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Scientific Data Documentation
National Ambulatory Medical Care Survey, 1975



This material provides documentation for users of the Micro-Data tapes of the
National Ambulatory Medical Care Survey (NAMCS) conducted by the National
Center for Health Statistics.  Section 1. "Description of the National
Ambulatory Medical Care Survey," includes information on the history of NAMCS,
the scope of the survey, the sample, data collection procedures, symptom
coding procedures, population estimates, and sampling errors.  Section II
provides technical details of the tape (number of tracks, record length, etc.).
Section III provides a detailed description of the contents of each data
record by location.  An appendix defines certain terms used in this


    INTRODUCTION.--These Micro-Data Tapes comprise the data collected by the
National Ambulatory Medical Care Survey (NAMCS) in 1975, conducted by the
National Center for Health Statistics (NCHS).  The National Ambulatory Medical
Care Survey'provides continuous data from samples of patient records selected
from a national sample of office-based physicians.  These national estimates
describe the utilization of ambulatory medical care services in the coterminous
United States.  In 1975 there were approximately 63,000 patient records
sampled from the 2,472 doctors that participated in the survey.  For a
description of the survey design and data collection procedures, see below.  For
a more detailed description of the survey design, data collection procedures,
and the estimation process see reference 1.

     HISTORY.--To provide more complete and precise information the the
utilization of the nation's ambulatory care resources and on the nature and
treatment of illness among the population seeking ambulatory care, the NCHS in
1967 began exploring possibilities for surveying morbidity in private physicians'
offices.  A national technical advisory group was established.  Initial
discussions resulted in a tentative protocol that called for periodic meetings
of a working group comprised of the Director of the NCHS Division of Health
Resources Statistics, the Project Officer and staff, the contractor's
representatives, and a consultant group from The Johns Hopkins University in
     The background and development of methods employed for the NAMCS required
exploratory and feasibility studies conducted over a period of 6 years.
Literature review and consultation documented needs and potential uses for
national ambulatory medical care statistics.  Information regarding accepted
definitions, uniform terminology, procedural experience,.or practical
classifications for the problems and conditions encountered in ambulatory care
settings was found to be limited.  First, data collection forms and procedures
were developed and tested by sample physicians in a national field survey, which
demonstrated the difficulty of achieving high levels of participation.  Refined
data collection forms and improved procedures were further tested by a second
sample of physicians in an extensive national survey lasting over 2 quarters in
1 year.  Results demonstrated the usefulness of professional endorsement,
procedural efficiency, and minimal work requirements in achieving physician-
participatibn levels exceeding 80 percent.
     Finally, with advice and support from the technical advisory group, the
American Medical Association, individual experts, other professional groups,
and elements of the Public Health Service, NCHS initiated the National Ambulatory
Medical Care Survey in 1973.

     SCOPE OF THE SURVEY.--The basic sampling unit for the NAMCS is the physician-
patient encounter or visit.  Only visits in the offices of nonfederally employed
physicians classified by the American Medical Association (,MIA) or the American
Osteopathic Association (AOA) as "office-based, patient carell were included in the
1975 NAMCS.  In addition, physicians in the specialties of anesthesiology,
pathology, and radiology were excluded from the physician universe.  Major
types of ambulatory encounters not included in the 1975 NAMCS were those made
by telephone, those made outside of the physician's office, and those made in
hospital or institutional settings.  It is planned to extend the NAMCS to
include these encounters in the future, though some complex methodological and
sampling problems must be resolved first.

    SAMPLING FRAME AND SIZE OF SAMPLE.--The sampling frame for the NAMCS is
composed of all physicians contained in the master files maintained by the
AMA and AOA as of December 31, 1974, who met the following criteria:

Office-based, as defined by the AMA and AOA;

Principally engaged in patient care activities;

Nonfederally employed;

Not in specialties of anesthesiology, pathology, clinical pathology,
forensic pathology, radiology, diagnostic radiology, pediatric
radiology, or therapeutic radiology.

