Series 1

No. 37

Plan and Operation of the

National Survey of Ambulatory

Surgery

October 1997

Vital and

Health Statistics

From the CENTERS FOR DISEASE CONTROL AND PREVENTION / National Center for Health Statistics

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Center for Health Statistics
Copyright information

All material appearing in this report is in the public domain and may be

reproduced or copied without permission; citation as to source, however, is

appreciated.

Suggested Citation

McLemore T, Lawrence L. Plan and operation of the National Survey of

Ambulatory Survey. National Center for Health Statistics. Vital Health Stat 1(37).

1997.

Library of Congress Catalog Card Number

McLemore, Thomas.

Plan and operation of the National Survey of Ambulatory Surgery (NSAS).

p. cm. — (Vital and health statistics. Ser. 1, Programs and collection

procedures ; no. 37) (DHHS publication ; no. (PHS) 98-1313)

By Thomas McLemore and Linda Lawrence.

Includes bibliographical references.

ISBN 0-8406-0532-3

1. National Survey of Ambulatory Surgery (U.S.) 2. Ambulatory

surgery—Research—United States—Methodology. 3. Medical care

surveys—United States. I. Lawrence, Linda, 1948- II. National Center for Health

Statistics (U.S.) III. Title. IV. Series. V. Series: DHHS publication ; no (PHS)

98-1313.

[DNLM: 1. National Survey of Ambulatory Surgery (U.S.) 2. Ambulatory

Care. 3. Data Collection—methods. 4. Health Surveys—United States. 5.

Research Design. W2 A N148va no. 37 1997]

RA409.U44 no. 37

[RD27.42]

362.1'07'23 s—dc21

[617'.007'23]

DNLM/DLC 97-36605

for Library of Congress CIP

For sale by the U.S. Government Printing Office

Superintendent of Documents

Mail Stop: SSOP

Washington, DC 20402-9328

Printed on acid-free paper.
Plan and Operation of the

National Survey of Ambulatory

Surgery

Series 1:

Programs and Collection Procedures

No. 37

Hyattsville, Maryland

October 1997

DHHS Publication No. (PHS) 98-1313

Vital and

Health Statistics

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Center for Health Statistics
National Center for Health Statistics

Edward J. Sondik, Ph.D., Director

Jack R. Anderson, Deputy Director

Jack R. Anderson, Acting Associate Director for

International Statistics

Lester R. Curtin, Ph.D., Acting Associate Director for

Research and Methodology

Jacob J. Feldman, Ph.D., Associate Director for Analysis,

Epidemiology, and Health Promotion

Gail F. Fisher, Ph.D., Associate Director for Data Standards,

Program Development, and Extramural Programs

Edward L. Hunter, Associate Director for Planning, Budget,

and Legislation

Jennifer H. Madans, Ph.D., Acting Associate Director for

Vital and Health Statistics Systems

Stephen E. Nieberding, Associate Director for

Management

Charles J. Rothwell, Associate Director for Data

Processing and Services

Division of Health Care Statistics

W. Edward Bacon, Ph.D., Director

Thomas McLemore, Deputy Director

Robert Pokras, Chief, Hospital Care Statistics Branch
Contents

Abstract ................................................................................................ 1

Introduction............................................................................................. 1

Background ............................................................................................. 1

Feasibility Study ......................................................................................... 2

Purpose .............................................................................................. 2

Design and Methods ................................................................................... 3

Recommendations ..................................................................................... 4

Pretest ................................................................................................. 5

Sampling Frame and Sample Selection .................................................................... 5

Data Collection Methods................................................................................ 5

Results .............................................................................................. 7

The 1994 National Survey of Ambulatory Surgery ............................................................. 7

Sample Design ........................................................................................ 7

Data Collection Procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Data Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Estimation Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Reliability of Estimates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Data Dissemination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Appendix I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Legislative Authorization. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Appendix II. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Members of the Technical Advisory Panel for the Feasibility Study of the National Survey of Ambulatory Surgery . . . . . 18

Appendix III . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Letters and Data Collection Instruments for the Feasibility Study of the National Survey of Ambulatory Surgery . . . . . . . 19

Appendix IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Letters and Data Collection Instruments for the Pretest of the National Survey of Ambulatory Surgery . . . . . . . . . . . . . . . 69

Appendix V. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Definitions of Terms Used in the Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Appendix VI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Data Collection Forms Used in the 1994 NSAS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Appendix VII . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Introductory Letter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

Appendix VIII. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

Endorsement Letters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

Appendix IX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

Definition of Terms Relating to the Medical Abstract Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

iii
Text Tables

A. Data elements collected in the Feasibility Study field test and recommended for use in the national survey........... 4

B. Number of hospitals and freestanding ambulatory surgery centers by facility specialty and region: National Survey of

Ambulatory Surgery, 1993 ............................................................................ 8

C. List of types of service to classify hospitals in the 1994 National Survey of Ambulatory Surgery................... 9

D. List of specialties used to classify the freestanding ambulatory surgery centers in the 1994 National Survey of

Ambulatory Surgery ................................................................................. 9

E. Number of facilities in the 1994 National Survey of Ambulatory Surgery sample, number and percent of in-scope

facilities, and response rates by type of facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

F. Parameters for use in the relative standard error formulae for the National Survey of Ambulatory Surgery by selected

characteristics: United States, 1994 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

iv
The National Survey of Ambulatory

Surgery (NSAS), a national probability

sample survey of ambulatory surgery

visits in hospitals and freestanding

ambulatory surgery centers, began

operation in 1994. This report traces

the development of the survey

instruments and procedures, and

presents the survey methodology for

the NSAS.

Keywords: National Survey of

Ambulatory Surgery c procedures c

outpatients c hospitals

Plan and Operation of the

National Survey of Ambulatory

Surgery

by Thomas McLemore, M.S.P.H., and Linda Lawrence, Division of

Health Care Statistics

Introduction

I

n April 1994, the National Center

for Health Statistics (NCHS)

initiated the National Survey of

Ambulatory Surgery (NSAS) to gather

and disseminate information about

ambulatory surgery occurring in

hospitals and freestanding ambulatory

surgery centers. NCHS has authority

under Section 306(b)(1)(F) of the Public

Health Service Act (42 USC 242k) to

collect data concerning the public’s use

of health care and services (see

appendix I). This report summarizes the

background and development of the

NSAS and provides the survey’s design

and methodologies.

Background

Since 1965, NCHS has provided

annual data on the Nation’s use of

inpatient medical and surgical care

provided in non-Federal, short-stay

hospitals. These data, collected through

the National Hospital Discharge Survey

(NHDS), have been extensively used in

monitoring changes and analyzing the

types of surgical treatment provided for

hospital inpatients. However, advances

in medical technology, such as

endoscopic techniques and new

anesthetic drugs that allow patients to

regain consciousness more quickly

following surgery, have increasingly

enabled many procedures to be

performed outside the hospital inpatient

setting (1).

In addition, concern about rising

health care costs led to changes in the

Medicare program that encouraged the

use of ambulatory surgery. In the early

1980’s, the Medicare program was

expanded to cover care in ambulatory

surgery centers, and a prospective

payment system based on

diagnosis-related groups (DRG’s) was

adopted for hospital inpatient care that

created strong financial incentives for

hospitals to shift less complex surgery

to outpatient settings. Many State

Medicaid plans and private insurers

followed the lead of the Medicare

program and adopted similar policies

(2).

In settings that provide ambulatory

surgery, the patient enters the facility;

receives care that includes at least one

surgical, diagnostic, or therapeutic

procedure; and (generally) leaves on the

Page 1

This report describes the development and methods used in the 1994 National Survey of Ambulatory

Surgery (NSAS). The design and execution of a large survey such as NSAS could not have been

accomplished without the participation of a large number of people. Many members of the staff of the

National Center for Health Statistics (NCHS), in particular, Iris Shimizu (Mathematical Statistician, Survey

Methods Staff, Office of Research and Methodology), Robert Pokras (Chief, Hospital Care Statistics

Branch), and many others outside NCHS, participated in the development of the NSAS. A private

contractor, the Center for Health Policy Studies, conducted the 1990 feasibility study. The Bureau of the

Census conducted the pretest and data collection for the 1994 study. Lastly, we are indebted to the sampled

facilities and their staffs without whose support and cooperation NSAS could not have been successfully

completed. This report was peer reviewed by Mary Moien, edited by Klaudia Cox, and typeset by

Annette F. Holman of the Publications Branch, Division of Data Services.
same day. Facilities providing this type

of care can be hospitals or freestanding

ambulatory surgery centers.

Ambulatory surgery settings provide

a wide variety of surgical procedures

including tonsillectomy, myringotomy,

lens extraction (and insertion of an

artificial lens), dilation and curettage of

uterus, repair of hernia, laparoscopic

tubal ligation, and arthroscopic surgery.

Also included are diagnostic procedures

including biopsy and endoscopic

examinations and therapeutic procedures

such as injection or infusion of cancer

chemotherapeutic substances. As

medical technology progresses,

additional procedures will move to

ambulatory surgery settings.

The growth of freestanding

ambulatory surgery facilities has been

dramatic. In 1983, there were 239

freestanding surgery centers performing

approximately 377,000 surgical

procedures (3). By 1993, there were

1,862 facilities performing more than

3.2 million procedures (4).

There has also been a rapid growth

of ambulatory surgery in hospitals. Data

from the SMG Marketing Group

indicate that in 1993, U.S. hospitals

performed about 12.4 million outpatient

surgical procedures and that about

52 percent of all surgery in hospitals

was ambulatory surgery (5). Together,

these figures indicate that ambulatory

surgery accounted for approximately

58 percent of all surgery in 1993.

The decline of selected procedures

(e.g., cataract extraction) on an inpatient

basis has been documented by the

NHDS (6). However, the concurrent

growth of ambulatory surgery has not

been documented, resulting in a serious

gap of information about surgical care.

Valid data about the medical and

surgical care provided in hospitals and

freestanding ambulatory surgery centers

are necessary to make national and local

decisions for the allocation of resources

and training of medical manpower, to

aid efforts to control medical costs, and

to plan for the provision of future

medical and surgical care. This need for

more complete surgery data is

accentuated by the rapidly aging

population and the introduction of new

medical technology. Therefore, the

omission of ambulatory surgery from

the surgical care database has left a

significant gap in coverage and limits

the utility of the current NHDS data.

The NSAS was developed to fill this

data gap and to respond to the

increasing demand for more complete

surgery data.

The NSAS is one of the NCHS

establishment surveys, collectively

called the National Health Care Survey

(NHCS). The NHCS was designed to

provide nationally representative data on

the use of health care resources for the

major sectors of the health care delivery

system and to address the dramatic

changes occurring in the health care

delivery system during the 1980’s. At

the request of NCHS, the plan to

develop the NHCS was evaluated by a

panel of experts convened by the

National Academy of Sciences and the

Institute of Medicine. In the final report

from the evaluation, Toward a National

Health Care Survey: A Data System for

the 21st Century, the panel stated that it

‘‘endorses the center’s plan to extend

coverage of the health care provider

surveys to include additional health care

settings that have emerged in recent

years.’’ The original NHCS plan

included the development of a survey of

hospital-based and freestanding

ambulatory surgery centers (7).

The plan for NHCS included a

restructuring of its ‘‘traditional’’ surveys

of health care providers and an

expansion of coverage to other major

sectors of the health care system. Under

this plan, the ‘‘traditional’’ surveys—the

NHDS, the National Ambulatory

Medical Care Survey (NAMCS), the

National Nursing Home Survey

(NNHS), and the National Master

Facility Inventory (NMFI)—were

modified to form an integrated NHCS.

Two new health care establishment

surveys, in addition to the NSAS, were

developed and fielded during the early

1990’s. These surveys included the

National Hospital Ambulatory Medical

Care Survey (NHAMCS), which

provides data on the utilization of

services in hospital emergency and

outpatient departments, and the National

Home and Hospice Care Survey

(NHHCS), which provides data on the

services and staff of home health

agencies and hospices and on the

personal and health characteristics of

current and discharged patients (8).

One of the first steps in developing

the NHCS was to identify viable

sampling frames or listings of health

care providers and establishments. These

listings could be based on available

databases and/or developed based on the

National Master Facility Inventory

(NMFI) methodology. The NMFI

methodology involved identifying

available facility files, compiling listings

by facility type, processing the lists to

create an unduplicated file, and

collecting additional information on

each facility through a mail survey. In

1986, an evaluation of the coverage of

the health facilities in the NMFI was

conducted under contract with La Jolla

Management Corporation (9). Results

applicable to the development of the

NSAS indicated that complete and

accurate lists of ambulatory surgery

facilities could be collected; however,

the NMFI methodology could not be

used exclusively for developing listings

of these facility types. Use of both the

NMFI methodology and data from the

SMG Marketing Group was

recommended for inventorying these

facilities. Study findings also included a

proposed definition of ‘‘ambulatory

surgery center’’ and suggested criteria

for differentiating ambulatory surgery

facilities from private physician

practices (10).

