Footnote 1 Publication

Number 383 + March 12, 2007

Table 2 has been revised (May 2007).

Office-based Medical Practices: Methods and

Estimates from the National Ambulatory

Medical Care Survey

by Esther Hing, M.P.H., and Catharine W. Burt, Ed.D., Division of Health Care Statistics

Abstract

Introduction

Objectives—The report uses a multiplicity estimator from a sample of officebased physicians to estimate the number and characteristics of medical practices in
the United States. Practice estimates are presented by characteristics of the practice
(solo or group, single, or multi-specialty group, size of practice, ownership,
location, number of managed care contracts, use of electronic medical records, and
use of computerized physician order entry systems).
Methods—Data presented in this report were collected during physician
induction interviews for the 2003–04 National Ambulatory Medical Care Survey
(NAMCS). The NAMCS is a national probability sample survey of nonfederal
physicians who see patients in an office setting in the United States. Radiologists,
anesthesiologists, and pathologists—as well as physicians who treat patients solely
in hospital, institutional, or occupational settings—are excluded. Sample weights for
physician data use information on the number of physicians in the sampled
physician’s practice to produce annual national estimates of medical practices.
Results—During 2003–04, an average of 311,200 office-based physicians
practiced in an estimated 161,200 medical practices in the United States. Medical
practice characteristics differed from physician characteristics. Although 35.8 percent
of office-based physicians were in solo practice, 69.2 percent of medical practices
consisted of solo practitioners. The one-fifth of medical practices with three or more
physicians (19.5 percent) contains about one-half of all office-based physicians
(52.4 percent). About 8.4 percent of medical practices involved multiple specialties.
Fifteen percent of medical practices, consisting of 19.0 percent of physicians, used
electronic medical records. Similarly, 6.5 percent of medical practices, consisting of
9.2 percent of physicians, used computerized prescription order entry systems.

In the United States, physician
offices are the most frequent location
where patients receive care (1). A
previous report (2) presented estimates
of physicians practicing in the United
States based on data collected during the
induction interview of the 2003–04
National Ambulatory Medical Care
Survey (NAMCS). Decisions affecting
patient care services, such as adoption
of evidence-based guidelines or use of
electronic medical records, however,
may be made at the organizational level
of the medical practice, rather than by
individual physicians. This report,
therefore, augments the previous report
by presenting estimates for medical
practices derived from the same data. To
make practices rather than physicians
the unit of analysis, it is necessary to
adjust the weighting scheme through the
use of a multiplicity estimator. Although
using a multiplicity estimator is not new
(3–6), the methodology has never been
applied to deriving practice estimates
from NAMCS physician data. Adjusting
the physician weight by the number of
physicians in the practice has the
mathematical effect of yielding only one
observation from each medical practice;
the sum of the adjusted weights yields a

Keywords: ambulatory care c physician medical practice c NAMCS

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Centers for Disease Control and Prevention
National Center for Health Statistics

2

Advance Data No. 383 + March 12, 2007

national estimate of the number of
medical practices. Practice estimates in
this report describe medical practice
characteristics and decisions made by
the practice that may affect patient care,
such as use of electronic medical record
systems.
The NAMCS is a nationally
representative survey of visits to
nonfederally employed, office-based
physicians conducted by the National
Center for Health Statistics (NCHS).
The NAMCS is part of the ambulatory
care component of the National Health
Care Survey, a family of provider-based
surveys that measures health care
utilization across various types of
settings. More information about the
National Health Care Survey can be
found at the NCHS Internet address:
www.cdc.gov/nchs/nhcs.htm.

Methods
The NAMCS is an annual national
probability sample survey of physicians
classified by the American Medical
Association (AMA) and the American
Osteopathic Association (AOA) as
primarily engaged in ‘‘office-based,
patient care.’’ Federally employed
physicians; those who specialize in
anesthesiology, radiology, or pathology;
and physicians who do not see patients
in an office, such as the majority of
emergency medicine physicians, are
excluded. The NAMCS utilizes a
multistage probability sample design
involving samples of 112 geographic
primary sampling units (PSUs),
physicians stratified by specialty and
sampled within PSUs, and patient visits
sampled within physician practices. The
PSUs are counties; groups of counties;
county equivalents, such as parishes or
independent cities; or towns and
townships, for some PSUs in New
England.
In the 2003–04 NAMCS, 6,000
physicians were sampled. During the
induction interview, physicians were
asked questions to determine their
eligibility for the survey, and to gather
information about their practice such as
size, ownership, and revenue sources. Of
3,968 physicians eligible for the survey,
2,235 physicians who saw patients
during their sampled weeks responded