     The 1975 NAMCS sample included 3,507 physicians: 3,366 Mb's and 141
 doctors of osteopathy.  Sample physicians were screened at the time of the
 survey to assure that they met the above-mentioned criteria; 438 physicians
 did not meet all of the criteria and were, therefore, ruled out of scope
 (ineligible) for the study.  The most frequent reasons for being out of scope
 were that the physician was retired, deceased, or employed in teaching,
 research, or administration.  Of the 3,069 in-scope (eligible) physicians,
 2,472 (80.5 percent) participated in the study.  The physician universe, sample
 size, and response rates by physician specialty are shown in table I. Of the
 participating physicians, 375 physicians saw no patients during their assigned
 reporting period because of vacations, illness, or other reasons for being
 temporarily not in practice.

     Sample Design.  The 1975 NAMCS utilized a multistage probability design
that involved probability samples of primary sampling units (PSU's), physician
practices within PSU'S, and patient visits within practices.  The first-stage
sample of 87 PSU's was selected by the National Opinion Research Center (NORC),
the organization responsible for field operations under contract to the NCHS.
A PSU is a county, a group of adjacent counties, or a standard metropolitan
statistical area (SMSA).  A modified probability proportional-to-size procedure
usinle separate sampling frames for SMSA's and for nonmetropolitan counties was
employed.  After sorting and stratifying by size, region, and demographic
characteristics, each frame was divided into sequential zones of 1 million
residents, and a random number was drawn to determine which PSU came into the
sample from each zone.

      The second stage consisted of a probability sample of practicing physicians
selected from the master files maintained by the American Medical Association
(AMA) and American Osteopathic Association (AOA).  Within each PSU, all eligible
physicians were arranged by nine specialty groups: general and family practice,
internal medicine, pediatrics, other medical specialties, general surgery,
obstetrics and gynecology, other surgical specialties, psychiatry, and other
specialties.  Then, within each PSU, a systematic random sample of physicians
was selected in such a way that the overall probability of selecting any
physician in the United States was approximately constant.  

     The final stage was the selection of patient visits within the annual
practices of sample physicians.  This involved two steps.. First, the total
physician sample was divided into 52 random subsamples of approximately equal
size, and each subsample was randomly assigned to 1 of the 52 weeks in the
survey year.  Second, a systematic random sample of visits was selected by
the physician during the assigned week.  The sampling rate varied for this
final step from a 100-percent sample for very small practices to a 20-percent
sample for very large practices as determined in a presurvey interview.  The
method by which the sampling rate was determined is described in reference 12.

     Data Collection.--The actual data collection for the NAMCS was carried
out by the physician aided by his office staff when possible.  Two data
collection forms were employed by the physician: the Patient Log and the
Patient Record (Figure 1).  The Patient Log is a sequential listing of patients
seen in the physician's office during his assigned reporting week.  This list
served as the sampling frame to indicate the visit for which data were to be
recorded.  A perforation between the patient names and patient visit
characteristics permitted the physician to remove patient names and protect

     Based on the physician's estimate of the expected number of office visits
each physician was assigned a patient-sampling ratio.  These ratios were
designed so that about 30 Patient Records were completed during the assigned
reporting week.  Physicians expecting 10 or fewer visits each day recorded
data for all of them, while those expecting more than 10 visits per day
recorded data for every second, third, or fifth visit based on the predetermined
sampling interval.  These procedures minimized the data collection workload and
maintained approximate equal reporting levels among sample physicians
regardless of practice size.  For physicians assigned a patient sampling ratio,
a random start was provided on the first page of the log, so that predesignated
sample visits on each succeeding page of the log provided a systematic
random sample of patient visits during the reporting period.

     Data Processing and Medical Coding.--In addition to the completeness checks
made by the field staff, clerical edits were performed upon receipt of the
data for central processing.  These procedures proved quite efficient, reducing
the item nonresponse rates to a negligible amount--2 percent or less for all
data items.

     Information contained in item 5 (patient's problem) of the Patient Record.was
coded according to a special classification system developed for that purpose.2
Diagnostic information, item 9 of the Patient Record, was coded according to
the Eighth Revision International classification of diseases, adapted for use in
the United States (ICDA).(3)  A maximum of three problems and three diagnoses
were coded.  A two-way independent verification procedure with 100-percent
verification was used to control the medical coding operation.  Differences
between coders were adjudicated at the National Center for Health Statistics.