Feasibility Study

The development of the National

Survey of Ambulatory Surgery

began with a study initiated in

1989 under contract to the Center for

Health Policy Studies to assess the

feasibility of collecting ambulatory

surgery data from hospitals and

freestanding ambulatory surgery centers

(11,12).

Purpose

With the NMFI evaluation study

providing the foundation, the objectives

of the feasibility study were to provide

detailed recommendations concerning

the design of a national survey of

Page 2 [ Series 1, No. 37
ambulatory surgery, including the

definitions of the universe of facilities

and of surgical visits within facilities to

be sampled, the data set to be collected,

the methodology to be employed, and

the estimated costs for a national survey.

A principal focus of the study was to

assess the extent to which the methods

used for the National Hospital Discharge

Survey (NHDS) could be adapted to a

national survey of ambulatory surgery.

Alternative methodologies were also

investigated.

An expert advisory panel consisting

of representatives of professional and

trade associations provided advice on

technical issues throughout the

feasibility study. The panel reviewed the

design and findings from the field test

and assisted in formulating

recommendations for a national survey.

Members of the feasibility study

technical advisory panel are listed in

appendix II.

Design and Methods

The feasibility study was conducted

in two phases. During Phase I, methods

and practices for collecting data

regarding ambulatory surgery and

ambulatory surgery facilities were

investigated. These activities included a

comprehensive literature review; an

evaluation of the number and

characteristics of hospital-based and

freestanding ambulatory surgery centers;

an investigation of the definition of the

universe of ambulatory surgical facilities

(hospital-based and freestanding) and of

potential sampling frames; an evaluation

of the accessibility and cost of selecting

samples and collecting data from these

sites, including an investigation of the

source, availability, and content of

computerized files for ambulatory

surgery centers; and an evaluation of the

comprehensiveness, availability, and

reliability of these data. A working

definition for identifying types of

ambulatory surgery visits to be included

and/or excluded from the sampling

frame within hospitals was also

developed.

Based on this background research,

it was hypothesized that a data

collection methodology similar to the

methodology employed in the National

Hospital Discharge Survey (NHDS), a

retrospective records review, could be

used to conduct a national survey of

ambulatory surgery. A preliminary

dataset was identified and operational

definitions were developed, sampling

frames and definitions were developed,

and the NHDS methodology was

modified to fit the data collection needs

of an ambulatory surgery survey. The

data collection plan consisted of a

detailed outline of the field test

methodology that included all survey

instruments, training materials, letters of

introduction, protocols, and sampling

methodologies.

During Phase II of the feasibility

study, a field test was conducted.

Because freestanding ambulatory

surgery centers tended to be located

primarily in medium-to-large cities, the

field test was restricted to metropolitan

statistical areas (MSA’s). The selection

of MSA’s was made to provide

geographic diversity, to provide a

sufficient pool of ambulatory surgery

centers, and to avoid MSA’s in which

the NHDS was being conducted. The

field test included two types of facilities

in which ambulatory surgery was

performed: hospitals and freestanding

ambulatory surgery centers (FSASC’s).

Out-of-scope and nonrespondent sample

facilities were replaced in an attempt to

include about 15 participating facilities,

half hospitals and half FSASC’s, in each

of the MSA’s. However, the smaller

number of FSASC’s available in these

areas compared with the numbers of

hospitals and the slightly higher refusal

rate among FSASC’s (particularly

among small eye surgery centers)

resulted in a final sample of 51

participating hospitals and 33

participating FSASC’s. The number of

facilities inducted was slightly larger,

but several facilities inducted did not

provide abstracts within the data

collection time period, and several

facilities visited for induction

subsequently refused to participate.

The facility induction and data

collection phase of the field test

occurred in the first 6 months of 1991.

Appendix III contains selected forms

and letters used during this phase of the

field test. Using the initial telephone call

script, the contractor’s staff telephoned

each of the selected facilities to identify

an appropriate contact as well as his/her

mailing address and telephone number.

After this initial contact, an information

package was sent to each sampled

facility that included a letter of

introduction from NCHS, a description

of the feasibility study, letters of

endorsement from the Society for

Ambulatory Anesthesia and the

Federated Ambulatory Surgery

Association, and an NCHS fact sheet.

Approximately 10 days later, each

facility was recontacted by telephone to

verify that the facility met the criteria

for inclusion in the feasibility study and

to set up an appointment for an

induction interview (see ‘‘Script for

Second Phone Call to Facilities’’ in

appendix III). Facilities identified during

these screening calls that did not meet

the eligibility criteria were excluded

from the study. During the induction

interview, contractor staff met with the

appropriate facility contact(s) to explain

the purpose of the study, to explain what

facility participation involved, and to

ask for the facility’s cooperation. Once a

facility agreed to participate, the

remainder of the induction visit

consisted of collecting information on

the facility’s ambulatory surgery

activities, lists of surgical patients, and

medical records contents.

Facilities that participated in the

study were given the choice of having

their staff sample and abstract medical

records or having the contractor’s staff

perform these tasks. Because the

feasibility study was testing the

methodology for a national survey,

extensive sampling and data abstraction

were not required. The sampling was

conducted for a continuous 2-month

period and data were abstracted for one

month of the sampled visits

(approximately 20 visits). If facility staff

chose to perform the sampling and

abstracting, training was provided by

contractor staff.

After data collection was

completed, quality control visits

(consisting of resampling and

reabstracting all previously selected

sampled visits) were conducted in 18 of

the participating facilities. Debriefing

visits occurred in an additional 13

participating facilities in an effort to

Series 1, No. 37 [ Page 3
determine what worked well and what

was problematic with the study

methodology.

Recommendations

This study demonstrated the

feasibility of collecting ambulatory

surgery data from hospitals and

freestanding ambulatory surgery centers

using the NHDS methodology. In

addition to providing experience in

facility induction and data collection, the

field test identified problem areas and

provided insights into how well the

proposed model would work in a

national survey. These insights, as well

as the advice of a technical advisory

panel, were used to develop detailed

recommendations to assist with the

future design of a national survey.

Among the major recommendations

were the following:

The survey should include two

facility components—hospitals and

freestanding ambulatory surgery centers.

Facilities licensed as hospitals offering

outpatient surgery should represent the

hospital portion of the universe.

Ambulatory surgery centers licensed by

States and/or certified as ambulatory

surgery centers for Medicare

reimbursement should represent the

FSASC’s portion of the universe.

The hospital sampling frame should

include 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 is general (medical or surgical)

or children’s general should be included.

These hospitals should also have six

beds or more staffed for patient use.

This is the same sampling frame that is

used for the NHDS. The sampling frame

for the FSASC’s should be compiled

through a combination of two existing

regularly updated machine-readable

files: the Freestanding Outpatient

Surgery Center Database (sold by the

SMG Marketing Group) and the Health

Care Financing Administration Provider

of Services file. An annual update of the

sampling frames was recommended.

Because of the need for a high

participation rate, a number of methods

should be used to encourage and

enhance facility participation. Facilities

should be given the option of

conducting the sampling and abstracting

themselves, or of having trained data

abstractors do the sampling and

abstracting for them. Facilities should be

compensated for selecting the sample

and completing the abstracts when

facility personnel perform these tasks,

and for the effort of pulling and refiling

records when contract staff do the

abstracting. Extensive efforts should be

made to obtain letters of endorsement

from organizations representing the

facilities and the medical records

profession.

Identification of ambulatory surgery

visits within hospitals can be difficult,

particularly since hospitals may use

slightly different definitions of a surgical

visit. Generally, but not always,

hospitals base their definition of

ambulatory surgery on the location

where the procedure is performed. Use

of a definition that captures all surgery

visits performed in certain specified

types of special procedure units and in

operating suites is operationally easy to

apply and will provide reliable national

statistics.

The dataset used in the field test

should be simplified. Some data

elements should be deleted, others

should be redefined, and data collector

training should be revised to reflect field

test findings. Table A shows the list of

data elements that were field tested and

recommended for implementation in the

national survey. Two additional data

items, ‘‘Ethnicity’’ and ‘‘Total charges,’’

although not recommended for the

NSAS, were included in the pretest.

Although both data items had very low

response rates for the feasibility study

(4 percent and 57 percent), they were

included in the pretest dataset due to

their importance for health care policy

and research. The data elements that

were field tested but not recommended

for implementation included ‘‘SSN

absent/present,’’ ‘‘Marital status,’’ ‘‘Was

surgery cancelled or terminated?,’’

‘‘Place of service,’’ ‘‘Patient and visit

types,’’ ‘‘Post-op anesthesia

assessment,’’ ‘‘Assistants in surgery,’’

‘‘Other services provided as indicated in

the medical record,’’ and ‘‘Outcome

followup.’’

The field test and the deliberations

of the project’s technical advisory panel

provided the basis for recommending a

number of refinements to the field test

instruments and training materials.

Because of these refinements, a pretest

was recommended before implementing

a national survey. The testing could be

more limited than was attempted in the

feasibility study, but simplified versions

of the facility induction interview forms,

simplified instructions for sampling in

hospitals with multiple lists of surgical

patients, and the recommended changes

to shorten the dataset and abstract form

needed to be tested before full survey

implementation. These findings were

also needed to provide updated

estimates of time required to conduct

sampling and abstracting, to prepare the

full survey Office of Management and

Budget (OMB) approval request, to

support negotiations for reasonable

facility reimbursement and Bureau of

the Census (the proposed data collection

agent for the national survey)

interagency agreement funding levels,

and to prepare data collector training

materials.

Table A. Data elements collected in the

Feasibility Study field test and

recommended for use in the national

survey

Facility data and visit identification

Unique facility number

Separate unit (unique) number (if applicable)

Medical record ID number

Date of surgery

ZIP Code of patient’s customary residence

Patient characteristics

Date of birth or age

Sex

Race

Expected source of payment

Status/disposition of patient

Surgical visit and medical data

Times (operating room, recovery room, and

discharge)

Type of anesthesia

Anesthesia administered by (credentials)

ASA (American Society of Anesthesiologists)

classification of patient (if an anesthesiologist is

involved)

Surgical procedures performed

Diagnoses

Page 4 [ Series 1, No. 37
Pretest

The pretest for a National Survey

of Ambulatory Surgery was

conducted in 1993 by the U.S.

Bureau of the Census as the data

collection agent. The purpose of the

pretest was to test and finalize all

procedures, manuals, forms, instructions,

training, and data collection methods for

the NSAS.

Sampling Frame and

Sample Selection

The sample for the NSAS pretest

consisted of 80 facilities in five primary

sampling units (PSU’s): 45 hospitals

selected from the NHDS-eligible

hospitals listed in the 1991 SMG

Hospital Market Database (13) and 35

freestanding ambulatory surgery centers

(FSASC’s) selected from the 1991 SMG

Freestanding Outpatient Surgery Center

Database (14) and/or the Medicarecertified FSASC’s listed in the HCFA

Provider-of-Services (POS) file dated

February 1992 (15).

The five PSU’s were selected by the

following criteria: (a) at least one PSU was

selected from each of the four Census

regions (Northeast, Midwest, South, and

West); (b) selected PSU’s had large

numbers of FSASC’s relative to other

areas; and (c) to the extent possible, all

current NHDS PSU’s and areas used in the

NSAS feasibility study were excluded.

FSASC’s specializing in eye care

were oversampled to investigate

methods to improve participation rates

in these facilities. Hospitals with higher

ambulatory surgery volume were

oversampled to maximize opportunities

to test sampling procedures in facilities

that had ambulatory surgery in more

than one location. Additionally, to verify

data contained in the sampling frame, all

60 nonsampled facilities in the five

PSU’s were contacted by telephone and

asked to verify general information

about the facility.

Data Collection Methods

Materials and methodologies that

had been revised based on the feasibility

study results were provided to the U.S.

Bureau of the Census. Census staff

designed and printed the various survey

forms and questionnaires. Census staff

also prepared the regional office

instructions, the field representative

manual, the facility manual, the field

representative self-study, the induction

flashcard, the sampling table, the

training guide, the training aids, and the

regional office checklist.

Six site visits to ambulatory surgery

facilities in the Washington, D.C., area

were conducted by NCHS and Census

headquarters staff to help develop the

data collection procedures. During the

‘‘dry run’’ training, all materials were

reviewed and suggested changes were

incorporated into the final training

materials.

