilities. In-scope major ambulatory surgeries are defined as selected therapeutic Current Procedural Terminology (CPT)-coded procedures that require the use of an operating room, penetrate or break the skin, and involve regional anesthesia, general anesthesia, or sedation to control pain. These surgeries are flagged as narrow in the HCUP Surgery Flag Software.3 They also belong to a subset of Clinical Classifications Software for Services and Procedures (CCS-Services and Procedures)4 categories with a relatively high major ambulatory surgery volume or aggregate charge total, and evidence of reliable reporting from SASD/SEDD hospitals. Detailed major ambulatory surgery selection criteria are outlined in Section 3.2. A complete list of 2019 in-scope CCS-Services and Procedure categories is included in Appendix B. The 2019 sample includes 2,958 hospitals, 8,994,101 in-scope major ambulatory surgery encounters (unweighted), and 11,880,487 in-scope major ambulatory surgery encounters (weighted for national estimates).

Although encounters are limited to those with at least one in-scope major ambulatory surgery on the record, the NASS Supplemental File provides information on other surgical and nonsurgical procedures performed during these encounters (see Section 1.3).

1.2 Data Restrictions

Some HCUP Partner organizations that contributed data to the NASS imposed restrictions on the release of certain data elements. In addition, because of confidentiality laws, some data sources were prohibited from providing HCUP with encounter records that indicated specific medical conditions, such as HIV/AIDS or behavioral health conditions. Detailed information on these restrictions is available in Appendix C.

1.3 File Structure of the NASS

The NASS is delivered as a set of related files. A hospital file lists hospitals in the NASS along with hospital attributes (e.g., teaching status, bed size category) as well as the encounter weight and sample stratum information. An encounter file links to the hospital table and contains information on the major ambulatory surgery encounter (e.g., patient age, expected source of payment, diagnoses), including information about in-scope major ambulatory surgeries. A related supplemental file contains entries for out-of-scope procedures with a key linking to the encounter file. Finally, a diagnosis and procedure group file contains information about diagnosis groupings with a key linking to the encounter file. (Note that this file is not available in the 2016 or 2017 NASS.)

Hospital File: This hospital-level file contains one observation for each hospital included in the NASS, along with encounter weight and stratum data elements. For 2019, the NASS Hospital File has 2,958 hospital-specific records. A list of data elements in the Hospital File is provided in Appendix D, Table D.1.

Encounter File: This encounter-level file contains 100 percent of ambulatory surgery encounters containing a major ambulatory surgery from hospital-owned facilities in participating States and the District of Columbia that meet facility inclusion criteria. For 2019, the NASS Encounter File has about 9.0 million ambulatory surgery encounter records (unweighted). Refer to Appendix D, Table D.2 for a list of data elements in the NASS Encounter File.

Supplemental File: This encounter-level file contains information on procedures that were performed during encounters recorded in the Encounter File but not considered to be in-scope major ambulatory surgeries in the NASS. The Supplemental File contains about 6.4 million records for 2019. Procedures included on the Supplemental File are limited to Healthcare Common Procedure Coding System (HCPCS) Level I (CPT) procedure codes. HCPCS Level II codes were excluded from the Supplemental File. Refer to Appendix D, Table D.3 for a list of data elements in the NASS Supplemental File.

Diagnosis and Procedure Groups File: Available beginning with the 2019 NASS, this encounter-level file contains information about International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnosis groups and comorbidity indicators for all diagnoses associated with encounters recorded in the Encounter File, derived from the Clinical Classifications Software Refined (CCSR) for ICD-10-CM Tool5 and Elixhauser Comorbidity Software Refined for ICD-10-CM.6 Refer to Appendix D, Table D.4 for a list of data elements in the NASS Diagnosis and Procedure Groups File.

1.4 NASS Data Elements

The coding of data elements in the NASS is consistent with the coding in other HCUP databases. The following three objectives guided the definition of data elements in all HCUP databases:

  • Ensure usability without extensive editing by analysts
  • Retain the largest amount of information available from the original sources, while still maintaining consistency among sources
  • Structure the information for efficient storage, manipulation, and analysis

More information on the coding of HCUP data elements is available on the HCUP User Support (HCUP-US) website (www.hcup-us.ahrq.gov/db/coding.jsp).

After analyzing the availability of information from the HCUP Partner organizations, a set of common fields to be available in the NASS was created. The NASS contains more than 100 clinical and nonclinical variables, such as the following:

  • Patient demographics (e.g., sex, age, race and ethnicity, urban-rural designation of residence, national quartile of the median household annual income for the patient's ZIP Code)
  • HCPCS Level I, also known as CPT procedure codes
  • ICD-10-CM diagnosis codes
  • Total charges and expected payment source (e.g., Medicare, Medicaid, private insurance, self-pay)
  • Hospital characteristics (e.g., ownership, teaching status, region of the United States)

For comprehensive information about the NASS data elements, please refer to the NASS documentation on the HCUP-US website (www.hcup-us.ahrq.gov/db/nation/nass/nassdbdocumentation.jsp).

Return to Introduction

 

2 GETTING STARTED

The HCUP NASS is 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®.

The NASS product is downloaded in a single zipped file, which contains several data-related files and accompanying documentation. The three data-related files include the following compressed files:

  1. Hospital File (NASS_2019_Hospital.zip)
  2. Encounter File (NASS_2019 Encounter.zip)
  3. Supplemental File (NASS_2019_Supplemental.zip)
  4. Diagnosis and Procedure Groups File (NASS_2019_DX_PR_GRPS.zip)

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 gigabytes (GB) of space available
  • A third-party zip utility such as ZIP Reader, SecureZIP®, WinZip®, or Stuffit Expander®
  • SAS®, SPSS®, Stata®, or similar analysis software

The total size of the CSV version of the NASS is 14.2 GB. The NASS files loaded into SAS are about 6.3 GB. Most SAS data steps will require twice the storage space of the file so that the input and output files can coexist.

With a file of this size, space easily could become a problem in a multistep program. It is not unusual to have several versions of a file marking different steps while preparing it for analysis, and there may be more versions for the actual analyses. Therefore, users should plan carefully because the amount of space required could escalate rapidly.

2.1 Decompressing the NASS Files

To extract the data files from the compressed download file, follow these steps:

  1. Create a directory for the NASS on your hard drive.
  2. Unzip the compressed NASS product file into the new directory using a third-party zip utility. This will place three compressed, encrypted data-related files in the new directory. You will be prompted to enter the encryption password (sent separately by email) to decrypt the file.

    Please note that attempts to unzip encrypted files using the built-in zip utility in Windows® (Windows Explorer) or Macintosh® (Archive Utility) will produce an error message warning of an incorrect password and/or file or folder errors. The solution is to use a third-party zip utility.

    Third-party zip utilities are available from the following reputable vendors on their official websites.
    • ZIP Reader (Windows) (free download offered by the PKWARE corporation)
    • SecureZIP for Mac or Windows (free evaluation and licensed/fee software offered by the PKWARE corporation)
    • WinZip (Windows) (evaluation and fee versions offered by the WinZip corporation)
    • Stuffit Expander (Mac) (free evaluation and licensed/fee software offered by Smith Micro corporation)
  3. Unzip each of the compressed, encrypted data-related files using the same password and third-party zip utility method. This will place the data-related CSV files in the same directory by default.

2.2 Downloading and Running the Load Programs

Programs to load the data into SAS, SPSS, or Stata are available on the HCUP-US website. To download and run the load programs, follow these steps:

  1. Go to the NASS Database Documentation page on HCUP-US at www.hcup-us.ahrq.gov/db/nation/nass/nassdbdocumentation.jsp.
  2. Go to the "File Specifications and Load Programs" section on this page.
  3. Click on "Nationwide SAS Load Programs", "Nationwide SPSS Load Programs", or "Nationwide Stata Load Programs" to go to the corresponding Load Programs page.
  4. Select the data year and the database ("NASS") from the drop-down lists on this page.
  5. Select and save the load programs you need. The load programs are specific to the data year and data-related file. Save the load programs into the same directory as the NASS CSV files on your computer.
  6. Edit and run the load programs as appropriate for your computing environment to create the analysis files. For example, modify the directory paths to point to the location of your input and output files.

NOTE: The Encounter File and Supplemental File load programs use the same variable names for the array of CPT codes (e.g., CPT1) and their associated CCS-Services and Procedures categories (e.g., CCSCPT1). When merging the Encounter and Supplemental Files, this can result in CPT and CCS-Services and Procedures codes being overwritten unintentionally in one file or the other. To avoid this problem, edit the Supplemental File load program to change the names of the CPT and CCS variable names (e.g., change CPT1 to SUPPCPT1 and CCSCPT1 to SUPCCSCPT1).

2.3 NASS Documentation

Comprehensive documentation for the NASS files is available on the HCUP-US website (www.hcup-us.ahrq.gov/db/nation/nass/nassdbdocumentation.jsp). Users of the NASS can access complete file documentation, including variable notes, file layouts, summary statistics, and related technical reports. Similarly, data users can download SAS, SPSS, and Stata load programs. These important resources help the user understand the structure and content of the NASS and aid in using the database. Appendix A, Table A.2 details the comprehensive NASS documentation available on HCUP-US.

2.4 HCUP Online Tutorials

For additional assistance, the Agency for Healthcare Research and Quality (AHRQ) has created the HCUP Online Tutorial Series, a series of free, interactive courses that provide information on using HCUP data and tools and training on technical methods for conducting research with HCUP data. Topics include an HCUP Overview Course and these tutorials:

  • The Load and Check HCUP Data tutorial provides instructions on how to unzip (decompress) HCUP data, save it on your computer, and load the data into a standard statistical software package. This tutorial also describes how to verify that the data have loaded correctly.
  • The HCUP Sample Design tutorial is designed to help users learn how to account for sample design in their work with the HCUP nationwide databases. The tutorial will be updated in the future to directly address the NASS sampling design.
  • The Producing National HCUP Estimates tutorial is designed to help users understand how three of the nationwide databases—the National (Nationwide) (NIS), the Nationwide Emergency Department Sample (NEDS), and the Kids' Inpatient Database (KID)—can be used to produce national and regional estimates. A tutorial specific to the NASS database will be added in the future.
  • The Calculating Standard Errors tutorial shows how to accurately determine the precision of the estimates produced from the HCUP nationwide databases. Users will learn two methods for calculating standard errors for estimates produced from the HCUP nationwide databases.
  • The HCUP Software Tools tutorial introduces users to the HCUP software tools, which can be applied to HCUP and other administrative databases to create new data elements from existing data, thereby enhancing a researcher's ability to conduct analyses. There are four modules within this course grouping the HCUP tools by the following coding systems: ICD-10-CM diagnoses, ICD-10-PCS procedures, CPT and HCPCS Level II codes, and ICD-9-CM diagnoses and procedures. Users will learn about the purpose of each tool and receive technical guidance for applying the tools to their data.

