National Hospital Discharge and Ambulatory Surgery Data
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National Survey of Ambulatory Surgery Sample Design

The universe of eligible facilities for the National Survey of Ambulatory Surgery (NSAS) consists of hospitals and freestanding ambulatory surgery centers. The hospital universe includes 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 with an average length of stay for all patients of less than 30 days) or those whose specialty is general (medical or surgical) or children's general are included in the survey. Those hospitals must also have six beds or more staffed for patient use. The universe definition is the same as that used for the National Hospital Discharge Survey and the National Hospital Ambulatory Medical Care Survey. The sampling frame for the hospital universe consists of eligible hospitals listed in the 1993 SMG Hospital Market Database.

The universe of freestanding facilities includes the freestanding ambulatory surgery centers listed in the 1993 SMG Freestanding Outpatient Surgery Center Database and/or Medicare-certified facilities included in the Health Care Financing Administration Provider-of-Services file. Facilities specializing in dentistry, podiatry, abortion, family planning or birthing are excluded.

NSAS uses a multistage probability design with independent samples of hospitals and freestanding ambulatory surgery centers selected at the first of second stages and visits to these facilities selected at the final stage. The NSAS sample includes all facilities with a high annual volume of ambulatory procedures. The remaining sample of facilities is selected using a three-stage stratified cluster design.

The first stage consists of a selection of a subsample of the primary sampling units (PSU’s) used in the 1985-94 National Health Interview Survey. PSU’s are counties, a group of counties, county equivalents (such as parishes or independent cities), or towns and townships (for some PSU’s in New England). The second stage consists of a selection of facilities from the sample PSU’s.

At the third stage, a systematic random sample of ambulatory surgery visits is selected. Sampled visits are drawn from all locations within a facility where ambulatory surgery is performed, including main or general operating rooms, all dedicated ambulatory surgery rooms, cystoscopy and endoscopy units, cardiac catheterization labs, and laser procedure rooms (in-scope locations). However, locations within hospitals dedicated exclusively to abortion, dentistry, podiatry, pain block, or small procedures (sometimes referred to as "lump and bump" rooms) are not included. The exclusion of these specialty locations, as well as the exclusion of specialty facilities, were recommended based on the feasibility study for the NSAS. A detailed description of the design and development of the NSAS is included in "The Plan and Operation of the National Survey of Ambulatory Surgery," Vital and Health Statistics Series 1, Number 37.
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