    Information from the Induction Interview and Patient Record was
keypunched, with 100-percent vertification, and converted to computer tape.
At this time, extensive computer consistency and edit checks were performed.
Data items still unanswered at this point were imputed by assigning a value
from a Patient Record with similar characteristics; amputations were based
on physician specialty, major reason for visit, and broad diagnostic

     Population Figures.--The base population used in computing annual visit
rates is presented in table II.  These figures are based on provisional
estimates for the civilian noninstitutionalized population as of July 1, 1975,
provided by the U.S. Bureau of the Census.  Because the NAMCS includes data
for only the coterminous United States, the original census estimates were
modified to account for the exclusion of Alaska and Hawaii from the study.
For this reason the population estimates should not be considered as official
population estimates and are presented here solely for the purpose of providing
denominators for rate computations.

     Estimation Procedures.--Statistics produced from the 1976 National
Ambulatory Medical Care Survey were derived by a multistage estimating
procedure.  The procedure produces essentially unbiased national estimates
and has basically three components: (1) inflation by reciprocals of the
probabilities of selection, (2) adjustment for rionresponse, and (3) a ratio
adjustment to fixed totals.  Each of these components is described briefly

     Inflation by reciprocals of sampling probabilities. --Since the survey
utilized a three-stage sample design, there were three probabilities:
(1) The probability of selecting the PSU, (2) the probability of selecting
a physician within the PSU, and (3) the probability of selecting a patient
visit with the physician's practice.  The last probability was defined to be
the exact number of office visits during the physician's specified reporting
week divided by the number of Patient Records completed.  All weekly
estimates were inflated by a factor of 52 to derive annual estimates.

     Adjustment for nonresponse.--Estimates from the NAMCS data were adjusted
to account for sample physicians who did not participate in the study.  This
was done in such a manner as to minimize the impact of nonresponse on final
estimates by imputing to nonresponding physicians the practice characteristics
of similar responding physicians.  'For this purpose, similar physicians were
judged to be physicians having the same specialty designation and practicing
in the same PSU.

     Ratio adjustment.--A poststratification adjustment was made within each of
nine physician specialty groups.  The ratio adjustment was a multiplication
factor which had as its numerator the number of physicians in the universe in
each physician specialty group, and as its denominator the estimated number of
physicians in that particular specialty group.  The numerator was based on
figures obtained from the AMA-AOA master files, and the denominator was
based on data from the sample.

     Sampling Errors.--Procedures for calculating sampling errors as well as
estimates of standard errors of statistics derived from the NAMCS are described
in the "Technical Notes" section of references 4 through 11.

     Patient Weight.--The "patient weight" is a vital component in the process
of producing national estimates from sample data and its use should be clearly
understood by all micro-data tape users.  The statistics contained on the
micro-data tape reflect data concerning only a sample of patient visits--and-not
a complete count of all the visits that occurred in the United States.  The
"patient weight" is an inflation factor assigned to each patient record.  By
aggregating the "patient weights" an estimated complete count or national
estimate can be obtained.

     Questions.--Questions concerning data in the tapes should be directed to
Ambulatory Care Statistics Branch, Division of Health Resources Utilization
Statistics, National Center for Health Statistics, Room 212, 3700 East-West
Highway, Hyattsville, Maryland   20782.


    NCHS published statistics from the NAMCS in Series 13 of VITAL AND HEALTH
STATISTICS, PHS No. 1000, Public Health Service, Washington, U. S. Government
Printing Office.

     l.-National Center for Health Statistics: National Ambulatory Medical
Care Survey: Background and Methodology, United States.  VITAL AND HEALTH
STATISTICS.  Series 2-No. 61.  DHEW Pub.  No. (HRA) 74-1335.  Health Resources
Administration.  Washington.  U.S. Government Printing Office.  March 1974.

     2.-National Center for Health Statistics: The National Ambulatory
Medical Care Survey: Symptom Classification, United States.  VITAL AND
HEALTH STATISTICS.  Series 2-No. 63.  DHEW Pub.  No. (HRA) 74-1337.  Health
Resources Administration.  Washington.  U.S. Government Printing Office,
May 1974.

     3.--National Center for Health Statistics: Eighth Revision International
Classification of Diseases, Adapted for Use in the United States (ICDA).  PHS
Pub.  No. 1693.  Public Health Service.  Washington.  U.S. Government Printing
Office, 1967.