Other tutorials about the design or use of the HCUP databases are also available, and new tutorials are added periodically. The Online Tutorial Series is located on the HCUP-US website at www.hcup-us.ahrq.gov/tech_assist/tutorials.jsp.

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3 METHODS

3.1 Creation of the NASS

Creation of the NASS requires the following steps:

  • Identify in-scope major ambulatory surgeries. The HCUP Surgery Flag Software (see Section 3.2) is used to identify major ambulatory surgeries as those with a taxonomy category of narrow. These are major therapuetic procedures that require the use of an operating room, penetrate or break the skin, and involve regional anesthesia, general anesthesia, or sedation to control pain. Empirical selection criteria then are used to define qualifying CCS Services and Procedures7 categories or in-scope major ambulatory surgeries. Selection criteria for a given CCS-Services and Procedures category include meeting volume or charge thresholds (i.e., the surgery accounts for at least .05% of total major ambulatory surgery volume in the SASD/SEDD or at least .05% of total charges associated with major ambulatory surgery encounters in the SASD/SEDD) and evidence that SASD/SEDD hospitals are reliably reporting major ambulatory surgeries in the CCS-Services and Procedures category.
  • Build the NASS hospital sampling frame. The NASS sampling frame is limited to facilities owned by community hospitals (excluding rehabilitaiton and long-term acute care hospitals) in the SASD and SEDD8 that perform in-scope major ambulatory surgeries. Additional restrictions imposed for the NASS sampling frame were that the hospital have no gross irregularities in quarterly reporting volume, submit data to the SASD/SEDD in all four quarters of 2019, and not have an unusually low volume of encounters containing an in-scope major ambulatory surgery.
  • Build encounter predictive models. NASS sampling frame hospitals are used to create models for volumes of encounters containing in-scope major ambulatory surgeries. The predictive model can be applied to hospitals outside the NASS sampling frame.
  • Construct the universe of hospitals and ambulatory surgery encounters. A national list of all hospitals performing ambulatory surgeries is created using SASD/SEDD and the American Hospital Association (AHA), consistent with the approach used for other HCUP nationwide databases. (Prior to data year 2019, the Centers for Medicare & Medicaid Services (CMS) Provider of Services (POS) file was also used.) The encounter predictive model is applied to hospitals outside the NASS sampling frame and then combined with observed data from sampling frame hospitals to create national encounter volume estimates.
  • Develop NASS sample strata. Strata are created using hospital characteristics. When needed, strata are collapsed to achieve reasonable hospital sample/universe ratios.
  • Compute sample weights. Hospitals are sampled from the NASS sampling frame. For 2019, 100 percent of hospitals were sampled from the frame. On the basis of the set of NASS sample hospitals and universe totals, sample weights are computed that project the NASS encounters and major ambulatory surgeries to the universe.
  • Create the NASS database. All of the previous steps culminate in a NASS database, which is a set of four related files: Hospital, Encounter, Supplemental and Diagnosis and Procedure Groups.

The relationship between the NASS universe, the SASD/SEDD sample, and the NASS sampling frame is portrayed in Figure 1. The predictive model for hospital major ambulatory surgery encounter volume is developed using the NASS sampling frame hospitals and then is applied to all other hospitals not in the sampling frame to generate the encounter universe. In 2019, the NASS sample and sampling frame were identical because 100 percent of hospitals were sampled.

Figure 1. NASS Hospital Universe, SASD/SEDD Sample, and NASS Sampling Frame

Abbreviations: NASS, Nationwide Ambulatory Surgery Sample; SASD, State Ambulatory Surgery and Services Databases; SEDD, State Emergency Department Databases.

Four concentric circles illustrating the relationship between the NASS universe, the SASD sample, and the NASS sampling frame. The innermost, smallest circle represents the NASS hospital sample. The next largest circle represents the NASS hospital sampling frame. The next largest circle represents the SASD/SEDD hospital sample. The outermost, largest circle represents the NASS universe (i.e., the Ambulatory Surgery National Hospital List).

Return to Introduction

 

The following sections describe several of the NASS development steps in greater detail.

3.2 Selection of Major Ambulatory Surgeries

3.2.1 Definition of Major Ambulatory Surgery

HCUP Surgery Flag Software9 was used to identify surgical procedures of interest for the NASS. The Surgery Flag Software processes CPT procedure codes and classifies them as narrow, broad, or neither. The NASS will focus on surgeries in the narrow class, or major ambulatory surgeries. The narrowly defined flag (narrow flag) is the most targeted and restrictive surgical identifier, consisting of major therapeutic procedures. A narrow procedure (1) requires the use of an operating room, (2) penetrates or breaks the skin, and (3) involves regional anesthesia, general anesthesia, or sedation to control pain. Some common narrow procedures are cataract surgery and cholecystectomy. Other examples include appendectomy, gastric bypass, hysterectomy, hernia repair, and spinal fusion. 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: biopsies if the procedure is within an internal organ (e.g., brain, deep cervical node, stomach), thoracotomy with or without biopsy, and exploratory laparotomy with or without biopsy.

3.2.2 Selection of In-Scope Major Ambulatory Surgeries

Several selection criteria were used to define in-scope major ambulatory surgeries for the NASS. Prior to application of selection criteria, all major ambulatory surgeries identified by HCUP Surgery Flag Software were grouped in categories defined by CCS for Services and Procedures.10

Beginning with data year 2019, the following criteria were applied at the CCS-Services and Procedures category level:

  1. Volume and charges. The ambulatory surgery accounts for at least .05% of total major ambulatory surgery volume in the SASD/SEDD or at least .05% of total charges associated with major ambulatory surgery encounters in the SASD/SEDD.
  2. Reporting quality. Hospitals are reliably submitting major ambulatory surgery data. Four CCS-Services and Procedures categories are excluded because SASD/SEDD hospital data showed evidence of unreliable reporting or underreporting of dental services, skin grafts, wound debridement, and percutaneous transluminal coronary angioplasty (PTCA).11

The final set of included, or in-scope, CCS-Services and Procedures categories for the 2019 NASS is provided in Appendix B. Appendix B also includes a running list of changes to the in-scope procedure groups over time.

Prior to data year 2019, the following criteria were applied at the CCS-Services and Procedures category level:

  1. Hospital share. A substantial share of major ambulatory surgeries occurs in hospital-owned facilities (at least 25 percent of major ambulatory surgeries for the CCS-Services and Procedures category).
  2. Volume. A relatively high major ambulatory surgery volume is observed in the SASD (4,000 surgeries annually).
  3. Reporting quality. Hospitals are reliably submitting major ambulatory surgery data.

Figure 2 illustrates the relationship between ambulatory surgeries, major ambulatory surgeries, and in-scope major ambulatory surgeries.

Note that although encounters are limited to those with at least one in-scope major ambulatory surgery on the record, the NASS Supplemental File provides information on other (or out-of-scope) procedures performed during these encounters.

In the remainder of this document, we use the term major ambulatory surgery as synonymous with in-scope major ambulatory surgery for brevity.

Figure 2. Ambulatory Surgeries, Major Ambulatory Surgeries, and In-Scope Major Ambulatory Surgeries

Abbreviation: CPT, Current Procedural Terminology.

Three overlapping circles illustrating the relationship between ambulatory surgeries, major ambulatory surgeries, and in-scope major ambulatory surgeries. The smallest circle represents in-scope major ambulatory surgeries (i.e., those meeting volume or charge thresholds and passing reporting quality screens). The next largest circle, which encapsulates the smallest circle, represents major ambulatory surgeries (i.e., CPT codes tagged as "narrow"). The largest circle, which encapsulates the smaller two circles, represents ambulatory surgeries (i.e., CPT codes tagged as "narrow" or "broad" surgeries).

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3.3 Sampling Design of the NASS

The NASS is a stratified cluster sample of major ambulatory surgery encounters (see Section 3.2) occurring in hospital-owned facilities. The main objective of a stratified sample is to ensure that it is representative of the target universe with respect to factors in the stratification scheme. In this section, we summarize the NASS setting and universe definition, the process for constructing the sampling frame, the sample strata, the sampling plan, and the calculation of sample weights.

3.3.1 Ambulatory Surgery Setting and Universe Definition

Ambulatory surgeries are performed in hospital-owned facilities, nonhospital-owned ambulatory surgery centers (ASCs), or office settings. In this context, office means a place of service that is neither a hospital-owned facility nor an ASC. The office setting may include professional facilities with procedure rooms or surgical suites.

HCUP Partners provide information on ambulatory surgeries in hospital-owned facilities. About half of the Partners also provide ambulatory surgery data from facilities that are not hospital owned. The designation of a facility as hospital owned is specific to its financial relationship with a hospital that provides inpatient care and is not related to its physical location. Hospital-owned ambulatory surgery and other outpatient care facilities may be contained within the hospital, physically attached to the hospital, or located in a different geographic area. The designation as hospital owned means that HCUP can verify that the hospital is billing for this service.

The NASS is restricted to major ambulatory surgeries performed in the hospital-owned facilities, either in the hospital itself or in physically separate hospital-owned facilities. There are two reasons for this restriction. First, the SASD have more than twice as many hospital-owned facilities as facilities that are not hospital owned. Second, although the HCUP hospital sampling frame is well understood, much less is known about the HCUP sample of surgery facilities that are not hospital owned compared with all freestanding ASCs.

In addition to restricting attention to the hospital-owned facilities, facilities are limitied to U.S. community hospitals, defined as "all non-Federal, short-term, general, and other specialty hospitals, excluding hospital units of institutions."12 Noncommunity hospitals are excluded because of inconsistent capture of data across HCUP States. Additionally, community hospitals that are either rehabilitation or long-term acute care (LTAC) facilities are excluded because these hospitals treat a unique patient population that has longer stays and higher costs.