Field training for the pretest

included a one-half day supervisors’

conference, a 4-hour field representative

self-study, and a 11⁄2 day field

representative classroom training

session. During the supervisors’

conference, the background of the

NSAS was covered; the questionnaires,

sample listing operation, and sampling

and abstracting methods were reviewed;

and the field and office procedures were

discussed. Staff from NCHS and the

U.S. Bureau of the Census attended the

conference. The self-study covered

information about the purposes and

objectives of the pretest, defined

ambulatory surgery, presented the NSAS

forms, and discussed the composition of

the sample. The classroom training

included mock interviews to complete

the Telephone Screener Call to Sampled

Facilities and the Induction

Questionnaire, sampling exercises,

abstracting exercises, and role-playing

situations where survey respondent

questions and/or concerns about the

pretest were addressed.

The initial contact with each facility

was a telephone call made by a Census

regional office clerk. Form NSAS-1(X),

Initial Telephone Call To Sampled

Facilities, contains the script used for

this first call (see appendix IV). During

this call, the clerk:

Verified the name and address of

the facility

Gave a brief explanation of the

survey

Asked to whom an introductory

letter and information package

should be sent

Asked if the facility was a hospital,

a licensed and/or Medicare-certified

FSASC

The regional office staff then mailed

an introductory letter and information

package to the designated official at the

sample facility. This package included a

letter from NCHS that provided legally

required information to the facility and

informed them that a U.S. Bureau of the

Census representative would soon be

contacting them, a fact sheet that gave

additional information about the pretest,

six endorsement letters signed by

health-related organizations to encourage

the facility’s participation in the pretest,

the NCHS Staff Manual on

Confidentiality, and the National Center

for Health Statistics Organization and

Activities brochure.

The Census field representative’s

first contact with each facility was the

telephone screener call to the facility.

Form NSAS-2A(X), Telephone Screener

Call To Sampled Facilities, contains the

script used for this call (see appendix

IV). During this call, the field

representative:

Verified receipt of the introductory

letter and information package

Determined whether the facility was

in scope for the survey

Explained what type of information

would be collected during the

induction visit

Gave a brief explanation of the

survey

Requested an appointment for an

induction interview

The only contact with the

nonsampled facilities (see previous

section, ‘‘Sampling Frame and Sample

Selection’’) was the telephone call to the

facility made by the field representative.

Form NSAS-2B(X), Telephone Call To

Non-Sampled Facilities, contains the

script used for this call (see appendix

IV). During this call, the field

representative:

Series 1, No. 37 [ Page 5
Verified the name and address of

the facility

Gave a brief explanation of the

survey

Determined whether the facility was

in scope for the survey

Asked several questions about the

facility including how many patients

received ambulatory surgery during

the past 12 months

The Induction Questionnaire,

NSAS-3(X), is shown in appendix IV.

During the induction interview, the field

representative:

Completed an in-depth interview

about the facility with the

administrator or designated contact

person

Obtained information for each log

that contained the types of

ambulatory surgery specified during

the induction visit

Established the method of

participation

If necessary, trained designated

facility staff to sample and/or

abstract data

Gave a Thank You Letter to the

facility administrator

For each of the facilities that agreed

to participate in the pretest, a 2-month

sample (October–November 1992) of

approximately 20–25 ambulatory

surgery visits per month was selected

and recorded onto NSAS-4(X) forms,

and medical record data were abstracted

onto NSAS-5(X) forms for the first

month (October) of the sampled visits.

Both forms are included in appendix IV.

The Medical Abstract form, NSAS-5(X),

included all data items recommended in

the feasibility study. Two additional data

items were included: ethnicity and total

charges.

In the pretest, two methods of

sampling surgery logs that contained

both inpatient and ambulatory patients

were tested. In one method, every visit

was counted regardless of whether it

was inpatient or ambulatory surgery. For

selected surgical visits that were

inpatient surgery, only the case number

and inpatient status indicator were

recorded on the Sample Listing Sheet,

NSAS-4(X). No additional information

was recorded that would allow the

medical records to be pulled and data to

be abstracted. In the other method, only

ambulatory procedures were counted,

that is, inpatients were skipped during

the counting process.

There were two basic methods of

facility participation: primary and

alternate. In the primary method, facility

staff pulled the medical records and

abstracted medical record data for the

20–25 ambulatory surgery visits selected

for October. For the facilities that had a

single log or list of ambulatory surgery

visits from which to sample, the facility

staff also performed the sampling of

approximately 20–25 ambulatory

surgery visits per month for October and

November of 1992. For facilities that

had more than one log or list of

ambulatory surgery visits from which to

sample, the Census field representative

conducted the sampling. Although the

November sampled visits were not

abstracted, 2-month sampling was

necessary to test sampling methods

across months. For the primary method,

the field representative trained the

designated facility staff to abstract and,

if necessary, to sample the data. This

training was conducted during the

induction visit if possible. If not, the

field representative scheduled an

appointment to conduct the training

when facility staff were available. For

primary procedure facilities, the field

representative returned to the facility

after the abstracting was completed and

edited the forms before leaving.

In the alternate method of

participation, the Census field

representative conducted the 2-month

sampling and data abstracting for the

October sampled visits. Using this

method of participation, facility

personnel were only responsible for

pulling and refiling the medical records

for the October sampled visits. If

possible, the field representative selected

the October sample during the induction

visit so these sampled records could be

pulled and available for data abstracting

when they returned at the appointed day

and time to conduct the sampling for

November.

After sampling and abstracting were

completed for each facility, a Post Data

Collection Questionnaire was completed

and returned to NCHS. Form NSAS7A(X) was used by facility staff who

performed the abstracting (and sampling,

if applicable) in primary procedure

facilities to evaluate the pretest. Form

NSAS-7B(X) was completed by Census

field representatives who did the

sampling and abstracting in alternate

procedure facilities or the sampling for

primary facilities with multiple logs.

These forms (see appendix IV) were

useful in determining methods for

maximizing participation rates for the

national survey as well as providing an

accurate indication of the response

burden imposed on facility staff for

participation in the pretest.

The field representatives notified

their supervisors of any facilities that

initially refused to participate in the

pretest. The regional office supervisors

attempted to persuade the facility to

participate. Procedures were available to

provide compensation for facility

participation if they would not

participate without reimbursement.

Although none of the facilities requested

reimbursement for their activities in the

pretest, personnel at several facilities

indicated that compensation would be

necessary for participation in an ongoing

national survey.

NCHS selected 12 participating

facilities for quality control visits that

were conducted by Census program

supervisors. The quality control

operation included contacting the facility

to set up an appointment, resampling for

October and November, and

reabstracting 10 of the originally

sampled medical records.

Although there was no formal edit

of the NSAS forms in the regional

office, their staff checked in and

reviewed all completed materials,

verified that they received the correct

number of forms for each completed

induction questionnaire, and checked for

completion of a few specific items. The

regional office staff sent shipments of

completed induction forms to NCHS (in

Hyattsville, Maryland) on a flow basis

and sent completed sampling and

abstracting forms to the NCHS data

processing facility in Research Triangle

Page 6 [ Series 1, No. 37
Park, North Carolina, where the data

elements were coded and keyed.

A debriefing conference was held in

May 1993 that was attended by regional

office supervisors and staff from NCHS

and Census headquarters. The agenda

included discussions of the pretest

training, data collection activities, and

the forms used in the pretest. The

experiences of the pretest and the

debriefing discussions formed the basis

of the recommendations for the national

study.

Results

Of the 80 sample facilities, 5 were

determined to be out of scope. Of the 75

in-scope facilities, 68 submitted abstract

data, an overall response rate of

91 percent. Thirty-nine of the 42

in-scope hospitals participated for a

93 percent response rate; and 29 of the

33 in-scope FSASC’s participated for a

response rate of 88 percent. Of the 60

nonsampled facilities, 12 were

determined to be out of scope. Of the 48

in-scope facilities, 46 provided data, a

response rate of 96 percent.

Results of the field test of the two

methods for sampling from logs that

contained both inpatient and outpatient

procedures were inconclusive. The

Census field representatives did not

indicate a preference for either method.

Therefore, based on the expectation that

the nonsampling error would be less, the

method in which every visit was

counted regardless of whether it was an

inpatient or outpatient was

recommended for the national survey.

No major changes were

recommended as a result of the pretest

for a National Survey of Ambulatory

Surgery. Minor changes to the data

collection instruments and to the

procedures for the national survey were

recommended. They include:

Increase the classroom training time

by 1/2 day to allow more time for

the sampling exercises

Allow the field representatives

(instead of the regional office

clerks) to make the initial contact

with the facility

Include more instructions in the

field representative manual

regarding the determination of

in-scope and out-of-scope facilities

Provide procedures for handling

facilities that have merged or have

been purchased

Create an Annual Update form that

includes new ambulatory surgery

locations, dropped ambulatory

surgery locations, volume for all

new locations, and a new name of

the CEO and/or medical record

contact person

Change item 4 in the Telephone

Screener, NSAS-2A(X), to ‘‘In this

survey we are excluding facilities

that are exclusively a family

planning clinic, birthing center,

podiatry center, dentistry center, or

abortion clinic. Is (facility)

exclusively one of these?’’ Move

the ‘‘abortion clinic’’ category to the

middle or end of the response list

Revise the Induction Questionnaire,

NSAS-3(X), to accommodate

additional sets of section V for

facilities that have multiple logs

Delete the items that request

detailed information on Medicare

certification and state licensure

(items 5, 6, 8, 9, and 10) in the

Induction Questionnaire,

NSAS-3(X), because they were

confusing and unnecessary. Also,

clarify item 7, which requests the

names, addresses, and telephone

numbers of satellite or affiliated

facilities to include only those

facilities that perform ambulatory

surgery

Use a log identifier, such as the

alpha designation for each log from

the induction questionnaire (item

11), to link the sample visits to a

particular log. A log identifier (list

used) field should be added to the

Sample Listing Sheet, NSAS-4(X),

and the Medical Abstract form,

NSAS-5(X)

Add an ‘‘other’’ category to item

15, type of anesthesia, on the

NSAS-5(X)

The Pretest for a National Survey of

Ambulatory Surgery was completed in

June 1993. No major methodological

problems were encountered during the

pretest. Based on the successful

completion of the pretest, preparations

were begun to finalize the design

methodology and field the NSAS in

1994.

The 1994 National

Survey of Ambulatory

Surgery

The NSAS was first fielded as a

national survey in 1994. It was

anticipated that the design and

data collection activities for this survey

would remain constant for a number of

years; therefore, as an example of these

activities, the 1994 NSAS is described

below.

Sample Design

The 1994 NSAS used a multistage

probability design with samples of

hospitals and freestanding ambulatory

surgery centers selected at the first or

second stage and surgical visits selected

at the final stage. The NSAS was

designed to provide estimates of

ambulatory surgery based on the

following priority of survey objectives:

U.S., hospitals and freestanding

ambulatory surgery centers, and region

(16). The sampling frames and the

sample design are described in the

following text.

Sampling Frames

The universe of eligible facilities

for the 1994 NSAS consisted of

hospitals and freestanding ambulatory

surgery centers (FSASC’s). The

determination of whether an ambulatory

surgery facility was a hospital or a

freestanding center was based on the

sampling frame (SMG listing) from

which a facility was selected. The

hospital universe included all

noninstitutional, non-Federal hospitals in

the 50 states and the District of

Columbia that were either short-stay

(had an average length of stay for all

patients of less than 30 days) or a

general hospital (medical, surgical, or

Series 1, No. 37 [ Page 7
children’s) regardless of average length

of stay. The hospital also had to have

six or more beds staffed for inpatient

use (see appendix V). The hospital

eligibility definition is the same as that

used in the National Hospital Discharge

Survey and the National Hospital

Ambulatory Medical Care Survey.

The sampling frame for the hospital

universe was the April 1993 SMG

Hospital Market Data Base (17). As

shown in table B, the SMG Hospital

Market Data Base contained 6,267

hospitals meeting the NSAS eligibility

criteria. Of these hospitals, 5,252

(84 percent) indicated that ambulatory

surgery was performed in the hospital

and 1,015 (16 percent) indicated that no

ambulatory surgery was performed in

the facility. Hospitals were defined as

performing ambulatory surgery if the

hospital file indicated a nonzero number

of ambulatory surgeries.

The universe of freestanding

facilities consisted of freestanding

ambulatory surgery centers (FSASC’s)

that were regulated by States or were

certified for Medicare by the Health

Care Financing Administration (HCFA).

The sampling frame for the FSASC

universe consisted of facilities listed in

the 1993 SMG Freestanding Outpatient

Surgery Center Database (18) and/or

Medicare-certified facilities listed in

HCFA’s Provider-of-Services (POS) file

(see appendix V) (19). Facilities

specializing in dentistry, podiatry, pain

block, abortion, family planning, or

birthing were excluded. Duplicates in

the combined list were removed prior to

sampling. There were 1,732 eligible

facilities on the FSASC sampling frame.

