Scientific Data DocumentationNational Ambulatory Medical Care Survey, 19751975 NAMCS MICRO-DATA TAPE DOCUMENTATION ABSTRACT 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 document. I. DESCRIPTION OFI THE NATIONAL ANBUIATORY MEDICAL CARE SURVEY 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 Baltimore. 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 confidentiality. 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 categories. 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 below. 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. References 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 from VITAL AND HEALTH STATI.5TICS, No. 13. DHEW PUBLICATION No. (HRA) 77-1250. 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 III. TAPE RECORD FORMAT 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 1-Female 2=Male 4 1 10 Race 1-White 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 2-serious 3=slightly serious 4=not serious 7 1 24 Ever Seen Patient Before I=no 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 documentation. 14 1 88 Geographic Region (Based on actual location of physician's practice. 1=Northeast 2-North Central 3-South 4-West 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) 02=Non-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 2-Internal-Medicine 3-Pediatrics 4-Other SURGICAL SPECIALTIES 5-General Surgery 6-Obstetrics and Gynecology 7-Other OTHER SPECIALTIES 8-Psychiatry 9-Other 17 1 92 Type of practice (Derived from Induction Interview--see reference 12) 1-Solo 2-Partnership 3-Group 4-Other IV. APPENDIX 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 premises. 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 scope: 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, Wisconsin 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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