A key challenge for the NASS design is the creation of national major ambulatory surgery encounter volume estimates (encounter universe), tabulated in strata used in the sampling design. National estimates do not exist for several reasons, but the most important is the definition of ambulatory surgery itself. Organizations collecting survey information from hospitals, such as the AHA, rely on verbal descriptions of ambulatory surgery.13 These descriptions leave room for interpretation and result in significant variation in which encounters hospitals report as ambulatory surgeries. In contrast, hospitals generally report total inpatient admissions, births, and emergency department visits with reasonable accuracy.

The NASS uses a CPT code- and data-based definition of major ambulatory surgeries (see Section 3.2). Self-reported hospital ambulatory surgery volumes from the AHA may or may not be consistent with the HCUP definition of major ambulatory surgeries, and it is challenging to ascertain that consistency. Consequently, rather than using an external reference source for major ambulatory surgery volumes, the NASS universe of major ambulatory surgery encounters was constructed by combining observed encounter volumes for hospitals in the NASS sampling frame and estimated encounter volumes for all other hospitals performing major ambulatory surgeries. Estimated encounter volumes were generated using a predictive model, described in Section 3.4.1.

3.3.2 Generating the Ambulatory Surgery National Hospital List

A crucial step in developing the NASS was generating a list of hospitals performing major ambulatory surgery outside the NASS sampling frame and hospital-specific predictor variables to compute estimated encounters using a predictive model (see Section 3.4.1).

Hospitals were included in the national list if they were a community hospital. Rehabilitation and LTAC hospitals were excluded. Hospitals reporting no outpatient surgeries in the AHA Annual Survey were then excluded from the national list.

Model predictor variables were obtained from the AHA Annual Survey (for HCUP SASD/SEDD hospitals and hospitals reporting outpatient surgeries in the AHA Annual Survey). See Table 2 for a description of the predictor variables obtained from AHA.

3.4 NASS Sampling Frame

Selection of SASD/SEDD hospitals for the NASS sampling frame was limited to facilities owned by community hospitals, excluding rehabilitation and LTAC hospitals.14

Additional restrictions imposed for the NASS sampling frame were that the hospital (1) have no gross irregularities in quarterly reporting volume, (2) submit data to the SASD/SEDD in all four quarters of 2019, and (3) not have an unusually low volume of encounters containing an in-scope major ambulatory surgery.

A comparison between the NASS hospital universe and the final NASS sample is provided in Appendix A, Table A.3.

3.4.1 NASS Encounter Predictive Model

Creation of the major ambulatory surgery encounter universe requires a method for estimating the volume of encounters containing major ambulatory surgeries for hospitals outside the NASS sampling frame. This estimation was accomplished by building a predictive model for encounters using data for the 2,958 hospitals in the NASS sampling frame.

The hospital-specific number of encounters containing at least one in-scope major ambulatory surgery was the outcome variable in the model. A model predicting the number of major ambulatory surgery encounters per hospital was developed using the NASS sampling frame hospitals. Predictor variables used in the model are reported in Table 2.

Table 2. Independent Variables Included in Encounter Predictive Model

Category Independent Variable
Ownership Voluntary, not for profit
Proprietary, for profit
Local or State governments
Location and teaching status Rural location
Urban nonteaching
Urban teaching
Census region Midwest
Northeast
South
West
Hospital size (number of beds) 001-024
025-049
050-099
100-199
200-299
300-399
400-499
500+
AHA Annual Survey: self-reported outpatient surgery volume Log scale
Abbreviation: AHA, American Hospital Association.

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3.4.2 NASS Sampling Strata

Sampling strata were selected using results from the encounter predictive model (which quantify the importance of stratification factors in encounter volume variation) and a study of the current NIS and NEDS stratification schemes. Table 3 contains values for the NASS stratification variables: census region, bed size category,15 location and teaching status, and ownership.16 There are 108 possible strata (i.e., unique combinations of region, bed size, location/teaching status, and ownership categories).

Table 3. NASS Stratification Variables

Stratum Code Label
Census region 1 North
2 Midwest
3 South
4 West
Bed size category 1 Small (depends on region, location, and teaching status)
2 Medium (depends on region, location, and teaching status)
3 Large (depends on region, location, and teaching status)
Location and teaching status 1 Rural
2 Urban nonteaching
3 Urban teaching
Ownership 1 Local and State government
2 Voluntary, not for profit
3 Proprietary, for profit
Abbreviation: NASS, Nationwide Ambulatory Surgery Sample.

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A goal was established to have at least 10 hospitals assigned to each stratum with as many strata as possible having a sampling fraction greater than .20. 17

Assignment of hospitals to the initial stratification scheme of 108 levels results in a number of strata with fewer than 10 sampling frame hospitals small sampling fractions. In those cases, ownership category was collapsed, first by combining local and State government with voluntary hospitals.18 If the goal still was not achieved, all the ownership types were combined. Other stratum combinations were constructed manually, using the following conventions:

  • Require that the region dimension persist.
  • Examine the strata with deficient ratios or sample sizes. On the basis of the data, elect to collapse the location and teaching or bed size dimensions using the following rules:
    • Collapse location and teaching into two categories (urban and rural nonteaching vs. teaching) or collapse the entire location and teaching dimension if required.
    • Collapse the entire bed size dimension if required.

We used judgment when manually collapsing the strata to ensure that no single stratum had a large percentage of total encounter or hospital volume. This led to relaxing the number of sampling frame hospitals or sampling fraction criteria for some strata. After manual adjustments, the NASS had 62 strata for 2019. In the end, all strata had at least 10 hospitals and sampling fractions greater than .20.

3.5 Sample Weights

To obtain nationwide estimates, encounter weights were developed combining the NASS universe of hospitals and encounters with the NASS sample hospitals and encounters.

Computation of sample weights is straightforward. Given a universe of encounter volumes in stratum s, the sample weight is computed as the ratio of NASS universe to sample encounter volumes so that the sample volume is inflated to agree with the universe volume within the stratum.

3.5.1 Frame Sampling Rate

For the 2019 NASS, all hospitals in the sampling frame were selected for inclusion in the NASS, resulting in an approximate 68 percent sample of universe hospitals.

3.5.2 Encounter Weights

Encounter weights were calculated by stratum. Within stratum s for hospital i, the universe weight for each encounter in the NASS sample was calculated as follows:

Wis(universe) = [Ns(universe) ÷ Ns(sample)] * (4 ÷ Qi),

where Wis(universe) is the encounter weight, Ns(universe) represents the number of ambulatory surgery encounters in the universe within stratum s, Ns(sample) is the number of ambulatory surgery encounters from sample hospitals selected for the NASS, and Qi represents the number of quarters of ambulatory surgery encounters contributed by hospital i to the NASS (for the 2018 NASS, Qi = 4 for all hospitals). Thus, each encounter's weight is equal to the number of universe ambulatory surgery encounters it represents in stratum s during that year. Wis(universe) is named DISCWT in the NASS encounter table (see Appendix D, Table D.2).

The 2019 NASS sampling frame required that all hospitals qualifying for the frame submit data in all four quarters of 2019.

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4 HOW TO USE THE NASS FOR DATA ANALYSIS

This section provides a synopsis of special considerations for using the NASS.

4.1 Calculating National Estimates

To produce national estimates, weights MUST be used.

The major ambulatory surgery encounter weight (DISCWT) in the NASS Encounter Table should be used for producing nationwide, encounter-level statistics where the ambulatory surgery encounter is the unit of analysis.

Because the NASS is a stratified cluster sample, proper statistical techniques must be used to calculate standard errors and confidence intervals. For detailed instructions, refer to the HCUP Methods Series report #2003-02, Calculating Nationwide Inpatient Sample (NIS) Variances for Data Years 2011 and Earlier, on the HCUP-US website (www.hcup-us.ahrq.gov/). The NASS used a stratified sampling design similar to the HCUP NIS prior to 2012, so techniques appropriate for the NIS prior to 2012 are also appropriate for the NASS

4.2 Choosing Data Elements for Analysis

For all data elements to be used in the analysis, the analyst first should perform descriptive statistics and examine the range of values, including the number of missing cases. When anomalies (such as large numbers of missing cases) are detected, descriptive statistics can be computed by region for that variable to determine whether there are region-specific differences. Sometimes, computing descriptive statistics by hospital can be helpful in detecting hospital-specific data anomalies.

4.3 ICD-10-CM Diagnosis Codes and CPT Procedure Codes

Each unique analysis should consider limitations related to ICD-10-CM and CPT procedure codes.

  • CPT procedure codes, which are copyrighted by the American Medical Association, can change each year in January. It is essential to check all procedure codes used for analysis to ensure that the codes are in effect during the time period(s) studied.
  • ICD-10-CM diagnosis codes provide valuable insights into the reasons for hospitalization and what procedures patients receive, but these codes need to be carefully used and interpreted. ICD-10-CM codes change every October as new codes are introduced and some codes are retired. It is critical to check all ICD-10-CM codes used for analysis to ensure that the codes are in effect during the time period studied.
  • The NASS contains fields for up to 15 diagnoses, up to 30 in-scope HCPCS Level I or CPT-coded procedures, and up to 30 out-of-scope HCPCS Level I or CPT-coded procedures, although the number of code fields populated varies by State because of reporting differences. Some States provide more than the maximum code fields retained on the NASS. To reduce the file size of the NASS, the number of codes retained was limited. For 2019, approximately 7.5 percent of all ambulatory surgery records report more fields than the maximum allowed on the NASS.

4.4 Missing Values

Missing data values can compromise the quality of estimates. For example, if the outcome for ambulatory surgery encounters with missing values is different from the outcome for ambulatory surgery encounters with valid values, then sample estimates for that outcome will be biased and inaccurately represent the ambulatory surgery utilization patterns. Several techniques are available to help overcome this bias. 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. Descriptions of such data preparation and adjustment are outside the scope of this report; however, it is recommended that researchers evaluate and adjust for missing data, if necessary.

Alternatively, if the cases with and without missing values are assumed to be similar with respect to their outcomes, no adjustment may be necessary for estimates of means and rates because the nonmissing cases would be representative of the missing cases. However, some adjustment still may be necessary for the estimates of totals. Sums of data elements (such as aggregate ambulatory surgery charges) containing missing values would be incomplete because cases with missing values would be omitted from the calculations. Estimates of the sum of charges should use the product of the number of cases times the average charge to account for records with missing information.