Each hospital was classified by its

type of service or specialty. Each facility

on the FSASC sampling frame was

assigned a facility specialty based on the

specialty data from the SMG or POS

file. If only one specialty was listed, the

facility was assigned that specialty. If

two or more specialties were listed, the

facility was designated as multispecialty.

The 16 hospital types and 13 FSASC

specialty groups in the 1994 NSAS are

indicated in tables C and D, respectively.

Ninety-eight percent of the

NSAS-eligible hospitals were classified

as general (including medical, surgical,

or children’s). Sixty-eight percent of the

NSAS-eligible FSASC’s were classified

as general surgery or multispecialty and

18 percent were ophthalmic surgery (see

table B).

Facility Selection

The NSAS facility sample was

selected independently within two strata:

hospitals with ambulatory surgery and

Table B. Number of hospitals and freestanding ambulatory surgery centers by facility

specialty and region: National Survey of Ambulatory Surgery, 1993

Facility specialty and region

Number in

universe

Number in

sample

Hospitals with Ambulatory Surgery

All specialties:

All regions ..................................... 5,252 368

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 794 73

Midwest ..................................... 1,545 84

South ....................................... 1,958 146

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 955 65

General:

All regions ..................................... 5,104 259

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763 48

Midwest ..................................... 1,517 64

South ....................................... 1,894 104

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 930 43

All other specialties1

:

All regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 109

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 25

Midwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 20

South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 42

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 22

Hospitals without ambulatory surgery2

All regions ..................................... 1,015 50

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125 6

Midwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 8

South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 455 24

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 12

Freestanding ambulatory surgery centers

All specialties:

All regions ..................................... 1,732 333

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 61

Midwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304 64

South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 695 135

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563 73

General and multispecialty:

All regions ..................................... 1,184 153

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 29

Midwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 26

South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 478 63

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 398 35

Ophthalmic:

All regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 308 70

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 15

Midwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 14

South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 24

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 17

All other specialties3

:

All regions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240 110

Northeast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 17

Midwest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 24

South . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 48

West . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 21

1Includes alcohol and other chemical dependency; children’s general; children’s orthopedic; children’s other specialty; children’s

psychiatric; children’s rehabilitation; chronic disease; eye, ear, nose, and throat; institution for the mentally retarded; obstetrics

and gynecology; orthopedic; other specialty; psychiatric; rehabilitation; and tuberculosis and other respiratory diseases.

2This sample accounts for hospitals that create ambulatory surgery units and that had incorrect sampling frame data.

3Includes ear, nose, and throat; gastrointestinal; gynecological; hernia repair; laparoscopy/endoscopy; neurosurgery; orthopedic;

other; plastic; and urological.

Page 8 [ Series 1, No. 37
freestanding ambulatory surgery centers

(FSASC’s). Facilities with an extremely

high volume of ambulatory surgeries

were selected with certainty. As

described below, the remaining sample

was selected at either the first or second

stage in each of four regions. The stage

of selection for the noncertainty

facilities in each region depended on the

number of facilities in the specialty

within region and the number of those

facilities within primary sampling units

(PSU’s). The sample PSU’s used for the

NSAS were a probability subsample of

the PSU’s selected for the 1985–94

National Health Interview Survey

(NHIS).

The NHIS PSU sample was selected

from approximately 1,900

geographically defined PSU’s that

covered the 50 States and the District of

Columbia. A PSU consists of a county, a

group of counties, county equivalents

(such as parishes and independent

cities), towns, townships, minor civil

divisions (for some PSU’s in New

England), or a metropolitan statistical

area (MSA). MSA’s were defined by the

U.S. Office of Management and Budget

on the basis of the 1980 Census. The

1,900 PSU’s were stratified by

socioeconomic and demographic

variables and then selected with a

probability proportional to their size.

Stratification was done within four

geographical regions by MSA or

non-MSA status. Based on data from the

1980 Census of Population, a computer

program was used to minimize the

between-PSU variance for the

stratification variables. Because the

PSU’s were selected with a probability

proportional to size, the largest PSU’s in

the United States were selected with

certainty. Fifty-two PSU’s were selected

and referred to as self-representing

PSU’s. The remaining PSU’s, the

nonself-representing PSU’s, were

combined into 73 strata and 2 PSU’s

were selected without replacement and

with probability proportional to the

projected 1985 population within each

stratum. A detailed description of the

1985–94 NHIS PSU sample design is

presented in a Vital and Health Statistics

Series 2 report (20).

The sample PSU’s used for the

NSAS consisted of 112 of the 198

PSU’s used in the 1985–94 NHIS. The

NSAS PSU sample included with

certainty the 26 NHIS PSU’s with the

largest populations, half of the next 26

largest PSU’s, and one PSU from each

of the 73 PSU strata formed from the

remaining PSU’s.

Within selected specialty-region

groups, facilities were selected at the

first stage with certainty or were

selected using systematic random

sampling with probability proportional

to size where size was the number of

ambulatory surgeries. Forty-five

hospitals and 44 FSASC’s were selected

at this stage of sampling.

For the remaining specialty-region

groups, facilities were selected at the

second stage within the first stage

sample of 112 PSU’s used for the NSAS

using systematic random sampling with

probability proportional to size where

size was the number of ambulatory

surgeries. From noncertainty PSU’s, 229

hospitals and 160 FSASC’s were

selected. From the certainty PSU’s, 93

hospitals and 129 FSASC’s were

selected.

The number of facilities in the

NSAS universe and sample by facility

type and region are shown in table B; a

fixed panel of 368 hospitals and 333

FSASC’s was selected for the NSAS

sample.

Additionally, a small sample of

hospitals was selected from the stratum

of hospitals that indicated no ambulatory

surgery was performed at the facility

(according to the sampling frame data).

This sample was selected to permit

complete and current estimates of the

ambulatory surgery, thus accounting for

hospitals that create ambulatory surgery

units and for hospitals with incorrect

sampling frame data. There were 1,015

hospitals with no indication of

ambulatory surgery: 486 in NSAS

PSU’s and 529 outside NSAS PSU’s.

Hospitals in the NSAS PSU’s in this

stratum were arrayed by region,

specialty, ownership, Census stratum,

and number of beds. A systematic

random sample of 50 hospitals was

selected based on probability

proportional to size where size was the

PSU weight.

Ambulatory Surgery Visit

Selection

A sample of ambulatory surgery

visits was selected using systematic

random sampling techniques. Within

each facility, all locations where

ambulatory surgery was performed were

identified. All eligible locations were

included, i.e., there was no subsampling

of surgical locations. These locations

included, but were not limited to,

general or main operating room, satellite

operating room, cystoscopy room,

endoscopy room, cardiac catheterization

laboratory, and laser procedures room.

Eligibility criteria for the NSAS surgery

locations included:

The location was dedicated to

surgery or diagnostic procedures

A list of surgeries for the location

existed or could be created

Table C. List of types of service used to

classify hospitals in the 1994 National

Survey of Ambulatory Surgery

Alcohol and other chemical dependency

Children’s general

Children’s orthopedic

Children’s other specialty

Children’s psychiatric

Children’s rehabilitation

Chronic disease

Eye, ear, nose, and throat

General

Institution for the mentally retarded

Obstetrics and gynecology (maternity)

Orthopedic

Other specialty

Psychiatric

Rehabilitation

Tuberculosis and other respiratory diseases

Table D. List of specialties used to classify

the freestanding ambulatory surgery

centers in the 1994 National Survey of

Ambulatory Surgery

Ear, nose, and throat surgery

Gastrointestinal surgery

General surgery

Gynecological surgery

Hernia repair

Laparoscopy/endoscopy

Multispecialty surgery

Neurosurgery

Ophthalmic surgery

Orthopedic surgery

Other surgery

Plastic surgery

Urological surgery

Series 1, No. 37 [ Page 9
Ambulatory surgery, i.e., previously

scheduled outpatient surgery, was

performed

The location was not dedicated

exclusively to dentistry, podiatry,

abortion, pain block, or small

procedures

The location was recognized as

distinct from an outpatient

department (in hospitals).

For each location eligible for the

survey, a systematic random sample of

ambulatory surgery visits was selected.

A list or log that could be used for

sampling that location’s ambulatory

surgery visits was identified. The list

could be the operating room log for the

location or another available list of

surgery visits that included all the

ambulatory surgery performed at that

location. The list should also include the

medical record number, date of surgery,

inpatient or outpatient status (if

necessary), and any other information

required to locate the sample medical

records.

A log could include the surgeries

performed in more than one surgery

location or each operating room could

have a separate list. For example, one

list of surgeries could be available for a

suite of operating rooms instead of a

separate list for each room.

For each facility, the approximate

number of ambulatory surgery visits for

a 12-month period for all survey-eligible

locations was obtained from the facility

staff. Based on this number, a single

sampling interval was determined which,

if the facility-provided estimate of

annual ambulatory surgery volume was

accurate, would yield a total sample of

20–25 ambulatory surgery visits per

month for the entire facility. A

random-start number less than or equal

to the sampling interval was selected for

the first month of sampling for each log

or list and, along with the sampling

interval, was used to select surgery

visits from each designated log or list in

the facility. Although visit sampling was

done monthly, quarterly, or on some

other periodic interval at each facility,

the sampling was continued across time

for each sampling list as though the

entire sample from that list was selected

at one sampling session. That is, the

sampling interval across sampling

sessions was identical and the count for

each sampling session was continued

from where the count ended in the prior

session. This basic procedure was

adapted, as necessary, to the record

keeping systems of the particular

facilities. In an effort to reduce the

sampling error in surgery logs that

contained both inpatient and ambulatory

procedures, every visit was counted

regardless of whether it was inpatient or

ambulatory surgery. For the sampled

surgery visits that were inpatient

surgery, only the case number and

inpatient status indicator were recorded

on the Sample Listing Sheet, NSAS-4.

After all survey-eligible locations

were identified and sampling was begun,

there were three possible reasons for a

visit to be designated out of scope for

the NSAS:

The patient was originally admitted

as a hospital inpatient

The patient was admitted through

the emergency room

The patient was a ‘‘no-show’’ or left

the facility prior to the receipt of

anesthesia and/or the

commencement of the procedure.

Auxiliary information (e.g., number

of ambulatory surgery visits in the

sampling frame for the month) needed

for estimation purposes was also

obtained from each sampling list. Data

from the medical record was abstracted

for each ambulatory surgery visit thus

selected. During 1994, data for 117,861

sample visits were included in the final

NSAS database.

Data Collection Procedures

The Bureau of the Census was the

data collection agent for the 1994

NSAS. Census headquarters staff

designed and printed the survey

questionnaires and telephone call scripts,

which are included in appendix VI.

Census staff also prepared and printed

the regional office instruction manual,

the field representative self-study

materials, the field representative

manual, and the facility manual. In

addition, Census headquarters staff

prepared all training materials, including

the training guide and all training aids.

Field Training

Training for the 1994 NSAS was

conducted in March of 1994. During

this month, the Census field

representatives underwent extensive

training in survey procedures, using

self-study materials and classroom

training. In addition, each field

representative was given a manual that

contained detailed instructions and

information necessary to induct facilities

into the survey, conduct the sampling

and abstracting, and edit and transmit

completed forms. In selecting field

representatives for the NSAS, every

effort was made to choose experienced

staff who had also worked on the

National Hospital Discharge Survey or

the National Hospital Ambulatory

Medical Care Survey. These field

representatives were most familiar with

working with medical professionals and

had an understanding of medical

terminology and procedures.

Training for the NSAS was

conducted by the Census staff and

consisted of a supervisor’s conference, a

self-study session, and a classroom

training session. The 1-day supervisor’s

conference was held in Alexandria,

Virginia, and attended by NSAS

supervisors from the 12 Census regional

offices. During the conference, NSAS

procedures, sampling, and data

abstracting were described in detail.

NSAS field representative training

included both self-study and classroom

training. The self-study took

approximately 4 hours to complete and

was used to introduce the field

representatives to NSAS concepts and to

give a general overview of the NSAS

forms and procedures. The 2-day

classroom training, attended by field

representatives and regional office

clerks, was held in four locations:

Seattle, Dallas, Atlanta, and

Philadelphia. Each of these sessions was

conducted by Census supervisors who

had attended the supervisor’s

conference. To ensure uniformity of

training, each supervisor followed a

written script that was prepared by

Census headquarters staff. The training

Page 10 [ Series 1, No. 37
covered the following topics: inducting

facilities; sampling procedures, including

determination of the random start and

‘‘take every’’ numbers; data abstracting;

training facility staff; and editing

completed forms. In addition to

providing a detailed explanation and

illustration of the forms and procedures,

a major focus of the training was on

gaining facility cooperation. The training

utilized many interactive techniques

such as role playing, practice interviews,

and discussion groups. NHDS field

representatives attended many of the

sessions to point out obstacles in dealing

with medical staff and to make

suggestions for overcoming them.

