At A Glance

Noteworthy Characteristics

  • Provides information on surgical procedures for obesity-related conditions.
  • Includes children’s general hospitals.
  • Nationally representative sample of ASCs.
  • Health Resources and Services Administration (HRSA) data linkage can provide information about county poverty level for the facilities and U.S. Census Bureau data linkage can provide similar information at the zip code level.



To collect data about visits to hospital-based and freestanding ambulatory surgery centers (ASCs) by patients in the United States (U.S.).

Target Population

Patients seen at hospital-based and free-standing ASCs in the U.S. The survey targets noninstitutional hospitals exclusive of federal, military, and Department of Veterans Affairs hospitals located in the 50 states and the District of Columbia. Only short-stay hospitals (hospitals with an average length of stay for all patients of fewer than 30 days), or those whose specialty was general (medical or surgical), or children’s general were included in the survey. These hospitals also must have staffed six beds or more for patient use.


Began in 1994. Conducted at irregular intervals. Most recent year conducted was 2006. Survey ended in 2006.

NSAS was conducted from 1994 to 1996 and then again in 2006. Data collection for hospital-based ASCs was added to the National Hospital Ambulatory Medical Care Survey (NHAMCS) beginning in 2009. Annual data collection for both hospital-based and free-standing ASCs was part of the NHAMCS beginning in 2010.


National Center for Health Statistics, Centers for Disease Control and Prevention, U.S. Department of Health and Human Services

Special Note(s)

Unit of analysis is patient visit rather than the patient. Patients may visit an ASC multiple times during the survey period, but each visit is counted separately.

NSAS does not provide nationally representative children’s hospital data.


Sample Design

Cross-sectional, nationally representative, multi-stage probability design. The 1994–1996 NSAS used a three-stage stratified cluster design. The 2006 NSAS used a two-stage list-based sample design. Learn more about the sampling design.

Sample Size

Approximately 142 hospital-based and 295 freestanding ambulatory surgery centers (ASCs) in 2006. Approximately 52,000 ambulatory surgery cases in 2006.

Special Note(s)

Because this is a sample survey, estimates for common surgeries are most likely to be reliable. Rare events may not be picked up on an annual basis but may yield reliable estimates if data for multiple years are combined. Ambulatory surgery for children with obesity-related conditions may not have been common enough to yield reliable national estimates.

Key Variables


NameMethods of Assessment
Expected source of paymentAmbulatory surgery center (ASC) medical records
Patient age in years and date of birthASC medical records
Patient race/ethnicityASC medical records
Patient sexASC medical records


NameMethods of Assessment
Metropolitan statistical area, provider/facility county (restricted), zip code (restricted)N/A


NameMethods of Assessment
DiagnosesASC medical records
DispositionASC medical records
Surgical and non-surgical proceduresASC medical records
Symptoms present during or after surgeryASC medical records

Data Access and Cost

Data Availability

Download public-use data files. NSAS data also are available on CD. Call +1 (301) 458-4321 for more information.


Free of charge. Restricted data are fee-based. Set-up fee is $750 per day. Learn more about additional fees.

Special Note(s)

Identification of facilities and patients cannot be made from the public-use data files. Restricted files can be accessed by applying to the National Center for Health Statistics (NCHS) Research Data Center. Researchers who wish to use restricted data must submit a proposal to be reviewed by NCHS staff. Learn more about requesting restricted data.

The 2006 NSAS public-use data set was revised in April 2010 to correct an error in the calculation of the “age in years” variable for children younger than 1 year of age. The days and months of these children were incorrectly counted as years in the previously released public-use NSAS file. The revised 2006 NSAS public-use data set is now available for downloading.


Geocode Variable(s)

Metropolitan statistical area, provider/facility county (restricted), zip code (restricted)

Existing Linkages

These NSAS data can be linked to Health Resources and Services Administration (HRSA) Area Resource File data on county poverty level using provider/facility county and to Census Bureau data at the zip code level.

Selected Publications


Bernstein AB, Hing E, Burt CW, Hall MJ. Trend data on medical encounters: Tracking a moving target. Health Affairs 2001;20(2):58-72.

Cullen KA, Hall MJ, Golosinskiy A. Ambulatory Surgery in the United States, 2006. National Health Statistics Reports. No 11. Revised. Hyattsville (MD): U.S. Centers for Disease Control and Prevention, National Center for Health Statistics, 2009.

Owings MF, Kozak LJ. Ambulatory and inpatient procedures in the United States, 1996. Vital and Health Statistics. Series 13, Data from the National Health Survey 1998 Nov;(139):1-119.


McLemore T, Lawrence l. Plan and operation of the National Survey of Ambulatory Surgery. Vital and Health Statistics 1 1997 Oct;(37):I-IV,1-124.