4.5 Variance Calculations

It may be important for researchers to calculate a measure of precision for some estimates based on the NASS sample data. Variance estimates must account for both the sampling design and the form of the statistic. The NASS sampling design consists of a stratified, single-stage cluster sample. A stratified random sample of hospitals (clusters) providing major ambulatory surgeries was drawn, and then all encounters with in-scope major ambulatory surgeries were included from each selected hospital. To accurately calculate variances from the NASS, appropriate statistical software and techniques must be used. For detailed instructions, refer to the HCUP Methods Series report #2003-02, Calculating Nationwide Inpatient Sample (NIS) Variances for Data Years 2011 and Earlier, on the HCUP-US website (www.hcup-us.ahrq.gov/). The NASS used a stratified sampling design similar to the HCUP NIS prior to 2012, so techniques appropriate for the NIS prior to 2012 are also appropriate for the NASS.

If hospitals inside the sampling frame are like hospitals outside the frame, the sample hospitals can be treated as if they were randomly selected from the entire universe of hospitals within each stratum. Standard formulas for a stratified, single-stage cluster sample without replacement could be used to calculate statistics and their variances in most applications.

A multitude of statistics can be estimated from the NASS data. Several computer programs that calculate statistics and their variances from sample survey data are listed in Section 4.6. Some of these programs use general methods of variance calculations (e.g., the jackknife and balanced half-sample replications) that account for the sampling design. However, it may be desirable to calculate variances using formulas specifically developed for certain statistics.

These variance calculations are based on finite-sample theory, which is an appropriate method for obtaining cross-sectional, nationwide estimates of outcomes. According to finite-sample theory, the intent of the estimation process is to obtain estimates that are precise representations of the nationwide population at a specific point in time. In the context of the NASS, any estimates that attempt to accurately describe characteristics and interrelationships among hospitals and ambulatory surgery encounters during a specific year should be governed by finite-sample theory. Examples include estimates of expenditure and utilization patterns.

Alternatively, in the study of hypothetical population outcomes not limited to a specific point in time, the concept of a superpopulation may be useful. Analysts may be less interested in specific characteristics of the finite population (and time period) from which the sample was drawn than they are in hypothetical characteristics of a conceptual superpopulation from which any particular finite population in a given year might have been drawn. According to this superpopulation model, the nationwide population in a given year is only a snapshot in time of the possible interrelationships among hospital, market, discharge, encounter, or visit characteristics. In a given year, all possible interactions between such characteristics may not have been observed, but analysts may wish to predict or simulate interrelationships that may occur in the future.

Under the finite-population model, the variances of estimates approach zero as the sampling fraction approaches one. This is the case because the population is defined at that point in time and because the estimate is for a characteristic as it existed when sampled. This is in contrast to the superpopulation model, which adopts a stochastic viewpoint rather than a deterministic viewpoint. That is, the nationwide population in a particular year is viewed as a random sample of some underlying superpopulation over time. Different methods are used for calculating variances under the two sample theories. The choice of an appropriate method for calculating variances for nationwide estimates depends on the type of measure and the intent of the estimation process.

4.6 Computer Software for Weighted and Variance Calculations

Computer programs are readily available to perform weighted variance calculations. Several statistical programming packages allow weighted analyses.19 For example, nearly all SAS procedures incorporate weights. In addition, several statistical analysis programs have been developed to specifically calculate statistics and their standard errors from survey data. Version 8 or later of SAS contains procedures (PROC SURVEYMEANS and PROC SURVEYREG) for calculating statistics on the basis of specific sampling designs. Stata and SUDAAN® are two other common statistical software packages that perform calculations for numerous statistics arising from the stratified, single-stage cluster sampling design. Examples of the use of SAS, SUDAAN, and Stata to calculate NIS variances are presented in the special report Calculating Nationwide Inpatient Sample (NIS) Variances for Data Years 2011 and Earlier, on the HCUP-US website (www.hcup-us.ahrq.gov/). For a helpful review of programs to calculate statistics from survey data, visit the following website: www.hcp.med.harvard.edu/statistics/survey-soft/. Exit Disclaimer

The NASS includes a Hospital File with variables required by these programs to calculate finite-population statistics. The file includes synthetic hospital identifiers (Primary Sampling Units, or PSUs), stratification variables, and stratum-specific totals for the numbers of ambulatory surgery encounters and hospitals so that finite-population corrections can be applied to variance estimates.

In addition to these subroutines, standard errors can be estimated by validation and cross-validation techniques. Depending on the analysis problem, a large number of observations may be available, and it may be feasible to set aside a part of the data for validation purposes. Standard errors and confidence intervals then can be calculated from the validation data.

If the analytic file is too small to set aside a large validation sample, cross-validation techniques may be used. For example, tenfold cross-validation would split the data into 10 subsets of equal size. The estimation would take place in 10 iterations. In each iteration, the outcome of interest is predicted for one-tenth of the observations by an estimate based on a model that is fit to the other nine-tenths of the observations. Unbiased estimates of error variance then are obtained by comparing the actual values to the predicted values obtained in this manner.

4.7 Limitations of the NASS

The NASS contains about 9.0 million ambulatory surgery encounter records and many clinical and nonclinical data elements. Many research studies can be conducted with the data, but some limitations should be considered:

The NASS contains encounter-level records, not patient-level records. This means that individual patients who visit a hospital facility for ambulatory surgery multiple times in 1 year may be present in the NASS multiple times. No uniform patient identifier is available that would allow a patient-level analysis to identify individuals with more than one ambulatory surgery encounter or to track outcomes or additional follow-up care received after an encounter. In contrast, some HCUP State databases may be used for this type of analysis.

The database includes only HCPCS Level I or CPT codes. HCPCS Level II codes are excluded.

4.8 Considerations for Trending Over Time

When conducting longitudinal analyses, users should exercise caution and consider several aspects of the NASS design and changes to the design over time.

  • Procedures considered in scope for the NASS sample can change from year to year (see Appendix B). These changes may result from an increase or decrease in the volume of procedures performed in the outpatient setting, as this determines whether a CCS-Services and Procedures category meets the threshold for inclusion in the NASS sample. Additionally, the 2018 NASS applied updated versions of the HCUP Surgery Flag Software for Services and Procedures (that expanded the range of possible CPT codes) and the CCS Services and Procedures Tool. Combined, these updates contributed to changes in the CPT procedures and CCS-Services and Procedures categories defined as in-scope for the NASS sample. The NASS in-scope procedure criteria also changed between data years 2018 and 2019, which resulted in some changes to the CCS-Services and Procedures categories considered in scope for the NASS.

  • Earlier years of the NASS (2016-2018) undercount certain emergent surgeries. 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.

  • The hospital-owned facility universe for the NASS changed between data years 2018 and 2019. First, the universe was expanded to include specialty hospitals such as surgical, cancer, heart, and orthopedic facilities owned by community hospitals that performed in-scope major ambulatory surgeries. This resulted in volume increases in certain surgeries commonly performed in these types of facilities. Second, the universe was limited to hospitals included in the AHA Annual Survey that reported performing outpatient surgeries. In prior years, the CMS POS data were used to augment the information.

Refer to Appendix E for a summary of CCS-Services and Procedures procedure category totals in the 2016-2019 NASS and contributing reasons for large changes over time. For the subset of CCS categories affected by NASS design changes, trend analyses based on CCS-Services and Procedure category are not recommended.

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5 USER FEEDBACK AND QUESTIONS

The NASS is a new HCUP database, with 2016-2019 as the first publicly available data years. To optimize the usefulness of the data and related documentation, HCUP would like to hear from data users regarding any suggestions, comments, or issues. Please contact HCUP User Support at hcup@ahrq.gov or (866) 290-HCUP (4287).

Appendix A: NASS Introductory Information

Table A.1. HCUP Partner Organizations Participating in the 2019 NASS

State Data Organization
Alaska Alaska Department of Health and Social Services
Alaska State Hospital and Nursing Home Association
California California Office of Statewide Health Planning & Development
Colorado Colorado Hospital Association
Connecticut Connecticut Hospital Association
District of Columbia District of Columbia Hospital Association
Florida Florida Agency for Health Care Administration
Georgia Georgia Hospital Association
Hawaii Hawaii Laulima Data Alliance
Hawaii University of Hawai'i at Hilo
Iowa Iowa Hospital Association
Illinois Illinois Department of Public Health
Indiana Indiana Hospital Association
Kansas Kansas Hospital Association
Kentucky Kentucky Cabinet for Health and Family Services
Maryland Maryland Health Services Cost Review Commission
Maine Maine Health Data Organization
Michigan Michigan Health & Hospital Association
Minnesota Minnesota Hospital Association
Missouri Missouri Hospital Industry Data Institute
North Carolina North Carolina Department of Health and Human Services
North Dakota North Dakota (data provided by the Minnesota Hospital Association)
Nebraska Nebraska Hospital Association
New Jersey New Jersey Department of Health
Nevada Nevada Department of Health and Human Services
New York New York State Department of Health
Ohio Ohio Hospital Association
Oklahoma Oklahoma State Department of Health
Oregon Oregon Association of Hospitals and Health Systems
Oregon Office of Health Analytics
Pennsylvania Pennsylvania Health Care Cost Containment Council
South Carolina South Carolina Revenue and Fiscal Affairs Office
South Dakota South Dakota Association of Healthcare Organizations
Tennessee Tennessee Hospital Association
Texas Texas Department of State Health Services
Utah Utah Department of Health
Vermont Vermont Association of Hospitals and Health Systems
Wisconsin Wisconsin Department of Health Services
Abbreviation: HCUP, Healthcare Cost and Utilization Project; NASS, Nationwide Ambulatory Surgery Sample.

 

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Figure A.1. HCUP States and the District of Columbia Included in the 2018 NASS

Abbreviation: HCUP, Healthcare Cost and Utilization Project; NASS, Nationwide Ambulatory Surgery Sample.

(A map of the United States showing which HCUP States are included in the 2019 NASS. Shaded States represent the 35 Partners that provide Ambulatory Surgery and Services data included in the NASS (Alaska, California, Colorado, Connecticut, District of Columbia, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kansas, Kentucky, Maine, Maryland, Michigan, Minnesota, Missouri, Nebraska, Nevada, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, and Wisconsin.) Non participating states are Alabama, Arkansas, Arizona, Delaware, Idaho, Louisiana, Massachusetts, Mississippi, Montana, New Hampshire, New Mexico, Rhode Island, Virginia, West Virginia, Washington, Wyoming,.