Facility Induction

Initial contact with the sample

hospitals and freestanding ambulatory

surgery centers was made through a

telephone call, using the telephone

number provided in the 1993 listing

from SMG Marketing, Inc. (16,17) or

HCFA’s POS file (18). The call was

made to identify to whom a letter about

the study should be sent and to validate

information used to sample the facility.

The script for this initial telephone call

appears in the NSAS-1 (see appendix

VI).

Shortly after this initial telephone

call, a letter from the National Center

for Health Statistics was sent to the

designated official of the facility. The

letter (see appendix VII) introduced the

study and indicated that the facility had

been selected to participate in the

NSAS. Included with the letter were a

summary of NCHS activities, a general

description of the study, and letters of

endorsement for the study from the

American Hospital Association, the

American Academy of Ophthalmology,

the American Health Information

Management Association, the Federated

Ambulatory Surgery Association, and

the American College of Surgeons. The

letters endorsing the NSAS are shown in

appendix VIII.

Approximately 10 days after

sending the letter, a telephone screener

call was made to the recipient of the

letter. The NSAS-2 contains the script

for this call (see appendix VI). During

this call, the field representative verified

receipt of the introductory letter and

information package, determined

whether the facility was in scope,

answered any general questions about

the NSAS, and arranged an appointment

for a facility induction interview.

The purposes of the induction

interview were to:

Explain the study and urge

participation

Establish the method of

participation

Determine the locations in the

facility where ambulatory surgery

was performed

Implement an ambulatory surgery

sampling plan

Determine the location of medical

records and other sources of needed

data

Train the facility staff, if necessary

The length of time needed for the

induction interview varied by the size

and complexity of the facility and

whether it was necessary to train facility

staff. It generally took from 1⁄2 to 11⁄2

hours. Information about the sample

facility obtained by the Census field

representative was recorded on the

induction questionnaire, NSAS-3 (see

appendix VI). If, during the induction

interview, it was determined that fewer

than 50 ambulatory surgery procedures

were conducted in the previous year, the

facility was deemed to be ineligible for

the 1994 NSAS.

During the induction process, the

method of participation for each facility

was determined. There were two

methods of facility participation for the

NSAS: primary and alternate. For the

primary method, facility staff agreed to

pull the medical records and abstract

medical record data onto the medical

abstract forms (NSAS-5, which is shown

in appendix VI) for a sample of

approximately 20–25 ambulatory

surgery visits per month, and to refile

the medical records. For facilities that

had a single log or list of ambulatory

surgery visits from which to sample,

facility staff also performed the

sampling. For facilities that had more

than one log or list from which to

sample, the Census field representative

conducted the sampling because of the

difficulty and need for special training to

perform this task.

The primary method of participation

was preferred for several reasons. First,

the lack of standard medical record

forms or formats across facilities and

the individuality of the record keeping

made field representative training

difficult. Second, for confidentiality

reasons some facilities did not want the

field representatives to review surgical

logs or see actual medical records.

Third, facility staff knew the particular

record-keeping systems of the facility

and were familiar with medical terms

and coding.

For the primary method facilities,

the field representative trained the

designated facility staff to abstract and,

if necessary, to sample the data. This

training was conducted during the

induction visit if at all possible. If not,

the field representative scheduled an

appointment to conduct the training

when facility staff were available. A

Facility Manual containing detailed

sampling and abstracting instructions

similar to those in the Field

Representative’s Manual provided the

basis for training facility staff and was

left with the staff member(s) for future

reference. The field representatives

maintained communication with staff at

primary facilities to ensure that data

abstraction was on schedule and to

answer any questions that arose.

The alternate method of

participation was selected when facility

staff were unwilling to perform the

sampling and abstracting activities. For

this method, the field representative

selected the sample of 20–25 visits per

month and performed the data

abstracting activities, thus reducing

facility staff responsibilities to pulling

and refiling specified medical records. If

possible, the field representative selected

a 2-month sample of ambulatory surgery

visits during the induction visit and

made an appointment to return to the

facility to perform the abstracting for

these sampled visits. For alternate

method facilities, the field representative

attempted to visit the facility bimonthly,

complete the abstract forms for the

records selected during the previous

visit, and select records for abstracting

Series 1, No. 37 [ Page 11
at the next visit. This process, which has

been used successfully for the NHDS

for many years, allowed time for

medical records to be completed and

pulled from the medical records room

prior to the visit.

Facilities still refusing to participate

at this point in the induction process

were offered monetary compensation for

participation. If compensation proved

necessary, the field representative

completed a Memorandum of

Agreement (NSAS-6, which is shown in

appendix VI). This established the

amount of reimbursement the facility or

facility staff would receive for the

method of participation selected. If the

amount requested was higher than the

field representative was authorized to

approve, a call was placed to the Census

regional office to request authorization

from the NSAS supervisor. In 1994,

approximately 40 percent of the sample

facilities requested compensation.

The number and percent of in-scope

and responding facilities in the 1994

NSAS are presented in table E. Of the

751 hospitals and freestanding

ambulatory surgery centers in the 1994

sample, 134 were found to be out of

scope (ineligible) because they went out

of business, performed less than 50

ambulatory surgeries in the previous

year, or otherwise failed to meet the

criteria for the NSAS. Of the 617

in-scope (eligible) facilities, 494

responded to the survey. The response

rate was 88 percent for the hospitals and

70 percent for the freestanding facilities.

Data Collection

To initiate the actual data collection

process for the 1994 NSAS and, if

necessary, to train facility staff, the field

representative requested the facility’s

January 1994 log(s) or list(s) that were

specified in item 7 of the NSAS-3,

Induction Questionnaire (see appendix

VI). The determination of locations and

ambulatory surgery visits that were

eligible for the NSAS have been

discussed in a previous section of this

report entitled ‘‘Ambulatory Surgery

Visit Selection.’’ Within participating

facilities, a systematic random sampling

technique was used to select a sample of

ambulatory surgery visits. Using the

Sampling Table/Random Number Tables

(see appendix VI) and the estimate of

annual ambulatory surgery volume

provided during the induction process,

the field representative determined the

sampling interval (‘‘Take Every’’) for

the facility and selected a random ‘‘Start

With’’ number for each NSAS-eligible

January log.

The Sample Listing Sheet,

(NSAS-4, which is shown in appendix

VI) was used to list the monthly sample

selected from each operating room log

or other acceptable list. A separate

NSAS-4 was used for each month

sampled and for each log or list

identified during the induction process

(see NSAS-3, item 7).

A medical abstract form (NSAS-5,

which is shown in appendix VI) was

completed for each in-scope sample

visit. Terms and definitions relating to

the medical abstract form are shown in

appendix IX. The NSAS medical

abstract form is based on the medical

abstract used in the National Hospital

Discharge Survey, which is based on the

Uniform Hospital Discharge Data Set,

(UHDDS). Therefore, many of the

variables in these surveys are similar or

identical. Two of the UHDDS items

(‘‘principal expected source of payment’’

and ‘‘status/disposition of patient’’) have

been slightly revised to make them

applicable to ambulatory surgery visits.

The remainder of the data items on

the abstract form were strongly

recommended for inclusion by the

feasibility study technical advisory panel

with the exception of ‘‘total charges.’’

Although this data item was not usually

available in the medical record and the

nonresponse rate for this item was high

in the pretest (32 percent nonresponse),

it was included in the NSAS because of

its importance for health care policy

issues. With the exception of ‘‘total

charges,’’ all data items on the NSAS

abstract form were successfully tested in

the pretest with a response rate higher

than 70 percent. One data item, ‘‘ASA

classification,’’ which was recommended

by the technical advisory panel, was

excluded from the NSAS due to an

exceedingly high (56 percent)

nonresponse rate in the pretest. As a

result of the pretest, minor revisions in

the data items were recommended by

the Bureau of the Census and were

incorporated into the NSAS Medical

Abstract form.

All completed Sample Listing

Sheets and Medical Abstract forms were

reviewed by the field representatives at

the facility before being submitted on a

flow basis to the regional offices.

Attempts were made at that time to

retrieve missing data or to correct

inconsistent data. The NSAS-16,

Transmittal Notice, (see appendix VI)

was used by the field representatives for

transmitting all completed work to the

Census regional offices. All forms were

reviewed for completeness at the

regional office. Forms that failed this

preliminary edit were returned to the

field representative for additional

information. All forms were transmitted

on a flow basis to the NCHS data

processing center in Research Triangle

Park, North Carolina, for data entry,

medical coding, and editing.

1995 Data Collection Activities

Prior to beginning the 1995 NSAS

data collection, field representatives

contacted all of the 1994 participating

facilities to update information on the

locations where ambulatory surgery was

currently being performed to obtain

estimated annual ambulatory surgery

volumes of any new locations and to

verify administrative information. The

Annual Update Form (NSAS-7) was

Table E. Number of facilities in the 1994 National Survey of Ambulatory Surgery sample,

number and percent of in-scope facilities, and response rates by type of facility

Type of facility Sample

In

scope Percent Respondents

Response

rate

Hospitals1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 368 333 90.5 293 88.0

Hospitals2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 3 6.0 3 100.0

Freestanding ambulatory surgery centers . . . . . . . 333 281 84.4 198 70.5

1Hospitals selected from the stratum of hospitals that the sampling frame data indicated performed ambulatory surgery.

2Hospitals selected from the stratum of hospitals that the sampling frame data indicated performed no ambulatory surgery.

Page 12 [ Series 1, No. 37
used to record this information (see

appendix VI). Due to the dynamic

nature of ambulatory surgery, this

process was essential to ensure accurate

sampling of ambulatory surgery visits

for the 1995 NSAS. In addition,

facilities determined to be out of scope

for the 1994 NSAS were contacted prior

to beginning the 1995 NSAS data

collection to determine whether they

were still ineligible for the study.

Attempts were made to induct and

collect data from any of these facilities

determined to be in scope for the 1995

NSAS as well as from those facilities

that refused to participate in 1994.

Confidentiality

Assurance of confidentiality was

provided to all facilities according to

Section 308(d) of the Public Health

Service Act (42 USC 242m), which

states that:

‘‘No information, if an establishment

or person supplying the information or

described in it is identifiable, obtained in

the course of activities undertaken or

supported under section . . . 306, . . . may be

used for any purpose other than the

purpose for which it was supplied unless

such establishment or person has consented

(as determined under regulations of the

Secretary) to its use for such other purpose

and (1) in the case of information obtained

in the course of health statistical or

epidemiological activities under section

. . . 306, such information may not be

published or released in other form if the

particular establishment or person

supplying the information or described in it

is identifiable unless such establishment or

person has consented (as determined under

regulations of the Secretary) to its

publication or release in other form, . . .’’

Strict procedures were utilized to

prevent disclosure of NSAS data. All

information that could identify the

facilities was confidential, was seen only

by persons engaged in the NSAS, and

was not disclosed or released to others

for any other purpose. As the data

collection agent for the NSAS, the

Bureau of the Census agreed to comply

with NCHS laws while engaged directly

in survey activities.

Names or other identifying

information for individual patients were

not removed from the sample facility.

To ensure patient confidentiality, the

sampled facility kept the top copy of the

completed 1994 Sample Listing Sheets,

NSAS-4, which could have contained

the names of the patients or other

personal identifying information. Such

information did not appear on the other

copies of the NSAS-4’s that were

transmitted to NCHS.

Data Processing

Data from the NSAS medical

abstract forms were coded by trained

medical coding personnel from the

Division of Data Processing at the

NCHS Computer Facility in Research

Triangle Park, North Carolina. The

International Classification of Diseases,

9th Revision, Clinical Modification

(ICD–9–CM) was used to code the final

diagnoses (item 17) and the surgical and

diagnostic procedures (item 18) on the

medical abstract form (21). A maximum

of seven diagnostic codes and six

surgical or nonsurgical procedure codes

were assigned for each sample abstract.

In addition to followups for missing

and inconsistent data made by the field

staff, numerous clerical edits were

performed on data received for central

data processing. Detailed editing

instructions were provided to manually

review the medical abstract forms to

reclassify or recode ‘‘other’’ entries, as

well as revising entries on the sample

listing sheets. Online keying edits for

code ranges and inconsistencies were

also performed.

All medical coding and keying

operations were subject to quality

control procedures. These procedures

utilized a two-way 10 percent

independent verification procedure in

which abstracts were independently

recoded by a second coder, with

discrepancies resolved by the chief

coder. Error rates for 1994 NSAS data

were less than 5 percent for medical

coding of diagnoses and procedures and

less than 0.5 percent for the entry of the

nonmedical data.