     4.--National Center for Health Statistics: Ambulatory Medical Care
Rendered in Physicians' Offices, United States, 1975.  Advance Data from
Vital and Health Statistics, No. 12.  DHEW Pub.  No. (HRA) 77-1250.  Health
Risources Administration.  Hyattsville, Maryland.  October 12, 1977.

     5.-National Center fgr Health Statistics: Ambulatory Medical Care
Rendered in Pediatricians' Offices During 1975, United States.  Advance Data
Health Resources Administration.  Hyattsville, Maryland.  October 13, 1977.

     6.--National Center for Health Statistics: National Ambulatory Medical
Care Survey of Visits to General and Family Practitioners, January-December 1975,
United States.  Advance Data from Vital and Health Statistics, No. 15.  DHEW
Pub.  No. (HRA) 78-1250.  Public Health Service.  Hyattsville, Maryland.
December 14, 1977.

     7.--National Center for Health Statistics: Office Visits to Internists:
National Ambulatory Medical Care Survey, United States, 1975.  Advance Data
from VITAL AND HEALTH STATISTICS, No. 16.  DHEW Pub.  No. (HRA) 78-1250.  Public
Heilth Service.  Hyattsville, Maryland.  February 7, 1978.

     8.--National Center for Health Statistics: Office Visits to Obstetrician-
 gynecologists: National Ambulatory Medical Care Survey, United States, 1975.
Advance Data from Vital AVD HEALTH STATISTICS, No. 20.  DHEW Pub.  No. (HRA)
78-1250.  Public Health Service.  Hyattsville, Md.  March 22, 1978.

     9.--National Center for Health Statistics: Office visits to general
surgeons: National Ambulatory Medical Care Survey, United States, 1975.
Advance Data from VitaZ and Health Statistics, No. 23.  DHEW Pub.  No. (HRA)
78-1250.  Public Health Service.  Hyattsville, Maryland.  March 24, 1978.

    10.--National Center for Health Statistics: Office visits to doctors
of osteopathy: National Ambulatory Medical Care Survey, United States, 1975.
Advance Data from VitaZ and Health Statistics, No. 25.  DHEW Pub. 14o. (HRA)
78-1250.  Public Health Service.  Hyattsville, Maryland.  March 22, 1978.

    ll.--National Center for Health Statistics: officc visits by persons
aged 65 and over: National Ambulatory Medical Care Survey, United States,
1975.  Advance Data from Vi'tal and Health Statistics; No. 22.  DHEW Pub.  No.
(HRA) 78-1250.  Public Health Service.  Hyattsville, Md-.  March 22, 1978.

    12.--Induction Interview Form.  National Ambulatory Medical Care Survey.
National Opinion Research Center.  University of Chicago.  OMB No. 68Rl498.

    13.-National Center for Health Statistics: The National Ambulatory
Medical Care Survey: 1975 Summary, United States, January-December 1975.
VitaZ and Health Statistics.  Series-13, No. 33.  DHEW Pub.  No. (pHs)
78-1784.  Public Health Service.  Washington.  U.S. Government Printing
6ffice, January 1978.

    14.--National Center for Health Statistics: Ambulatory Care Utilization
Patterns of Children and Young Adults: National Ambulatory Medical Care
Survey, United States, January-December 1975.  VitaL and Health Statistics.
Series 13, No. 39.  DHEW Pub.  No. (PHS) 78-1790.  Public Health Service.
Washington.  U.S. Government Printing Office, August 1978.

II. Technical Description of Tapes 

Number of Recording Tracks:            9
Density (bpi):                      1600
Language:                         EBEDIC
Parity:                              ODD
Record Length:                        92
Blocksize:                         4,600
Number of Records:                62,697


 This section consists of a detailed breakdown of each tape record, providing
 a brief description of each item of data included in the records.  The data ar
 arranged sequentially according to their physical location on the tape record.
 Unless otherwise stated in the "Item Description" column, the data are derived
 from the patient record (figure 1).  The AMA and the induction interview
 (reference 12)are alternate sources of data, while the computer generates
 other items by recoding selected data items.