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Table A.2. NASS-Related Reports and Database Documentation Available on the HCUP-US Website

Description of the NASS Database
Restrictions on the Use
File Specifications and Load Programs
Data Elements
Additional Resources for NASS Data Elements
  NASS

HCUP Tools: Labels and Formats


Obtaining HCUP Data

Abbreviation: CCSR, Clinical Classification Software Refined; HCUP, Healthcare Cost and Utilization Project; ICD-10-CM/PCS, International Classification of Diseases, Tenth Revision, Clinical Modification/Procedure Coding System; NASS, Nationwide Ambulatory Surgery Sample; US, User Support.

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Table A.3. NASS Target Universe, Sampling Frame, and Final Sample Characteristics, 2019

Sample Description Number of Hospitals Providing Outpatient Surgery Number of In-Scope Major Ambulatory Surgery Encounters
2019 target universe Community hospitals (excluding rehabilitation and LTAC) 4,362a 11,880,487b
2019 NASS Sample of target universe drawn from the sampling frame 2,958 8,994,101
Abbreviation: LTAC, long-term acute care; NASS, Nationwide Ambulatory Surgery Sample.
a Estimated. See section 3.3.2.
b Estimated. See section 3.4.1.

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Table A.4. Percentage of Population in NASS Sample, by Census Region, 2019

Census Region 2019 Population States in NASS Sampling Frame 2019 Population States Not in NASS Sampling Frame Total 2019 Population States in NASS Sampling Frame: Percent of Total Population
Northeast 46,671,228 9,311,575 55,982,803 83.4
Midwest 68,329,004 0 68,329,004 100.0
South 98,733,086 26,847,362 125,580,448 78.6
West 57,922,227 20,425,041 78,347,268 73.9
Total 271,655,545 56,583,978 328,239,523 82.8
Abbreviation: NASS, Nationwide Ambulatory Surgery Sample.
Source: Annual Estimates of the Resident Population for the United States, Regions, States, and Puerto Rico: April 1, 2010 to July 1, 2020 (NST-EST2020), Population Division, U.S. Census Bureau, www.census.gov/programs-surveys/popest/technical-documentation/research/evaluation-estimates/2020-evaluation-estimates/2010s-state-total.html. Exit Disclaimer Accessed September 14, 2021.

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Table A.5. Percentage of Encounters and Facilities in NASS Sample, by Census Region, 2019



Census Region Encounters Facilities
No. of Ambulatory Surgery Encounters (Unweighted) No. of Ambulatory Surgery Encounters (Weighted)a Unweighted Encounters: Weighted Encounters, % No. of NASS Sample Hospitals No. of Hospitals Performing Ambulatory Surgeryb NASS Sample Hospitals: Hospitals Performing Ambulatory Surgery, %
Northeast 1,624,579 2,051,945 79.2 416 548 75.9
Midwest 2,833,662 3,064,307 92.5 1,095 1,336 82.0
South 2,978,600 4,332,331 68.8 956 1,591 60.1
West 1,557,260 2,431,904 64.0 491 887 55.4
Total 8,994,101 11,880,487 75.7 2,958 4,362 67.8
Abbreviation: NASS, Nationwide Ambulatory Surgery Sample.
a Estimated. See section 3.3.2.
b Estimated. See section 3.4.1.

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Appendix B: 2019 NASS In-Scope Major Ambulatory Surgeries

Table B.1. NASS In-Scope Clinical Classifications Software (CCS) for Services and Procedure Categories

CCS for Services and Procedures Category Description Data Years in Scope for the NASS Sample
003 Laminectomy, excision intervertebral disc 2016-2019
006 Decompression peripheral nerve 2016-2019
009 Other OR therapeutic nervous system procedures 2016-2019
010 Thyroidectomy, partial or complete 2016-2019
012 Other therapeutic endocrine procedures 2016-2019
013 Corneal transplant 2016-2019
014 Glaucoma procedures 2018-2019
015 Lens and cataract procedures 2016-2019
016 Repair of retinal tear, detachment 2016-2019
017 Destruction of lesion of retina and choroid 2019
019 Other therapeutic procedures on eyelids, conjunctiva, cornea 2018-2019
020 Other intraocular therapeutic procedures 2018-2019
021 Other extraocular muscle and orbit therapeutic procedures 2016-2019
022 Tympanoplasty 2016-2019
023 Myringotomy 2016-2019
024 Mastoidectomy 2016-2019
026 Other therapeutic ear procedures 2016-2019
028 Plastic procedures on nose 2016-2019
030 Tonsillectomy and/or adenoidectomy 2016-2019
033 Other OR therapeutic procedures on nose, mouth and pharynx 2016-2019
042 Other OR therapeutic procedures on respiratory system 2016-2019
043 Heart valve procedures 2019
045 Percutaneous transluminal coronary angioplasty (PTCA)a 2016, 2017
048 Insertion, revision, replacement, removal of cardiac pacemaker or cardioverter/defibrillator 2016-2019
049 Other OR heart procedures 2016-2019
053 Varicose vein stripping, lower limb 2016-2018
057 Creation, revision and removal of arteriovenous fistula or vessel-to-vessel cannula for dialysis 2016-2019
061 Other OR procedures on vessels other than head and neck 2016-2019
063 Other non-OR therapeutic cardiovascular proceduresa 2016, 2017
067 Other therapeutic procedures, hemic and lymphatic system 2016-2019
078 Colorectal resection 2016-2019
080 Appendectomy 2016-2019
081 Hemorrhoid procedures 2018-2019
084 Cholecystectomy and common duct exploration 2016-2019
085 Inguinal and femoral hernia repair 2016-2019
086 Other hernia repair 2016-2019
087 Laparoscopy 2016-2019
094 Other OR upper GI therapeutic procedures 2016-2019
095 Other non-OR lower GI therapeutic proceduresa 2016, 2017
096 Other OR lower GI therapeutic procedures 2016-2019
099 Other OR gastrointestinal therapeutic procedures 2016-2019
100 Endoscopy and endoscopic biopsy of the urinary tract 2016, 2017
101 Transurethral excision, drainage, or removal urinary obstruction 2018-2019
104 Nephrectomy, partial or complete 2018-2019
106 Genitourinary incontinence procedures 2016-2019
109 Procedures on the urethra 2016-2019
112 Other OR therapeutic procedures of urinary tract 2016-2019
113 Transurethral resection of prostate (TURP) 2016-2019
114 Open prostatectomy 2016-2019
117 Other non-OR therapeutic procedures, male genitala 2016, 2017
118 Other OR therapeutic procedures, male genital 2016-2019
119 Oophorectomy, unilateral and bilateral 2016-2019
120 Other operations on ovary 2016, 2019
121 Ligation of fallopian tubes 2016-2019
122 Removal of ectopic pregnancy 2016-2019
124 Hysterectomy, abdominal and vaginal 2016-2019
125 Other excision of cervix and uterus 2016-2019
129 Repair of cystocele and rectocele, obliteration of vaginal vault 2016-2019
130 Other diagnostic procedures, female organs 2018
132 Other OR therapeutic procedures, female organs 2016-2019
141 Other therapeutic obstetrical procedures 2016, 2017
142 Partial excision bone 2016-2019
143 Bunionectomy or repair of toe deformities 2016-2019
144 Treatment, facial fracture or dislocation 2016-2019
145 Treatment, fracture or dislocation of radius and ulna 2016-2019
146 Treatment, fracture or dislocation of hip and femur 2017, 2019
147 Treatment, fracture or dislocation of lower extremity (other than hip or femur) 2016-2019
148 Other fracture and dislocation procedure 2016-2019
149 Arthroscopy 2016-2019
150 Division of joint capsule, ligament or cartilage 2016-2019
151 Excision of semilunar cartilage of knee 2016-2019
152 Arthroplasty knee 2016-2019
153 Hip replacement, total and partial 2016-2019
154 Arthroplasty other than hip or knee 2016-2019
157 Amputation of lower extremity 2016-2019
158 Spinal fusion 2016-2019
160 Other therapeutic procedures on muscles and tendons 2016-2019
161 Other OR therapeutic procedures on bone 2016-2019
162 Other OR therapeutic procedures on joints 2016-2019
164 Other OR therapeutic procedures on musculoskeletal system 2016-2019
166 Lumpectomy, quadrantectomy of breast 2016-2019
167 Mastectomy 2016-2019
170 Excision of skin lesion 2019
171 Suture of skin and subcutaneous tissue 2016-2017, 2019
174 Other non-OR therapeutic procedures on skin and breasta 2016, 2017
175 Other OR therapeutic procedures on skin and breast 2016-2019
225 Conversion of cardiac rhythm 2016-2019
244 Gastric bypass and volume reduction 2016-2019
Abbreviations: GI, gastrointestinal; NASS, Nationwide Ambulatory Surgery Sample; OR, operating room.
a CCS-Services and Procedures categories 63, 95, 117, and 174 did not meet the criteria for inclusion beginning with the 2018 NASS because all major surgeries were moved from these "non-OR therapeutic procedure" categories in an update to the CCS Services and Procedures Tool. For example, all major surgeries in CCS 95, Other non-OR lower GI therapeutic procedures were reassigned to CCS 96, Other OR lower GI therapeutic procedures. Therefore, these major surgeries are still included in the NASS, but with a different CCS-Services and Procedures category assignment than in previous NASS data years. CCS 45, PTCA, was removed from the NASS beginning with data year 2018 based on evidence of underreporting by NASS sample hospitals.
Notes: Updates to the HCUP Surgery Flag Software for Services and Procedures changed the designation of several surgeries from major ("narrow") to not major ("broad" or "neither"), and vice versa. This affected the major surgery volume and hospital-owned facility outpatient market share for multiple CCS-Services and Procedures categories, resulting in additions to and deletions from the NASS in-scope CCS-Services and Procedures categories between data year 2017 and 2018. See www.hcup-us.ahrq.gov/toolssoftware/ccs_svcsproc/ccssvcproc.jsp for more information on CCS Services and Procedures.