After medical coding and keying

were completed, extensive computer

editing was conducted to ensure that all

responses were accurate, consistent,

logical, and complete. When necessary,

records were reviewed manually to

resolve inconsistencies. Missing age and

sex data were imputed using a hot deck

procedure. This imputation was required

for less than 2 percent of the 1994

NSAS records and all imputed data

were identified as such on the data tapes

to enable the analyst to distinguish

between imputed and reported data.

Estimation Procedures

The probability sample design of

the NSAS allows the sample data to be

weighted to produce national estimates

for the United States. Unweighted data

are not used for analysis because

unweighted data ignore the

disproportionate sampling used in

NSAS.

Statistics from the NSAS are

derived by a multistage estimation

procedure that produces essentially

unbiased national estimates for hospitals

and FSASC’s. The weight includes three

basic components: (a) inflation by

reciprocals of the probabilities of

selection, (b) adjustment for

nonresponse, and (c) ratio adjustment to

fixed totals (22). Each component is

briefly described below.

Inflation by Reciprocals of

Probabilities of Selection

Because the survey utilized a

multistage sample design, there were

several probabilities of selection. They

were: (a) the probability of selecting the

PSU, (b) the probability of selecting the

facility, and (c) the probability of

selecting the ambulatory surgery visit

within the facility. The overall

probability of selection was the product

of the probabilities at each stage. The

inverse of the overall selection

probability was the basic inflation

weight.

Adjustment for Nonresponse

Estimates from NSAS data were

adjusted to account for sample units that

were in scope, but did not participate in

the study. These adjustments were

calculated to minimize the impact of

nonresponse on final estimates by

imputing to nonresponding units the

Series 1, No. 37 [ Page 13
characteristics of similar responding

units. As nonresponse may occur at each

stage of sampling, adjustments were

included for the month within log, log

within facility-specialty strata, and

facility within specialty strata. A final

adjustment was included to account for

sample facilities involved in mergers.

Ratio Adjustment

NSAS estimates included

poststratification adjustments within 20

strata defined by five facility-specialty

groups and four Census regions. The

five groups, defined by facility type

(hospital or freestanding surgery center)

and the facility specialty (as designated

on the sampling frame), are:

Hospitals specializing in children’s

care (general, orthopedic,

psychiatric, rehabilitation, and

other)

All other hospitals

Freestanding ambulatory surgery

centers with a single specialty in

plastic surgery

Freestanding ambulatory surgery

centers with a single specialty in

eye surgery

All other freestanding ambulatory

surgery centers

The ratio adjustment was a

multiplication factor whose numerator

was the total of annual numbers of

outpatient surgeries in the sampling

frame facilities in each stratum and

whose denominator was the estimated

number of surgeries in that stratum

based on the sample facilities. The

numerator for the hospitals was based

on figures from the SMG Hospital

Market Database and the numerator for

the freestanding ambulatory surgery

centers was based on figures from the

SMG Freestanding Outpatient Surgery

Centers Database. These adjustment

ratios are updated annually.

Smoothing of Weights

Procedures were used to reduce

excessively large weights while

maintaining the total visit estimates

within each of the 20 post-stratification

strata. In each strata the weights were

inflated to account for the reductions in

excess weights.

Reliability of Estimates

Because statistics from the NSAS

are based on a sample, they may differ

somewhat from the figures that would

be obtained if a complete census were

taken using the same forms, definitions,

instructions, and procedures. However,

the probability design of NSAS permits

the calculation of sampling errors. The

standard error is primarily a measure of

sampling variability that occurs by

chance because only a sample rather

than the entire population is surveyed.

The standard error, as calculated for the

NSAS, also reflects part of the variation

that arises in the measurement process,

but does not include estimates of any

systematic biases that may be in the

data. The relative standard error (RSE)

of an estimate is obtained by dividing

the standard error by the estimate itself

and is expressed as a percent of the

estimate. Generally in NCHS published

data reports, an asterisk (*) indicates

any estimate with more than a

30 percent relative standard error.

In repeated samples using the same

forms and procedures, the chances are

about 68 of 100 that an estimate from

the sample would differ from a complete

census by less than the standard error.

The chances are about 95 of 100 that

the difference would be less than twice

the standard error, and about 99 of 100

that it would be less than 2 1/2 times as

large.

Estimation of Standard Errors

Estimates of sampling variability for

the 1994 NSAS data presented in NCHS

publications were computed using a

first-order Taylor Series approximation

of the deviation of estimates from their

expected values. The SUDAAN

software was used to compute the

standard errors. A description of this

software and the approach it uses has

been published (23).

Standard Error Approximations

The SUDAAN procedure can be

used to compute directly the standard

errors for NSAS estimates. However,

this procedure is not practical or feasible

for all users of the data. Therefore, a

generalized procedure for approximating

the relative standard errors for the

NSAS estimates was developed.

Relative standard errors were

computed for a range of estimates.

Regression techniques were used to

produce generalized variance equations

from which a relative standard error for

any estimate may be approximated. The

parameters of the equations are

presented in table F. Rules explaining

the use of these equations are presented

in the section below.

To derive error estimates that would

be applicable to a wide variety of

statistics and that could be prepared at

moderate cost, several approximations

were required. As a result, standard

errors computed using this procedure

should be interpreted as approximate

rather than exact for any specific

estimate.

Standard Error Applications

Estimates of Standard Errors for

Aggregate Estimates

The approximate standard errors for

estimates of the number of ambulatory

surgery visits with a particular

characteristic may be computed using

the following formula, where x is the

aggregate estimate of interest, and a and

b are the appropriate parameters from

table F.

SE (x) = √ ax2 + bx

The approximate relative standard

error, expressed as a percent, for the

estimated number of ambulatory surgery

visits with a particular characteristic,

may be computed as follows:

RSE (x) = 100√ a + b/x

Estimates of Rates Where the

Denominator Is Assumed to have

Negligible Error

The approximate relative standard

error for a rate in which the

denominator is the total U.S. population

or one or more of the age-sex-race

groups of the total population is

equivalent to the relative standard error

Page 14 [ Series 1, No. 37
elative standard error formulae for the National Survey of Ambulatory Surgery by selected characteristics: United States, 1994

Hospital Freestanding ambulatory surgery center

sted diagnoses All-listed procedures First-listed diagnoses All-listed procedures First-listed diagnoses All-listed procedures

ba b aba b abab

6 936.205191 0.003143 1042.899257 0.003650 937.293644 0.003981 979.807673 0.018124 246.302178 0.014197 756.815317

9 243.411027 0.005227 374.804666 0.007031 259.440811 0.006342 400.073897 0.019727 148.897701 0.023878 111.243531

744.535434 0.003760 617.095495 0.003961 765.853727 0.004677 609.191764 0.018401 194.160869 0.011996 581.791449

2 65.117616 0.009705 221.957283 0.016933 94.811756 0.011514 232.068663 0.058636 9.701896 0.031078 86.170098

422.786841 0.004022 650.559674 0.005294 467.627182 0.004410 733.863531 0.019479 148.963379 0.019803 208.035925

7 376.824342 0.007491 168.934774 0.006899 395.110427 0.009749 118.058917 0.019775 133.575567 0.024660 131.182539

9 406.831653 0.004337 572.576624 0.007944 425.446412 0.005884 577.976415 0.030411 111.597626 0.044043 1.418704

2 985.293492 0.013715 857.242241 0.016358 999.602588 0.018943 737.602043 0.078174 58.884015 0.068619 58.777503

8 644.607350 0.012862 551.969910 0.010397 653.985309 0.012367 593.605658 0.091502 165.353131 0.095489 216.628667

404.799980 0.009781 527.582934 0.012810 346.621537 0.012582 522.614760 0.046815 90.105394 0.044594 114.414283

4 457.195190 0.016911 649.373580 0.025218 411.895141 0.022593 618.081376 0.062637 174.678875 0.068295 256.6595774 31.834535 0.039479 32.664023 0.051251 26.389558 0.050903 25.226753 0.098051 6.783712 0.086114 8.3417232 211.166190 0.009182 136.913855 0.012175 213.793529 0.011596 137.695464 0.033715 98.594817 0.047121 1.438590

3 250.110798 0.010870 344.151585 0.015120 260.182032 0.012655 348.099706 0.071604 66.821585 0.079337 36.770793

3 157.029327 0.039332 226.739743 0.056480 173.181171 0.051979 215.742100 0.215697 1.579989 0.159833 23.4754895 777.159166 0.003298 1127.335798 0.003948 835.302005 0.003895 1209.832213 0.013969 252.712881 0.010695 605.398623

0 151.933029 0.020759 293.084515 0.036721 129.202040 0.027805 246.162675 0.100330 35.442751 0.109715 39.262511

0 145.424550 0.073571 149.427354 0.088636 123.675106 0.083409 154.663601 0.256708 0.770684 0.222136 0.791779

0 318.862505 0.066747 381.219870 0.080030 325.472443 0.085397 297.598560 0.204803 56.398866 0.220737 115.011084

Series 1, No. 37 [ Page 15
of the numerator that can be obtained

using the relative standard error formula

above and the appropriate coefficients

from table F. The standard error is then

given by:

SE (r) = r c RSE(r)

The population figures used in

computing annual rates for the NSAS

are based on the July 1 estimates of the

civilian population, including

institutionalized persons, of the United

States.

Estimates of Standard Errors of

Percents Where Both the Numerator

and Denominator are Estimated from

the Same Sample and the Numerator

Is a Subclass of the Denominator

The approximate relative standard

errors, expressed as a percent, for

estimates of percents may be computed

using the appropriate relative standard

errors for the aggregate statistics as

follows: Obtain the relative standard

error of the numerator and denominator

of the percent. Square each of the

relative standard errors, subtract the

resulting value for the denominator from

the resulting value for the numerator,

extract the square root, and multiply by

100.

RSE (p) = RSE(x/y) = 100√ RSE2 (x) – RSE2(y)

Alternatively, if both numerator and

denominator are estimated from the

same sample, approximate relative

standard errors, expressed as a percent,

for estimates of percents may be

computed using the following formula,

where p is the percent of interest and y

is the denominator of the percent using

the appropriate parameters.

RSE (p) = 100√ b(1–p)/py

For standard errors, the appropriate

formula is:

SE (p) = √ bp(1–p)/y

The approximation of the absolute

or relative standard error is valid if the

relative standard error of the

denominator is less than 0.05 (24) or if

the relative standard errors of the

numerator and denominator are both less

than 0.10 (25).

Estimates of Ratios (r = x/y) Where

the Numerator Is not a Subclass of

the Denominator

The standard error for a ratio may

be approximated by

SE~r! = r v RSE(r)

where

RSE (r) =

RSE(x/y) =100 √RSE 2(x)+ RSE 2(y)

This approximation is valid if the

relative standard error of the

denominator is less than 0.05 (24) or if

the relative standard errors of the

numerator and denominator are both less

than 0.10 (25).

Estimates of Differences Between Two

Statistics

The standard error of the difference

between two statistics is approximated

by:

SE (x1 – x2) = √ SE 2(x1) + SE2 (x2)

where SE(x1) and SE(x2) are computed

using the appropriate directions. This

formulation represents the standard error

for the difference between separate and

uncorrelated characteristics, although it

is only a rough approximation in most

other cases.

Nonsampling Error

Estimates based on the 1994 NSAS

are subject to nonsampling as well as

sampling errors. Nonsampling errors

include reporting and processing errors

as well as biases due to nonresponse or

incomplete response. Although the

magnitude of the nonsampling errors

cannot be computed, these errors are

kept to a minimum by procedures built

into the operation of the survey. To

eliminate ambiguities and encourage

uniform reporting, careful attention was

given to the phrasing of questions,

terms, and definitions. Also, extensive

pretesting of most data items and survey

procedures were performed. The steps

taken to reduce bias in the data are

discussed in the sections on data

collection and data processing. Quality

control procedures and edit checks

discussed in the data processing section

reduced errors in data coding and

processing. Because survey results are

subject to sampling and nonsampling

errors, the total error will be larger than

the error due to sampling variability

alone.

Data Dissemination

This report presents a description of

the NSAS through its first year of

operation (1994). Summary data from

the 1994 NSAS is available in an

Advance Data from Vital and Health

Statistics report entitled Ambulatory

Surgery in the United States, 1994 (26).

A Vital and Health Statistics report that

combines data on ambulatory surgery

and surgery performed on hospital

inpatients during 1994 is forthcoming.

Additional descriptive, analytical, and

methodological reports will be published

in Vital and Health Statistics, Series 1,

2, and 13. Brief reports on topics of

special interest will be published in

Advance Data From Vital and Health

Statistics. Information will also be

presented in journal articles and in

papers presented at professional

meetings. As resources permit, special

tabulations and analyses will beprovided

to both public and private data

requestors.