 Item    Field        Tape
 NO.     Length    Location              Item Description and Codes

    1       4         1-4        Date of visit
 1.1        2         1-2          Month of visit
                                    01-12: January-December
 1.2        2         2-4          Year of visit
                                    Last 2 digits of year

    2       4         5-8        Date of birth
 2.1        2         5-6          Month of birth
                                    01-12: January-December
 2.2        2         7-8          Year of birth
                                    Last 2 digits bf year

    3       1           9        Sex

    4       1         10         Race
                                   2-All other

    5      12         11-22      Patient Problems (see reference 2 for codes)
 5.1        4         11-14        Most important problem #1
 5.2        4         15-18        Most important problem #2
 5.3        4         19-22        Other problem

    6       1         23         Seriousness of Problem
                                   1-very serious
                                   3=slightly serious
                                   4=not serious

    7       1         24         Ever Seen Patient Before
                                   2-yes, for problem in item 5
                                   3-yes, but not for problem in item 5

   8         14        25-38    Major reason for this visit
               1        25         Acute problem                (1=yes and 2=no)
               1        26         Acute problem, follow-up              "
               1        27         Chronic problem, routine              "
               1        28         Chronic problem, flare-up             "
               1        29         Prenatal care			 "
               1        30         Postnatal care			 "
               1        31         Postoperative care			 "
               1        32         Well adult/child exam		 "
               1        33         Family planning			 "
               1        34         Counseling or advice			 "
               1        35         Immunization				 "
               1        36         Referral				 "
               1        37         Administrative			 "
               1        38         Other				 "

   9         12        39-50      Physician's principal diagnosis 
                                    (see reference 3 for
  9.1          4       39-42         First diagnosis associated with item 5A
  9.2          4       43-46         Second diagnosis associated with item 5A
  9.3          4       47-50         Other significant current diagnoses

  10         18        51-68       Diagnostic/therapeutic services ordered/provided
  10.1         1         51          None   (1=yes and 2=no)
  10.2         1         52          Limited history/exam                      "

10.3         1         53          General history/exam                      " 
  10.4         1         54          Clinical lab. test                        "

10.5         1         55          Blood pressure check		       "
  10.6         1         56          EKG				       "
  10.7         1         57          Hearing test			       "
  10.8         1         58          Vision test			       "
  10.9         1         59          Endoscopy				       "
  10.10        1         60          Office surgery			       "
  10.11        1         61          Drug prescribed or dispensed	       "
  10.12        1         62          X-ray				       "
  10-13        1         63          Injection				       "
  10.14:-      1         64          Immunization/desensitization	       "
  10.15        1         65          Physiotherapy			       "
  10.16        1         66          @tedical counseling		       "
  10.17        1         67          Psychotherapy/therapeutic listening       "
  10.18        1         68          Other				       "

 Item    Field        Tape
 No.     Length    Location         Item Description and Codes

  11         8        69-76       Disposition of visit
 11.1        1         69           No follow-up planned          (1=yes and 2=no)
 11.2        1         70           Return at specified time             "
 11.3        1         71-          Return if needed			 "
 11.4        1         72           Telephone follow-up			 "
 11.5        i         73           Referral				 "
 11.6        1         74           Return to referring physician	 "
 11.7        1         75           #.dmit to hospital			 "
 11.8        1         76           Other				 "

  12         1         77           Duration of visit
                                      1=0 minutes
                                      2-1-5 minutes
                                      3-6-10 minutes
                                      .4-11-15 minutes
                                      5=16-30 minutes
                                      6-31-60 minutes
                                      7-60+ minutes

   13        10        78-87        Patient Weight

                                    A right justified, alphanumeric integer developed
                                    by the NAMCS staff.for the purpose of producing
                                    national estimates from sample estimates.  
                                    See section on "Estimation Procedures" on page 43 
                                    of reference 13. and also notes on page 6 of these 

   14        1         88           Geographic Region (Based on actual location of
                                    physician's practice.
                                      2-North Central

    15       2         89-90        Metropolitan/Nonmetropolitan (Based on actual
                                    location in conjunction with the Bureau of the
                                    Census definition.)
                                      Ol=Standard Metropolitan Statistical Area (SMSA)

  Item    Field	      Tape	          Item Description and Codes
  No.     Length     Location
  16 	    1	       91            Physician Specialty Group (Derived from Induction
                                     Interview-see reference 12).