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Appendix C: Data Restrictions

Table C.1 enumerates the types of restrictions applied to the 2019 Nationwide Ambulatory Surgery Sample. Restrictions include the following types:

  • Confidentiality of hospitals
  • Confidentiality of records
  • Limited reporting of diagnosis codes for medical misadventures and adverse effects
  • Missing encounters for specific populations of patients

Table C.1. Data Restrictions

Confidentiality of Hospitals
Limitations on sampling to ensure hospital confidentiality:
  • Hospital identifiers are removed from NASS records.
  • State identifiers are removed from NASS records.
Confidentiality of Records
Limitations on selected data elements to ensure patient confidentiality:
  • Age (AGE) values greater than 90 are set to 90 for all NASS records.
  • At least one HCUP Partner required ages in years (AGE) to be set to the midpoints of age ranges.
  • At least one HCUP Partner requires that admission month (AMONTH) is set to missing on all records.
Limited Reporting of Diagnosis Codes for Medical Misadventures and Adverse Effects
  • At least one HCUP Partner removes diagnosis codes for medical misadventures and adverse effects from the data files supplied to HCUP.
Missing Information for Specific Populations of Patients
  • Human Immunodeficiency Virus (HIV)
  • At least one HCUP Partner excludes records for HIV patients from the files provided to HCUP. Therefore, these records are not included in the NASS.
  • Alternatively, at least one HCUP Partner includes records for HIV patients in the data provided to HCUP but removes the diagnosis codes identifying HIV.
  • At least one HCUP Partner masks the type of abortion (e.g., spontaneous, legally induced) by setting all abortion-specific diagnosis and procedure codes to "unspecified" abortions.
Abbreviations: HCUP, Healthcare Cost and Utilization Project; NASS, Nationwide Ambulatory Surgery Sample.

Appendix D: NASS Files and Data Elements

Table D.1. NASS Hospital File Data Elements, 2019

Type of Data Element HCUP Data Element Coding Notes
Encounter counts TOTAL_AS_ENCOUNTERS SASD/SEDD encounters with at least one narrow surgery for this hospitala
N_DISC_U Number of ambulatory surgery encounters for all hospitals in the stratum
S_DISC_U Number of ambulatory surgery encounters for sampled hospitals in the stratum
Encounter weight DISCWT Encounter weight used to calculate national estimates
Encounter year YEAR Encounter year
Hospital characteristics HOSP_BEDSIZE_CAT Hospital bed size category: (1) 00-99, (2) 100-299, (3) 300+
HOSP_CONTROL Control/ownership of hospital: (1) public, (2) voluntary, (3) proprietary
HOSP_LOCATION Location of hospital: (0) rural, (1) urban
HOSP_LOCTEACH Location/teaching status of hospital: (1) rural, (2) urban nonteaching, (3) urban teaching
HOSP_REGION Region of hospital: (1) Northeast, (2) Midwest, (3) South, (4) West
HOSP_TEACH Teaching status of hospital: (0) nonteaching, (1) teaching
NASS_STRATUM Stratum used to sample hospital-owned facilities, includes geographic region, bed size category, location/teaching status, and control/ownership
Hospital counts N_HOSP_U Number of hospitals in the stratum
S_HOSP_U Number of sampled hospitals in the stratum
NASS hospital identifier, synthetic HOSP_NASS Unique HCUP NASS hospital number, links to other NASS files, but not to other HCUP databases
Abbreviations: HCUP, Healthcare Cost and Utilization Project; NASS, Nationwide Ambulatory Surgery Sample.
a Surgeries flagged as "narrow" in the HCUP Surgery Flag Software are defined as invasive therapeutic surgical procedures that typically require the use of an operating room and regional anesthesia, general anesthesia, or sedation.

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Table D.2. NASS Encounter File Data Elements, 2019

Type of Data Element HCUP Data Element Coding Notes
Admission timing AMONTH Admission month coded from (1) January to (12) December
AWEEKEND Admission on weekend: (0) admission on Monday-Friday, (1) admission on Saturday-Sunday
Age at admission AGE Age in years coded 0-90 years. Any ages greater than 90 years were set to 90.
CPT procedure information CPT1-CPT30 In-scope CPT procedures on the record (maximum of 30)
CPTCCS1-CPTCCS30 Clinical Classifications Software (CCS) for Services and Procedures category for in-scope CPT procedures
NCPT_INSCOPE Number of in-scope CPT procedures for this encounter
Diagnosis information I10_DX1-I10DX15 ICD-10-CM diagnoses (maximum of 15)
I10_NDX Number of diagnoses for this encounter
I10_INJURY Injury ICD-10-CM diagnosis reported on record
I10_MULTINJURY Multiple ICD-10-CM injuries reported on record
Disposition of the patient DISPUNIFORM Disposition of patient, uniform coding: (1) routine; (2) transfer to short-term hospital; (5) other transfers, including skilled nursing facility, intermediate care, and another type of facility; (6) home healthcare; (7) against medical advice; (20) died in hospital; (99) discharged alive, destination unknown
Encounter timing DQTR Encounter quarter
YEAR Encounter year
Encounter weight DISCWT Encounter weight used to calculate national estimates
Identifiers, synthetic HOSP_NASS Unique HCUP NASS hospital number, links to other NASS files but not to other HCUP databases
KEY_NASS Unique HCUP NASS record number, links to NASS Supplemental and Diagnosis and Procedure Groups Files, but not to other HCUP databases
National quartile for median household income of patient's ZIP Code ZIPINC_QRTL Median household income quartiles for patient's ZIP Code. For 2018, the median income quartiles are defined as (1) $1-$42,999, (2) $43,000-$53,999, (3) $54,000-$70,999, and (4) $71,000 or more.
Payer information PAY1 Expected primary payer, uniform: (1) Medicare, (2) Medicaid, (3) private including HMO, (4) self-pay, (5) no charge, (6) other
Race and ethnicity of patient RACE Race and ethnicity, uniform: (1) White, (2) Black, (3) Hispanic, (4) Asian or Pacific Islander, (5) Native American, (6) other
Sex of patient FEMALE Indicator of sex: (0) male, (1) female
Total charges TOTCHG Total charges for AS services, edited
Urban-rural location of patient's residence PL_NCHS Urban-rural designation for patient's county of residence: (1) large central metropolitan, (2) large fringe metropolitan, (3) medium metropolitan, (4) small metropolitan, (5) micropolitan, (6) not metropolitan or micropolitan
Abbreviations: AS, ambulatory surgery; CPT, Current Procedural Terminology; HCUP, Healthcare Cost and Utilization Project; HMO, health maintenance organization; ICD-10-CM, International Classification of Diseases, Tenth Revision, Clinical Modification; NASS, Nationwide Ambulatory Surgery Sample.

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Table D.3. NASS Supplemental File Data Elements, 2019

Type of Data Element HCUP Data Element Coding Notes
CPT procedure informationa CPT1-CPT30b Out-of-scope CPT procedures on the record (maximum of 30)
CPTCCS1-CPTCCS30b Clinical Classifications Software (CCS) for Services and Procedures category for out-of-scope CPT procedures
NCPT_NOTINSCOPE Number of out-of-scope CPT procedures for this encounter
Encounter year YEAR Encounter year
Identifiers, synthetic HOSP_NASS Unique HCUP NASS hospital number, links to other NASS files but not to other HCUP databases
KEY_NASS Unique HCUP NASS record number, links to NASS Encounter and Diagnosis and Procedure Groups Files but not to other HCUP databases
Abbreviations: CPT, Current Procedural Terminology; HCUP, Healthcare Cost and Utilization Project; NASS, Nationwide Ambulatory Surgery Sample.
a Although some encounter records may have included Level II Healthcare Common Procedure Coding System (HCPCS) codes, this procedure information is limited to Level I HCPCS codes (i.e., CPT codes).
b Data elements with the same names (representing in-scope, rather than out-of-scope procedures) appear in the Encounter file. Please refer to Section 2.2 of this document for recommendations about how to rename the data elements to avoid issues when merging the Encounter and Supplemental files.

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Table D.4. NASS Diagnosis and Procedure Groups File Data Elements, 2019

Type of Data Element HCUP Data Element Coding Notes
CCSR for ICD-10-CM diagnoses DXCCSR_AAAnnn1 Indication that at least one ICD-10-CM diagnosis on the record is included in CCSR AAAnnn
DXCCSR_DEFAULT_DX1 Default CCSR for first-listed ICD-10-CM diagnosis
DXCCSR_VERSION Version of CCSR for ICD-10-CM diagnoses
Elixhauser Comorbidity Software Refined for ICD-10-CM CMR_aaa2 Comorbidity measures (aaa) identified by the AHRQ Elixhauser Comorbidity Software Refined for ICD-10-CM diagnosis codes
CMR_VERSION Version of the Elixhauser Comorbidity Measure Refined for ICD-10-CM
Identifiers, synthetic HOSP_NASS Unique HCUP NASS hospital number, links to other NASS files but not to other HCUP databases
KEY_NASS Unique HCUP NASS record number, links to NASS Encounter and Supplemental Files but not to other HCUP databases
Abbreviations: CCSR, Clinical Classifications Software Refined; CPT, Current Procedural Terminology; HCUP, Healthcare Cost and Utilization Project; ICD-10-CM, International Classification of Diseases, Tenth Revision, Clinical Modification; NASS, Nationwide Ambulatory Surgery Sample.
1 Where aaa denotes the body system and nnn denotes the CCSR number within the body system.
2 Where aaa denotes the specific comorbidity measure.