To obtain general information on

published reports or to receive The

Catalog of Publications, Catalog of

Electronic Data Products, and Catalog

of Public-Use Data Tapes, contact:

Data Dissemination Branch (DDB)

National Center for Health Statistics

Centers for Disease Control and

Prevention

6525 Belcrest Road, Room 1064

Hyattsville, Maryland 20782

(301) 436–8500

E-mail: nchsquery@cdc.gov

Internet: http://www.cdc.gov/nchswww/

nchshome.htm

For each data year, a public-use

data tape and data tape documentation

will be prepared for distribution through

the National Technical Information

Service (NTIS). The public-use data

tape for the 1994 NSAS is currently

available. There are also plans to release

NSAS data on diskette and CD-ROM

using the Statistical Export and

Page 16 [ Series 1, No. 37
Tabulation System (SETS) database

software. To purchase data tapes and

other computer products, contact:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

(703) 487–4650

References

1. New surgical technologies reshape

hospital strategies. Hospitals

66(9):30–36, 38, 40–42. 1992.

2. Leader S, Moon M. Medicare trends in

ambulatory surgery. Health Affairs

Spring: 158–170. 1989.

3. Durant G. Ambulatory surgery centers:

surviving, thriving into the 1990’s.

Medical Group Management Journal

36(2):16–18, 20. 1989.

4. SMG Marketing Group, Inc. Outpatient

surgery centers exceed three million

cases. SMG Market Letter 8(6). 1994.

5. SMG Marketing Group, Inc. U.S.

hospitals performed 23.7 million

surgical procedures in 1993. SMG

Market Letter 8(3). 1994.

6. Kozak LJ. Hospital inpatient surgery:

United States, 1983–87. Advance data

from vital and health statistics; no 169.

Hyattsville, Maryland: National Center

for Health Statistics. 1989.

7. Institute of Medicine. Toward a

National Health Care Survey: A Data

System for the 21st Century.

Washington. National Academy Press.

1992.

8. McLemore T, Bacon WE.

Establishment Surveys of the National

Center for Health Statistics.

Proceedings of the International

Conference on Establishment Surveys.

Buffalo, New York. pp. 93–98. 1993.

9. Evaluation of the coverage of health

facilities of the National Master Facility

Inventory. Contract No. 282–86-0103.

Columbia, Maryland. La Jolla

Management Corporation. 1989.

10. Duggar, BC. Defining and finding

ambulatory surgery facilities. J

Ambulatory Care Management. Vol. 13.

No. 1. Jan. 1990.

11. Feasibility study of a national survey of

ambulatory surgery. Contract No.

200–89-8022. Columbia, Maryland.

Center for Health Policy Studies. 1991.

12. Duggar BC, Balicki, B. The feasibility

of collecting data in a national survey

of ambulatory surgery. Proceedings of

the 1991 Public Health Conference on

Records and Statistics. pp. 48–53.

Washington. 1992.

13. SMG Marketing Group, Inc. Hospital

Market Database. Chicago: Healthcare

Information Specialists. April 1991.

14. SMG Marketing Group, Inc.

Freestanding Outpatient Surgery

Centers Database. Chicago: Healthcare

Information Specialists. 1991.

15. Health Care Financing Administration.

Provider of Services Public Use File.

Baltimore: 1992.

16. Shimizu, I. Specifications for selecting

the NSAS sample facilities.

Unpublished document. 1993.

17. SMG Marketing Group, Inc. Hospital

Market Database. Chicago: Healthcare

Information Specialists. April 1993.

18. SMG Marketing Group, Inc.

Freestanding Outpatient Surgery

Centers Database. Chicago: Healthcare

Information Specialists. 1993.

19. Health Care Financing Administration.

Provider of Services Public Use File.

Baltimore: 1993.

20. Massey JT, Moore TF, Parsons VL,

Tadros W. Design and estimation for

the National Health Interview Survey,

1985–94. National Center for Health

Statistics. Vital Health Stat 2(110).

1989.

21. Public Health Service and Health Care

Financing Administration. International

Classification of Diseases, 9th Revision,

Clinical Modification. Washington:

Public Health Service. 4th ed. 1991.

22. Shimizu I. Estimators for NSAS.

Unpublished document. 1995.

23. Shah BV, Barnwell BG, Biegler GS.

SUDAAN User’s Manual: Software for

Analysis of Correlated Data, Release

6.40. Research Triangle Park, NC:

Research Triangle Institute. 1995.

24. Hansen MH, Hurwitz WN, Madow

WG. Sample survey methods and

theory vol. 1. New York: John Wiley

and Sons. Wiley Classics Library

Edition, 1993.

25. Cochran WG. Sampling techniques.

New York: John Wiley and Sons. 1953.

26. Kozak LJ, Hall MJ, Pokras R,

Lawrence L. Ambulatory Surgery in the

United States, 1994. Advance data from

vital and health statistics; no 283.

Hyattsville, Maryland: National Center

for Health Statistics. 1997.

Series 1, No. 37[ Page 17
Appendix I

Legislative Authorization

Public Health Service Act

Section 306(a) & (b)

NATIONAL CENTER FOR

HEALTH STATISTICS

Section 306. [242k](a) There is

established in the Department of Health

and Human Services the National Center

for Health Statistics (hereinafter in this

section referred to as the ‘‘Center’’)

which shall be under the direction of a

Director who shall be appointed by the

Secretary and supervised by the

Assistant Secretary for Health (or such

officer of the Department as may be

designated by the Secretary as the

principal adviser to him for health

programs).

(b) In carrying out section 304(a),

the Secretary, acting through the

Center—

(1) shall collect statistics on—

(A) the extent and nature of

illness and disability of the

population of the United States

(or any groupings of people

included in the population),

including life expectancy, the

incidence of various acute and

chronic illnesses, and infant and

maternal morbidity and mortality,

(B) the impact of illness and

disability of the population on

the economy of the United States

and on other aspects of the

well-being of its population (or

of such groupings),

(C) environmental, social, and

other health hazards,

(D) determinants of health,

(E) health resources, including

physicians, dentists, nurses, and

other health professionals by

specialty and type of practice and

supply of services by hospitals,

extended care facilities, home

health agencies, and other health

institutions,

(F) utilization of health care,

including utilization of (i)

ambulatory health services by

specialties and type of practice of

health professionals providing

such service, and (ii) services of

hospitals, extended care facilities,

home health agencies, and other

institutions,

(G) health care costs and

financing, including the trends in

health care prices and costs, the

sources of payments for health

care services, and Federal, State,

and local governmental

expenditures for health care

services, and

(H) family formation, growth,

and dissolution;

(2) shall undertake and support

(by grant or contract) research,

demonstrations, and evaluations

respecting new or improved

methods for obtaining current data

on the matters referred to in

paragraph (1);

(3) may undertake and support

(by grant or contract) epidemiologic

research, demonstrations, and

evaluations on the matters referred

to in paragraph (1); and

(4) may collect, furnish, tabulate,

and analyze statistics, and prepare

studies, on matters referred to in

paragraph (1) upon request of

public and nonprofit private entities

under arrangements under which

the entities will pay the cost of the

service provided.

Amounts appropriated to the Secretary

from payments made under

arrangements made under paragraph (4)

shall be available to the Secretary for

obligation until expended.

Appendix II

Members of the Technical

Advisory Panel for the

Feasibility Study of a

National Survey of

Ambulatory Surgery

Dr. James Aquavella

Clinical Director of Ophthalmology and

Director of the Cornea Research

Laboratory

University of Rochester Medical Center

Medical Director

Rochester Ophthalmic Associates

Ambulatory Surgery Center

Dr. Enzo DiGiacomo

President

Society for Ambulatory Care

Professionals of the American Hospital

Association

Past-President

Massachusetts Chapter of the American

College of Emergency Physicians

Ms. Irene Fraser

Director

Division of Ambulatory Care and

Emergency Services

American Hospital Association

Mr. George Greenberg

Program Analyst

Department of Health and Human

Services

Office of the Assistant Secretary for

Planning and Evaluation

Mr. Allen D. Hecht

Executive Vice President

Medical Management and Development

Corporation

Dr. John Henley

President

Federated Ambulatory Surgery

Association

Administrator

Fayetteville Ambulatory Services, Inc.

Dr. Marie-Louise Levy

Associate Professor of Anesthesia

Director of In-and-Out Anesthesia

George Washington University Hospital

Mr. Ted Matson1

Senior Staff Specialist

Division of Ambulatory Care and

Emergency Services

American Hospital Association

Mr. Kurt Price1

Deputy Director

Maryland Health Services Cost Review

Commission

Ms. Jane Ruseski

Associate Director of Methodology

Maryland Health Services Cost Review

Commission

1

Mr. Kurt Price and Mr. Ted Matson were replaced

by Ms. Jane Ruseski and Ms. Irene Fraser,

respectively, between the first and second TAP

meetings.

Page 18 [ Series 1, No. 37
Appendix III

Letters and Data Collection Instruments for the

Feasibility Study of a National Survey of

Ambulatory Surgery

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Appendix IV

Letters and Data Collection

Instruments for the Pretest of

the National Survey of

Ambulatory Surgery

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Appendix V

Definitions of Terms Used

in the Survey

Ambulatory surgery—Previously

scheduled surgical and nonsurgical

procedures performed on an outpatient

basis in a hospital or freestanding

ambulatory surgery center.

In scope—Ambulatory surgery

conducted in general or main

operating rooms, satellite operating

rooms, cystoscopy rooms,

endoscopy rooms, cardiac

catheterization labs, and laser

procedure rooms.

Out of scope—Ambulatory surgery

conducted in locations dedicated

exclusively to dentistry, podiatry,

abortion, pain block, or small

procedures (i.e., lump and bump

procedure rooms).

Hospital—A hospital with an average

length of stay for all patients of less

than 30 days (short stay) or a hospital

whose specialty is general (medical or

surgical) or children’s general, except

Federal hospitals and hospital units of

institutions and hospitals with less than

six beds staffed for patient use.

Out of scope—Hospitals where less

than 50 ambulatory surgery

procedures were conducted in the

previous year.

Freestanding Ambulatory Surgery

Center—A freestanding ambulatory

surgery facility that was regulated by a

State or was certified for Medicare by

the Health Care Financing

Administration (HCFA). For the 1994

NSAS, a facility listed in the 1993 SMG

Freestanding Outpatient Surgery Center

Database and/or a Medicare-certified

facility included in the Health Care

Financing Administration’s Provider of

Services (POS) file.

Out of scope—Facilities where less

than 50 ambulatory surgery

procedures were conducted in the

previous year as well as facilities

specializing in dentistry, podiatry,

pain block, abortion, family

planning, or birthing.

Ambulatory surgery visit—A visit by a

person to a hospital or freestanding

ambulatory surgery center to receive

previously scheduled surgical or

nonsurgical procedures on an outpatient

basis. Each appearance of an outpatient

to a surgical location constitutes one

visit regardless of the number of

procedures the patient receives.

Procedures—All surgical procedures

(such as tonsillectomy), diagnostic

procedures (such as cystoscopy), and

other therapeutic procedures (such as

injection or infusion of cancer

chemotherapeutic substance) reported on

the patient’s medical record are included

in the NSAS. A maximum of six

procedures per ambulatory surgery visit

are coded.

Region—Facilities are classified by

location in one of the four geographic

regions of the United States that

correspond to those used by the U.S.

Bureau of the Census.

Region States included

Northeast Connecticut, Maine,

Massachusetts, New

Hampshire, New Jersey, New

York, Pennsylvania, Rhode

Island, and Vermont

Midwest Illinois, Indiana, Iowa,

Kansas, Michigan, Minnesota,

Missouri, Nebraska, North

Dakota, Ohio, South Dakota,

and Wisconsin

South Alabama, Arkansas,

Delaware, District of

Columbia, Florida, Georgia,

Kentucky, Louisiana,

Maryland, Mississippi, North

Carolina, Oklahoma, South

Carolina, Tennessee, Texas,

Virginia, and West Virginia

West Alaska, Arizona, California,

Colorado, Hawaii, Idaho,

Montana, Nevada, New

Mexico, Oregon, Utah,

Washington, and Wyoming

Page 92 [ Series 1, No. 37
Appendix VI

Data Collection Forms Used in

the 1994 NSAS

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Appendix VII

Introductory Letter

Series 1, No. 37[ Page 117
Appendix VIII

Endorsement Letters

Page 118 [ Series 1, No. 37
Series 1, No. 37 [ Page 119
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Series 1, No. 37 [ Page 121
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Appendix IX

Definitions of Terms

Relating to the Medical

Abstract Form

Facility number—The uniquely

assigned four-digit facility identification

number.