                                        I-General/Family Practice

                                              MEDICAL SPECIALTIES


                                              SURGICAL SPECIALTIES

                                           5-General Surgery
                                           6-Obstetrics and Gynecology

                                              OTHER SPECIALTIES


  17        1          92         Type of practice (Derived from Induction
                                  Interview--see reference 12)



 Definitions of Certain Terms Used in this Document.

    Office(s).--Premises that the physician identifies as locations for his
ambulatory practice.  Responsibility over time for patient care and professional
services rendered there generally resides with the individual physician rather
than with any institution.

    Ambulatory patient.--An individ'ual presenting for personal health services,
neither bedridden nor currently admitted to any health care institution on the

    Physician.--Can be classified as either:

    In-Scope: All duly licensed doctors of medicine and doctors of osteopathy
    currently in practice who spend some time in caring for ambulatory patients
    at an office location.

    out-of-scope: Those physicians who treat patients only indirectly,
    including specialists in anesthesiology, pathology, forensic pathology,
    radiology, therapeutic radiology, and diagnostic radiology, and the
    following physicians.

        physicians in military service

        physicians who treat patients only in an institutional setting
        (e.g., patients in nursing homes and hospitals)

        physicians employed full time by an industry or institution and
        having no private practice (e.g., physicians who work for the VA,
        the Ford Motor Company, etc.)

        physicians who spend no time seeing ambulatory patients (e.g.,
        physicians who only teach, are engaged in research, or are retired).

    Patients.--Can be classified as either:

    In-scope: All patients seen by the physician or member of his staff in
    his office(s).

    Out-of-scope: Patients seen by the physician in a hospital, nursing home,
    or other extended care insitution, or the patient's home. (Note: If the
    doctor has a private office (which fits definition of "office") located
    in a hospital, the ambulatory patients seen there would be considered
    "in-scope."] The following types of patients are also considered out of

       patients seen by the physician in any institution (including outpatient
       clinics of hospitals) for which the institution has the primary
       responsibility for the care of the patient over time

       patients who telephone and receive advice from the physician

       patients who come to the office only to leave a specimen, pick up
       insurance forms, or pay their bills

       patients who come to the office only to pick up medications previously
       prescribed by the physician.

    Visit.--A direct, personal exchange between ambulatory patient and the
    physician (or members of his staff) for the purpose of seeking care
    and rendering health services.

    Physician specialty.--Principal specialty (including general practice) as
    designated by the physician at the time of the survey.  Those physicians
    for whom a specialty was not obtained were assigned the principal specialty
    recorded in the Master Physician files maintained by the AMA or AOA.

    Region of practice location.--The four geographic regions, excluding Alaska
    and Hawaii, which correspond to those used by the U.S. Bureau of the Census,
    are as follows:

        Region                         States Included

    Northeast .............. Connecticut, Maine, Massachusetts,
                             New Hampshire, New Jersey, New York,
                             Pennsylvania, Rhode Island, Vermont

    North Central  .......... Illinois, Indiana, Iowa, Kansas,
                              Michigan, Minnesota, Missouri, Nebraska,
                              North Dakota, Ohio, South Dakota,

    South  .................. Alabama, Arkansas, Delaware,
                              District of Columbia, Florida,
                              Georgia, Kentucky, Louisiana,
                              Maryland, Mississippi, North Carolina,
                              Oklahoma, South Carolina,
                              Tennessee, Texas, Virginia,
                              West Virginia

    West ................... Arizona, California, Colorado,
                             Idaho, Montana, Nevada, New Mexico,
                             Oregon, Utah, Washington, Wyoming

    Metropolitan status of practice location.--Physician's practice is
classified by its location in metropolitan or nonmetropolitan areas.  Metro-
politan areas are standard metropolitan statistical areas (SMSA's) as
defined by the U.S. Office of Management and Budget.

    The definition of an individual SMSA involves two considerations:
first, a city or cities of specified population which constitute the
central city and identify the county in which it is located as the central
county; second, economic and social relationships with "contiguous"
counties which are metropolitan in character, so that the periphery of the
specific metropolitan area may be determined.  SMSA's may cross State lines.
In New England SMSA's consist of cities and towns, rather than counties.

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