Appendix E: Encounter Totals by In-Scope Clinical Classifications Software (CCS) for Services and Procedure Category, 2016-2019

Table E.1. Encounter Totals by In-Scope Clinical Classifications Software (CCS) for Services and Procedure Category, 2016-2019

CCS Category Description Total Encounters, N Percentage Change, % Potential Contributing Reasons for Change Over Time
2016 2017 2018 2019 2016-2017 2017-2018 2018-2019
003 Laminectomy, excision intervertebral disc 234,832 236,796 235,412 276,727 0.8 -0.6 17.6  
006 Decompression peripheral nerve 357,868 353,638 351,346 387,555 -1.2 -0.6 10.3  
009 Other OR therapeutic nervous system procedures 95,818 102,208 95,330 115,510 6.7 -6.7 21.2 Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
010 Thyroidectomy, partial or complete 108,583 105,810 101,233 104,081 -2.6 -4.3 2.8  
012 Other therapeutic endocrine procedures 51,516 51,495 53,825 59,067 0.0 4.5 9.7  
013 Corneal transplant 19,656 18,961 19,091 20,318 -3.5 0.7 6.4  
014 Glaucoma procedures     51,990 63,511     22.2 Surgery Flag Software update: added as in scope for 2018; satisfied hospital outpatient market share threshold after reclassification of 2 CPT codes
Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
015 Lens and cataract procedures 1,284,125 1,239,408 1,149,894 1,172,830 -3.5 -7.2 -2.0  
016 Repair of retinal tear, detachment 97,996 96,040 96,345 99,232 -2.0 0.3 3.0  
017 Destruction of lesion of retina and choroid       5,365       Change to in-scope procedure criteria: in scope in 2019 because market share criterion was removed
019 Other therapeutic procedures on eyelids, conjunctiva, cornea     133,758 152,854     14.3 Surgery Flag Software update: added as in scope for 2018; satisfied hospital outpatient market share threshold after reclassification of several CPT codes
020 Other intraocular therapeutic procedures     85,208 88,491     3.9 Surgery Flag Software update: added as in scope for 2018; satisfied hospital outpatient market share threshold after reclassification of 3 CPT codes
021 Other extraocular muscle and orbit therapeutic procedures 61,677 62,476 56,429 60,624 1.3 -9.7 7.4  
022 Tympanoplasty 25,591 29,354 48,328 52,013 -0.8 64.6 7.6 Surgery Flag Software update: volume increase in 2018 after 2 CPT codes were reclassified from broad to narrow
023 Myringotomy 330,666 324,996 308,451 335,397 -1.7 -5.1 8.7  
024 Mastoidectomy 18,284 17,510 17,505 17,616 -4.2 0.0 0.6  
026 Other therapeutic ear procedures 29,016 29,497 31,174 32,892 1.7 5.7 5.5  
028 Plastic procedures on nose 168,446 164,723 155,805 167,263 -2.2 -5.4 7.4  
030 Tonsillectomy and/or adenoidectomy 447,101 436,314 395,401 422,137 -2.4 -9.4 6.8  
033 Other OR therapeutic procedures on nose, mouth and pharynx 243,975 218,489 268,994 292,706 -10.4 23.1 8.8 Surgery Flag Software and CCS for Services and Procedures updates: overall volume increase in 2018 after reclassification of several CPT codes and reassignment of 1 narrow CPT code to CCS 33 from CCS 32
042 Other OR therapeutic procedures on respiratory system 38,833 36,275 34,996 35,164 -6.6 -3.5 0.5  
043 Heart valve procedures       4,609       Change to in-scope procedure criteria: in scope in 2019 because this procedure exceeded the minimum threshold for total charges
045 Percutaneous transluminal coronary angioplasty (PTCA) 97,895 97,567     -0.3%     Evidence of underreporting in 2016 and 2017 SASD: removed from in scope list for 2018
048 Insertion, revision, replacement, removal of cardiac pacemaker or cardioverter/defibrillator 286,008 276,914 260,471 265,870 -3.2 -5.9 2.1  
049 Other OR heart procedures 8,048 9,997 13,449 16,297 24.2 34.5 21.2 Surgery Flag Software and CCS for Services and Procedures updates: overall volume increase in 2018 after 4 CPT codes were reclassified from narrow to broad and 1 narrow CPT code was reassigned to CCS 49 from CCS 63
Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
Addition of emergent surgeries: increase in 2019 after SEDD encounters were added to the NASS
053 Varicose vein stripping, lower limb 31,981 29,557 7,865   -7.6 -73.4   Surgery Flag Software update: volume decrease in 2018 after 2 CPT codes were reclassified from narrow to broad
Change to in-scope procedure criteria: no longer in scope in 2019 because procedure did not meet minimum volume or charge threshold
057 Creation, revision and removal of arteriovenous fistula or vessel-to-vessel cannula for dialysis 157,200 155,498 158,579 156,776 -1.1 2.0 -1.1  
061 Other OR procedures on vessels other than head and neck 197,486 191,088 230,710 233,450 -3.2 20.7 1.2  
063 Other non-OR therapeutic cardiovascular procedures 30,133 28,156     -6.6     CCS for Services and Procedures update: not in scope for 2018 after all narrow procedures were reassigned from CCS 63 to other CCS categories
067 Other therapeutic procedures, hemic and lymphatic system 198,459 211,123 239,130 282,313 6.4 13.3 18.1  
078 Colorectal resection 12,472 12,590 13,077 13,810 0.9 3.9 5.6  
080 Appendectomy 170,057 174,534 170,952 278,430 2.6 -2.1 62.9 Addition of emergent surgeries: increase in 2019 after SEDD encounters were added to the NASS
081 Hemorrhoid procedures     51,396 53,967     5.0 Surgery Flag Software update: added as in scope for 2018; satisfied volume threshold after 6 CPT codes were reclassified to narrow
084 Cholecystectomy and common duct exploration 599,503 585,438 559,911 606,943 -2.3 -4.4 8.4  
085 Inguinal and femoral hernia repair 462,360 458,721 446,464 456,556 -0.8 -2.7 2.3  
086 Other hernia repair 425,538 432,115 424,602 434,222 1.5 -1.7 2.3  
087 Laparoscopy 73,341 72,596 76,976 81,420 -1.0 6.0 5.8  
094 Other OR upper GI therapeutic procedures 11,613 12,619 21,563 26,266 8.7 70.9 21.8 Surgery Flag Software and CCS for Services and Procedures updates: volume increase in 2018 after 1 CPT code was reclassified from broad to narrow and 1 narrow CPT code was reassigned to CCS 94 from CCS 95
Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
095 Other non-OR lower GI therapeutic procedures 5,402 5,405     0.1     CCS for Services and Procedures update: not in scope for 2018 after all narrow procedures were reassigned from CCS 95 to CCS 96
096 Other OR lower GI therapeutic procedures 59,123 56,694 67,864 70,387 -4.1 19.7 3.7  
099 Other OR gastrointestinal therapeutic procedures 31,426 32,470 18,708 18,679 3.3 -42.4 -0.2 Surgery Flag Software update: volume decrease in 2018 after reclassification of 2 CPT codes were reclassified from narrow to broad
100 Endoscopy and endoscopic biopsy of the urinary tract 17,593 17,309     -1.6     CCS for Services and Procedures update: not in scope for 2018 after all narrow procedures were moved from CCS 100
101 Transurethral excision, drainage, or removal urinary obstruction     151,887 158,548     4.4 Surgery Flag Software update: added as in scope for 2018; satisfied volume threshold after reclassification of 8 CPT codes
104 Nephrectomy, partial or complete     6,841 8,895     30.0 Volume increase: added as in scope for 2018; satisfied volume threshold
Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
106 Genitourinary incontinence procedures 104,290 109,746 112,970 115,272 5.2 2.9 2.0  
109 Procedures on the urethra 24,010 24,127 18,228 18,925 0.5 -24.4 3.8 Surgery Flag Software update: volume decrease in 2018 after 5 CPT codes were reclassified from narrow to neither
112 Other OR therapeutic procedures of urinary tract 29,318 31,313 20,757 24,315 6.8 -33.7 17.1 Surgery Flag Software update and CCS for Services and Procedures update: volume decrease in 2018 after reclassification of 5 CPT codes and the reassignment of 1 narrow CPT code to CCS 112 from CCS 111
113 Transurethral resection of prostate (TURP) 77,962 81,782 84,765 89,707 4.9 3.6 5.8  
114 Open prostatectomy 7,211 7,914 28,017 38 ,372 9.7 254.0 37.0 Reimbursement change: increased volume in 2018 after CMS removed laparoscopic prostatectomy from the inpatient-only lista
Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
117 Other non-OR therapeutic procedures, male genital 6,841 8,412     23.0     CCS for Services and Procedures update: not in scope for 2018 after all narrow procedures were reassigned from CCS 117 to CCS 118
118 Other OR therapeutic procedures, male genital 208,611 208,931 170,126 177,839 0.2 -18.6 4.5  
119 Oophorectomy, unilateral and bilateral 146,930 156,208 167,501 184,111 6.3 7.2 9.9  
120 Other operations on ovary 15,105 15,109   14,865 0.0     Not in scope for 2018; did not satisfy outpatient hospital market share threshold
Change to in-scope procedure criteria: in scope in 2019 because market share criterion was removed
121 Ligation of fallopian tubes 100,709 89,543 76,698 65,993 -11.1 -14.3 -14.0  
122 Removal of ectopic pregnancy 20,922 19,621 18,687 28,143 -6.2 -4.8 50.6 Addition of emergent surgeries: increase in 2019 after SEDD encounters were added to the NASS
124 Hysterectomy, abdominal and vaginal 353,477 372,422 382,869 399,826 5.4 2.8 4.4  
125 Other excision of cervix and uterus 18,923 19,695 51,258 51,878 4.1 160.3 1.2 Surgery Flag Software update: volume increase in 2018 after 1 CPT code was reclassified from broad to narrow
129 Repair of cystocele and rectocele, obliteration of vaginal vault 90,843 97,167 99,669 101,688 7.0 2.6 2.0  
130 Other diagnostic procedures, female organs     5,436         Surgery Flag Software update: added as in scope for 2018; satisfied volume threshold after 2 CPT codes were reclassified from neither to narrow
Change to in-scope procedure criteria: no longer in scope in 2019 because procedure did not meet minimum volume or charge threshold
132 Other OR therapeutic procedures, female organs 222,486 227,217 226,192 236,321 2.1 -0.5 4.5  
141 Other therapeutic obstetrical procedures 17,071 17,887     4.8     Surgery Flag Software update: not in scope for 2018; did not satisfy volume threshold after reclassification of 2 CPT codes
142 Partial excision bone 258,034 266,434 249,908 277,326 3.3 -6.2 11.0  
143 Bunionectomy or repair of toe deformities 188,512 181,576 173,820 179,157 -3.7 -4.3 3.1  
144 Treatment, facial fracture or dislocation 47,292 48,670 43,661 43,810 2.9 -10.3 0.3  
145 Treatment, fracture or dislocation of radius and ulna 115,218 115,873 117,738 135,863 0.6 1.6 15.4  
146 Treatment, fracture or dislocation of hip and femur   5,391 5,835 7,355   8.2 26.0 Volume increase: added as in scope in 2017; satisfied volume threshold
Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
Addition of emergent surgeries: increase in 2019 after SEDD encounters were added to the NASS
147 Treatment, fracture or dislocation of lower extremity (other than hip or femur) 155,318 160,680 165,542 193,280 3.5 3.0 16.8  
148 Other fracture and dislocation procedure 109,868 110,565 106,409 126,658 0.6 -3.8 19.0  
149 Arthroscopy 100,325 94,560 88,407 99,699 -5.7 -6.5 12.8  
150 Division of joint capsule, ligament or cartilage 60,423 55,031 51,386 54,569 -8.9 -6.6 -6.2  
151 Excision of semilunar cartilage of knee 453,421 417,884 383,794 404,838 -7.8 -8.2 5.5  
152 Arthroplasty knee 44,230 49,456 192,752 301,910 11.8 289.7 56.6 Reimbursement change: increased volume in 2018 after CMS removed total knee arthroplasty from the inpatient-only lista
Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
153 Hip replacement, total and partial 26,406 30,308 36,846 49,826 14.8 21.6 35.2 Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
154 Arthroplasty other than hip or knee 55,131 55,611 57,934 66,148 0.9 4.2 14.2  
157 Amputation of lower extremity 39,518 40,221 42,802 48,970 1.8 6.4 14.4  
158 Spinal fusion 52,558 75,851 82,506 93,850 44.3 8.8 13.7 Reimbursement change: increased volume in 2017 after CMS added several surgical spine codes to the list of approved ambulatory surgery center covered proceduresb
160 Other therapeutic procedures on muscles and tendons 811,537 808,426 806,409 911,712 -0.4 -0.2 13.1  
161 Other OR therapeutic procedures on bone 277,895 281,555 259,817 288,187 1.3 -7.7 10.9  
162 Other OR therapeutic procedures on joints 524,408 513,561 463,114 518,568 -2.1 -9.8 12.0  
164 Other OR therapeutic procedures on musculoskeletal system 42,909 43,797 43,683 55,635 2.1 -0.3 27.4 Inclusion of specialty hospitals: increase in 2019 after specialty hospitals were added to the NASS universe
166 Lumpectomy, quadrantectomy of breast 317,041 317,702 317,525 331,735 0.2 -0.1 4.5  
167 Mastectomy 74,635 79,764 86,585 94,411 6.9 8.6 9.0  
170 Excision of skin lesion       134,215     Change to in-scope procedure criteria: in scope in 2019 because market share criterion was removed
171 Suture of skin and subcutaneous tissue 7,867 8,247   120,030 4.8     Surgery Flag Software update: not in scope for 2018 because outpatient hospital share fell below threshold after reclassification of 6 CPT codes
Change to in-scope procedure criteria: in scope in 2019 because market share criterion was removed
174 Other non-OR therapeutic procedures on skin and breast 18,666 19,753     5.8     CCS for Services and Procedures update: not in scope for 2018 after all narrow procedures were reclassified from CCS 174 to CCS 175
175 Other OR therapeutic procedures on skin and breast 341,972 343,787 342,372 376,101 0.5 -0.4 9.9  
225 Conversion of cardiac rhythm 118,742 136,840 149,475 169,206 15.2 9.2 13.2  
244 Gastric bypass and volume reduction 26,390 24,600 24,709 24,658 -6.8 0.4 -0.2  
Abbreviations: CMS, Centers for Medicare & Medicaid Services; CPT, Current Procedural Terminology; GI, gastrointestinal; NASS, Nationwide Ambulatory Surgery Sample; OR, operating room; SEDD, State Emergency Department Databases
a See CMS Hospital Outpatient Prospective Payment- Notice of Final Rulemaking (NFRM) with Comment Period (CMS-1678-FC), available at:https://www.govinfo.gov/content/pkg/FR-2017-11-13/pdf/2017-23932.pdf.
b See CMS Hospital Outpatient Prospective Payment - Final Rule with Comment and Final CY2017 Payment Rates (CMS-1656-FC), available at:https://www.govinfo.gov/content/pkg/FR-2016-11-14/pdf/2016-26515.pdf.
Notes: Totals represent weighted estimates. CCS-Services and Procedures category totals are unduplicated, such that if two or more CPT codes on the same encounter record mapped to the same CCS-Services and Procedures category, the record was only counted once. Totals are missing if the CCS-Services and Procedures category was not in scope for the NASS sample. Contributing reasons for changes over time are reported only for categories that were added to or removed from the NASS in-scope procedure list and for categories with year-to-year percentage change in volume greater than 20 percent.