NSAS number and list used—The

uniquely assigned NSAS number for the

sampled ambulatory surgery visit and

the alphabetic identifier for the log or

list used for sampling.

Medical record number—The uniquely

assigned number within a facility that

identified the individual patient.

Date of surgery—The date on which the

sampled ambulatory surgery visit

occurred, using two digits for the month

and day, and four digits for the year.

Residence ZIP code—The nine-digit or

five-digit U.S. Postal Service ZIP code

that designates the patient’s residence.

Date of birth—The patient’s date of

birth, using two digits for the month and

day, and four digits for the year.

Age—Entered on the medical abstract

form only if the date of birth was

unavailable. The computed age that

appears in NSAS reports and data tapes

is calculated from the date of birth and

is the age at last birthday on the date of

surgery.

Race—This item was completed based

on information recorded in the patient’s

medical record.

White—Patient is white or

Caucasian.

Black—Patient is Negro, black, or

African American.

American Indian/Eskimo/Aleut—

Patient has American Indian origin

or is classified as Eskimo, Aleut, or

any other Alaskan Native origin.

Asian/Pacific Islander—Patient has

origins in the Far East, Southeast

Asia, the Indian subcontinent, or the

Pacific Islands. For example, this

area includes China, India, Japan,

Korea, the Philippine Islands, and

Samoa.

Other—Patient’s race does not fall

into a category described above.

Not stated—Patient’s race is not

indicated.

Ethnicity—Refers to the national or

cultural group from which the patient is

descended, i.e., the nationality or lineage

of the patient’s ancestors. Race and

ethnicity are considered separate

characteristics. The item was completed

from factual information from the

medical record and was not completed

based on the patient’s name or

birthplace.

Hispanic origin—Person is of

Mexican, Puerto Rican, Cuban,

Central or South American, or other

Spanish culture or origin, regardless

of race.

Non-Hispanic—All other persons.

Not stated—Patient’s ethnicity is

not indicated.

Status/disposition of patient—The

status/disposition of the patient upon

discharge from the facility.

Routine discharge to customary

residence—Patient was discharged

to return to his or her normal place

of residence, i.e., home, nursing

home, or prison.

Discharge to observation

status—Patient was kept at the

facility for up to 72 hours for

‘‘observation,’’ but was not

considered an inpatient.

Discharge to recovery care

center—Patient was discharged to

an organized recovery care center.

Admitted to hospital as

inpatient—Patient was admitted to

the hospital as an inpatient after

ambulatory surgery was performed.

Surgery canceled or

terminated—Patient was scheduled

for ambulatory surgery, appeared at

the designated time and received

anesthesia and/or began the

procedure, but the procedure was

terminated prior to completion.

Other-Specify—Patient’s status

and/or disposition was something

other than the above categories.

Status/disposition not

stated—Patient’s status and/or

disposition was not indicated.

Expected source(s) of payment—The

method of payment expected for the

ambulatory surgery visit. If only one

payment source was indicated in the

medical record, only the ‘‘Principal’’

column was marked and the ‘‘Other

additional sources’’ column was left

blank. If two or more payment sources

were indicated and one was designated

as the primary source, the ‘‘Principal’’

column was marked for the primary

source and the ‘‘Other additional

sources’’ column was marked for the

remaining indicated sources of payment.

Only one source of payment was

marked in the ‘‘Principal’’ column.

Worker’s Compensation—Expected

payment is a state or municipal

disability insurance or industrial

accident insurance. This category

does not include company health

insurance plans or direct payment

by an employer.

Medicare—Expected payment is

under the Health Care Financing

Administration’s health insurance

program for the aged and disabled.

This includes Medicare part A

and/or B and patients under the

renal dialysis program. If Medicare

benefits are available, they are

considered ‘‘Principal’’ unless

another source of payment is

specifically stated as the principal

source.

Medicaid—Expected payment is

made under Title XIX of the Social

Security Act, which gives Federal

assistance to states to provide health

care for medically indigent patients.

Medicaid may be known as public

aid, medical assistance, general

relief, or some title specific to the

State of residence, such as MediCal

in California.

CHAMPUS—Expected payment is

made by the Civilian Health and

Medical Program of the Uniformed

Services and is for military

Series 1, No. 37 [ Page 123
personnel and their families who

use civilian facilities.

Other government

payments—Expected payment under

the Title V Program, including

payment under the State-funded

Maternal and Child Health Program

and the State-funded Crippled

Children’s Program, or if the

expected payment cannot be

classified in one of the other four

government categories.

Blue Cross/Blue Shield—Expected

payment is made by a Blue Cross

insurance plan, Blue Cross

Association, or Blue Shield plan.

HMO/PPO—Charges are included

under a prepayment plan. Includes

health maintenance organizations

(HMO’s), independent practice

associations (IPA’s), preferred

provider organizations (PPO’s), etc.

Other private or commercial

insurance—Expected payment is

made by any private insurance plan

not included in the ‘‘Blue

Cross/Blue Shield’’ or ‘‘HMO/PPO’’

categories.

Self-pay—Charges are paid in part

or in full by the patient or the

patient’s family, which will not be

reimbursed by a third party.

Includes ‘‘co-payments’’ and

‘‘insurance deductibles.’’

No charge—Visits for which no fee

is charged.

Other—Any other source of

payment not covered in the

categories above.

Billing Number—Needed on occasion

to match medical record data to ‘‘total

charge’’ information.

Total charges—Total charges as

reported by the facility. In most cases,

this amount was the facility fee charged

for the procedure(s) performed that

excluded any professional (e.g., surgeon)

fees. However, some charges may have

included professional fees if facilities

were unable to separate them from an

all-inclusive bill or ‘‘flat fee’’ charged

for certain procedures.

Time—The appropriate clock time

(including a.m. or p.m.) was recorded or

the ‘‘not available’’ box was checked for

each phase of the ambulatory surgery

visit.

Type of anesthesia—All types of

anesthesia used during the ambulatory

surgery visit that were clearly

documented in the medical record

were included. If the type of

anesthesia was not clearly identified,

‘‘other’’ was marked. If the type of

anesthesia was not given, ‘‘none

specified’’ was marked.

Topical/local—Anesthesia that

numbs only part of the body;

feeling is numbed by temporarily

blocking nerves at or near the site

of the operation.

Regional—A special form of local

anesthesia; anesthetics are injected

around the main nerve to the

affected area. The anesthetics stop

sensation in a wide region of the

body. Regional anesthesia can be of

three types: epidural, spinal, and

block.

General—Anesthesia causing the

whole body to lose sensation. There

are two ways that general anesthesia

can be administered: inhalation of

gas or vapor; and intravenous.

Anesthesia administered by—The

specialty of the person(s) administering

the anesthesia.

Final diagnoses—The ‘‘final,’’

‘‘discharge,’’ ‘‘primary,’’ ‘‘secondary,’’

‘‘associated,’’ ‘‘additional,’’ and ‘‘other’’

diagnoses specifically identified and

clearly summarized on the discharge

summary of the patient’s medical record

was recorded. ‘‘Admitting,’’

‘‘preliminary,’’ ‘‘working,’’ ‘‘tentative,’’

or ‘‘provisional’’ diagnoses were not

abstracted. If the final diagnoses were

not specifically identified and clearly

summarized on the discharge summary,

the face sheet of the medical record was

used for abstracting the diagnostic

information. If the physician did not

specify a principal diagnosis (the

condition established after study to be

chiefly responsible for causing the

patient’s visit for ambulatory surgery),

the first final diagnosis listed on the

patient’s medical record was designated

on the medical abstract form as

principal.

Surgical and diagnostic

procedures—The physician’s exact

wording for each procedure as it

appeared in the patient’s medical record.

If a principal procedure was specified, it

was listed as ‘‘principal.’’ If a principal

procedure was not specified, the first

procedure listed in the medical record

was recorded as the principal procedure

on the abstract form.

Page 124 [ Series 1, No. 37
Vital and Health Statistics

series descriptions

SERIES 1. Programs and Collection Procedures—These reports describe

the data collection programs of the National Center for Health

Statistics. They include descriptions of the methods used to

collect and process the data, definitions, and other material

necessary for understanding the data.

SERIES 2. Data Evaluation and Methods Research—These reports are

studies of new statistical methods and include analytical

techniques, objective evaluations of reliability of collected data,

and contributions to statistical theory. These studies also include

experimental tests of new survey methods and comparisons of

U.S. methodology with those of other countries.

SERIES 3. Analytical and Epidemiological Studies—These reports present

analytical or interpretive studies based on vital and health

statistics. These reports carry the analyses further than the

expository types of reports in the other series.

SERIES 4. Documents and Committee Reports—These are final reports of

major committees concerned with vital and health statistics and

documents such as recommended model vital registration laws

and revised birth and death certificates.

SERIES 5. International Vital and Health Statistics Reports—These

reports are analytical or descriptive reports that compare U.S.

vital and health statistics with those of other countries or present

other international data of relevance to the health statistics

system of the United States.

SERIES 6. Cognition and Survey Measurement—These reports are from

the National Laboratory for Collaborative Research in Cognition

and Survey Measurement. They use methods of cognitive science

to design, evaluate, and test survey instruments.

SERIES 10. Data From the National Health Interview Survey—These

reports contain statistics on illness; unintentional injuries;

disability; use of hospital, medical, and other health services; and

a wide range of special current health topics covering many

aspects of health behaviors, health status, and health care

utilization. They are based on data collected in a continuing

national household interview survey.

SERIES 11. Data From the National Health Examination Survey, the

National Health and Nutrition Examination Surveys, and the

Hispanic Health and Nutrition Examination Survey—Data

from direct examination, testing, and measurement on

representative samples of the civilian noninstitutionalized

population provide the basis for (1) medically defined total

prevalence of specific diseases or conditions in the United States

and the distributions of the population with respect to physical,

physiological, and psychological characteristics, and (2) analyses

of trends and relationships among various measurements and

between survey periods.

SERIES 12. Data From the Institutionalized Population Surveys—

Discontinued in 1975. Reports from these surveys are included in

Series 13.

SERIES 13. Data From the National Health Care Survey—These reports

contain statistics on health resources and the public’s use of

health care resources including ambulatory, hospital, and

long-term care services based on data collected directly from

health care providers and provider records.

SERIES 14. Data on Health Resources: Manpower and Facilities—

Discontinued in 1990. Reports on the numbers, geographic

distribution, and characteristics of health resources are now

included in Series 13.

SERIES 15. Data From Special Surveys—These reports contain statistics on

health and health-related topics collected in special surveys that

are not part of the continuing data systems of the National

Center for Health Statistics.

SERIES 16. Compilations of Advance Data From Vital and Health

Statistics—Advance Data Reports provide early release of

information from the National Center for Health Statistics’ health

and demographic surveys. They are compiled in the order in

which they are published. Some of these releases may be

followed by detailed reports in Series 10–13.

SERIES 20. Data on Mortality—These reports contain statistics on mortality

that are not included in regular, annual, or monthly reports.

Special analyses by cause of death, age, other demographic

variables, and geographic and trend analyses are included.

SERIES 21. Data on Natality, Marriage, and Divorce—These reports

contain statistics on natality, marriage, and divorce that are not

included in regular, annual, or monthly reports. Special analyses

by health and demographic variables and geographic and trend

analyses are included.

SERIES 22. Data From the National Mortality and Natality

Surveys—Discontinued in 1975. Reports from these sample

surveys, based on vital records, are now published in Series 20

or 21.

SERIES 23. Data From the National Survey of Family Growth—These

reports contain statistics on factors that affect birth rates,

including contraception, infertility, cohabitation, marriage,

divorce, and remarriage; adoption; use of medical care for family

planning and infertility; and related maternal and infant health

topics. These statistics are based on national surveys of women

of childbearing age.

SERIES 24. Compilations of Data on Natality, Mortality, Marriage,

Divorce, and Induced Terminations of Pregnancy—These

include advance reports of births, deaths, marriages, and divorces

based on final data from the National Vital Statistics System that

were published as supplements to the Monthly Vital Statistics

Report (MVSR). These reports provide highlights and summaries

of detailed data subsequently published in Vital Statistics of the

United States. Other supplements to the MVSR published here

provide selected findings based on final data from the National

Vital Statistics System and may be followed by detailed reports

in Series 20 or 21.

For answers to questions about this report or for a list of reports published in

these series, contact:

Data Dissemination Branch

National Center for Health Statistics

Centers for Disease Control and Prevention

6525 Belcrest Road, Room 1064

Hyattsville, MD 20782-2003

(301) 436–8500

E-mail: nchsquery@cdc.gov

Internet: http://www.cdc.gov/nchswww/nchshome.htm
DHHS Publication No. (PHS) 98-1313, Series 1, No. 37

7-0673 (10/97)

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