1 SEDD encounters are included in the 2019 NASS only. Reference section 1.1 for more information about how this compares to previous data years.
2 The following States have at least one freestanding facility in the HCUP SASD: California, Florida, Illinois, Kentucky, Michigan, Missouri, North Carolina, Nevada, New York, Oklahoma, Oregon, Pennsylvania, South Carolina, Utah, and Wisconsin.
3 Agency for Healthcare Research and Quality. Surgery Flag Software for Services and Procedures. Healthcare Cost and Utilization Project (HCUP). Last modified May 25, 2021. www.hcup-us.ahrq.gov/toolssoftware/surgeryflags_svcproc/surgeryflagssvc_proc.jsp. Accessed September 16, 2021. The terms narrow and broad are specific to the Surgery Flag Software. The 2018 and 2019 NASS applied v2019.2, which included narrow surgeries identified in the following ranges of CPT codes: surgical (10004-69990), emerging technology (0100T-0588T), and cardiac-related medical (92920-93986). For more information, reference the Surgery Flag Software for Services and Procedures User Guide, available at: www.hcup-us.ahrq.gov/toolssoftware/surgeryflags_svcproc/surgeryflagssvc_proc.jsp#user.
4 The Clinical Classifications Software for Services and Procedures (CCS-Services and Procedures) is HCUP software that provides a method for classifying CPT codes and Healthcare Common Procedure Coding System (HCPCS) codes into clinically meaningful procedure categories. More than 9,000 CPT/HCPCS codes and 6,000 HCPCS codes are collapsed into 244 clinically meaningful categories that may be more useful for presenting descriptive statistics than are individual CPT or HCPCS codes. For more information, visit www.hcup-us.ahrq.gov/toolssoftware/ccs_svcsproc/ccssvcproc.jsp.
5Agency for Healthcare Research and Quality. Elixhauser Comorbidity Software Refined for ICD-10-CM. Last modified October 23, 2020 www.hcup-us.ahrq.gov/toolssoftware/comorbidityicd10/comorbidity_icd10.jsp. Accessed September 10, 2021.
6 Agency for Healthcare Research and Quality. Clinical Classifications Software Refined (CCSR) for ICD-10-CM Diagnoses. Last modified October 15, 2020. www.hcup-us.ahrq.gov/toolssoftware/ccsr/ccs_refined.jsp. Accessed October 19, 2020.
7 Agency for Healthcare Research and Quality. Clinical Classifications Software (CCS) for Services and Procedures. Healthcare Cost and Utilization Project (HCUP). Last modified April 26, 2021. www.hcup-us.ahrq.gov/toolssoftware/ccs_svcsproc/ccssvcproc.jsp. Accessed September 16, 2021.
8 The SEDD were sampled for data year 2019 only.
9Agency for Healthcare Research and Quality. Surgery Flag Software for Services and Procedures. Healthcare Cost and Utilization Project (HCUP). Last modified May 25, 2021. www.hcup-us.ahrq.gov/toolssoftware/surgeryflags_svcproc/surgeryflagssvc_proc.jsp. Accessed September 16,, 2021. The terms narrow and broad are specific to the Surgery Flag Software. The 2018 and 2019 NASS applied v2019.2, which included narrow surgeries identified in the following ranges of CPT codes: surgical (10004-69990), emerging technology (0100T-0588T), and cardiac-related medical (92920-93986).
10Agency for Healthcare Research and Quality. Clinical Classifications Software (CCS) for Services and Procedures. Healthcare Cost and Utilization Project (HCUP). Last modified April 26, 2021. www.hcup-us.ahrq.gov/toolssoftware/ccs_svcsproc/ccssvcproc.jsp. Accessed September 16, 2021.
11 Percutaneous transluminal coronary angioplasty (PTCA) was included in earlier years of the NASS (2016 and 2017).
12 See the AHA "community hospital designation" at www.ahadataviewer.com/glossary. Exit Disclaimer
13 The AHA Annual Survey definition for outpatient surgery is as follows: Scheduled surgical services provided to patients who do not remain in the hospital overnight. The surgery may be performed in operating suites also used for inpatient surgery, specially designated surgical suites for outpatient surgery, or procedure rooms within an outpatient care facility. (American Hospital Association. TrendWatch Chartbook 2019 - Glossary. www.aha.org/system/files/media/file/2019/10/AHA-TrendWatch-Chartbook-Glossary.pdf. Exit Disclaimer Accessed September 16, 2021.)
14 The HCUP SASD contain a number of hospital-owned facilities performing major ambulatory surgeries that are not inpatient hospitals. In the NASS, these facilities are assigned the identifier of the hospital owner. Stratification, sampling, weighting, and reporting are performed using the hospital owner identifier and hospital characteristics.
15 Bed size categories were originally established for the development of the Nationwide Inpatient Sample (NIS). Cutoff points were originally chosen so that approximately one-third of the hospitals in each region, location, and teaching status combination would fall within each bed size category (small, medium, or large). For more information, reference the Introduction to the NIS, available at www.hcup-us.ahrq.gov/db/nation/nis/nisdbdocumentation.jsp.
16 Hospital service type was not used as a stratum because of its weaker predictive power in the predictive models and the small number of children's hospitals in the sample.
17 The sampling fraction is defined as (number units in sample)/(number units in universe). The overall sampling fraction for the 2019 universe of hospitals and sampling frame is (2,958/4,362) = 0.68.
18 The ownership category was chosen for collapsing because it had lower explanatory power in the predictive models than did bed size or location and teaching status. Census region was considered as an essential stratum to include in the design.
19 Carlson BL, Johnson AE, Cohen SB. An evaluation of the use of personal computers for variance estimation with complex survey data. J Off Statistics. 1993;9(4):795-814.

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Internet Citation: 2019 Introduction to 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/NASS_Introduction_2019.jsp.
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