to the Physician Induction Interview
(PII), for an unweighted response rate of
56.3 percent.
Both the physician and office visit
estimation procedures have three basic
components:
1. Inflation by reciprocals of the
sampling selection probabilities
2. Adjustment for physician
nonresponse, and
3. A calibration ratio adjustment
between the number of physicians in
the sample frame when the sample
was selected and the number of
physicians when the NAMCS data
were collected.
For each physician, the sampling
selection probability reflects the
probability of PSU selection and
selection of physicians within each PSU.
The physician nonresponse adjustment
factor is the sample weight for
responding physicians augmented by a
factor accounting for the amount of
nonresponse by similar physicians.
Similar physicians were judged to be
physicians having the same specialty
designation and practicing in the same
PSU and/or region/metropolitan
statistical area (MSA) status. The
calibration ratio adjusts the number of
physicians based on the sample frame
within specialty stratum and region cells
to reflect the most recent universe
counts provided by AMA and AOA for
the NAMCS weights. For example, the
estimated number of physicians in 2003
increased from 280,500 to 312,400 after
calibration ratios were applied.
Similarly, the estimated number of
physicians in 2004 increased from
282,100 to 309,900 after application of
the calibration ratios. A previous report
presents information on physician
estimation, response rates, and survey
definitions in more detail (1).
The sample weights for office visits
include the same physician nonresponse
adjustment and calibration ratio
components utilized in the physician
weight. The major difference between
the physician and visit weight is in the
sampling probabilities for visits. That is,
the visit sample selection probabilities
reflect selection of PSUs, selection of
physicians within each PSU, as well as
selection of visits within each

physician’s practice. In addition, the
visit weights go through a smoothing
process such that excessively large visit
weights are truncated and a ratio
adjustment is performed. This technique
preserves the total estimated visit count
within each specialty by shifting the
‘‘excess’’ from visits with the largest
weights to visits with smaller weights.
More details on the NAMCS sampling
design and estimation process have been
published (7,8).

Medical practice estimates
In this report, the NAMCS
physician sampling weight is modified
to produce a medical practice estimator.
Multiplicity occurs within a sampling
frame when a member of the population
is linked to more than one entry on the
frame, so that the member has multiple
chances of being selected. In the
NAMCS sampling frame, multiplicity
exists among partnerships and group
practices because medical practices with
more physicians have a higher
probability of being selected than
practices with fewer physicians. Group
practices are defined as three or more
physicians practicing together with a
common billing and medical record
system (9). No sampling frame currently
exists for sampling all types of medical
practices, i.e., solo, partnership, and
group. Sampling frames for individual
physicians and for group practices exist,
but no sampling frame has all practices.
Modifying a physician survey to make
estimates of medical practices has the
advantage of using a single survey and
arithmetic manipulations to estimate
both physicians and practices. In this
report, nationally representative
estimates of medical practices were
derived using a ‘‘multiplicity estimator’’
to account for multiplicity in the
physician frame (4).
The multiplicity measure used in
this calculation was based on physician
response to the question ‘‘How many
other physicians are associated with you
(at this location)?’’ This question was
asked for a maximum of four office
locations at which the sample physician
saw ambulatory patients during his/her
sampled week (see Excerpts from the
2004 Physician Induction Interview (PII)

Advance Data No. 383 + March 12, 2007
form in ‘‘Technical Notes,’’ Figure I).
Practice size was assumed to be one
plus the number of other physicians
recorded at the first-listed location.
About 14.4 percent of physicians
reported that they saw patients at
multiple office locations. Medical
practices were estimated by adjusting
the physician sample weight by the
inverse of the multiplicity indicator
(number of physicians in the practice) to
account for the increased likelihood of
selection:
(Medical practice weight)ij=(Physician
sample weight)ij/Sij,
Where Sij = number of physicians
within practice j reported by physician i

(52.4 percent). The percentage of
practices that are multi-specialty groups
(8.4 percent) is smaller than the
percentage of physicians in these
practices (21.1 percent), although the
percentage of practices that are in solo
and single specialty groups
(91.6 percent) is larger than comparable
percentage of physicians in these
practices (78.9 percent). The percentage
of health maintenance organization
(HMO) practices is only 0.5 percent, but
the percentage of physicians in HMO
practices is 2.0 percent.
As would be expected, the percent
distribution of office visits by practice
size more closely resembles the
distribution of physicians than it does
medical practices. Practices involving 11
or more physicians constituted only
1.2 percent of practices, but 9.8 percent
of all visits occurred at these practices,
since 10.7 percent of all physicians are
employed there. In contrast, solo
physician practitioners, who constituted
69.2 percent of all practices but
35.8 percent of all physicians, had
36.8 percent of all office-based visits.
Similarly, solo and single-specialty
practices and multi-specialty group
practices constituted 91.6 and
8.4 percent of all practices, respectively,

using SUDAAN were performed to
detect significant associations among
practice characteristics. Tests of linear
trends, such as the percent of revenue
from managed care contracts by size of
practice, are based on a weighted linear
regression with significance at the 0.05
level. All other tests of statistical
significance among estimates are based
on the two-tailed t-test at the 0.05 level
of significance, unless otherwise noted.
Terms relating to differences, such as
‘‘greater than’’ or ‘‘less than,’’ indicate
that the difference is statistically
significant. A lack of comment
regarding the difference between any
two estimates does not mean that the
difference was not tested for
significance.

Analysis

Results
During 2003–04, there were, on
average, 161,200 office-based medical
practices in the United States involving
311,200 physicians (Table 1). Although
35.8 percent of office-based physicians
were in solo practice, 69.2 percent of
medical practices consisted of solo
practitioners (Figure 1). The one-fifth of
medical practices with three or more
physicians (19.5 percent) contains about
one-half of all office-based physicians

Practices1
100

Physicians
91.6
78.9

80
69.2

Percent

The PII form included questions
used to determine physician eligibility
for the survey as well as to gather
information about the practice, such as
size, ownership, and revenue sources.
The breadth of specialization for
practices was based on the questions,
‘‘Do you have a solo practice’’ and ‘‘Is
this a single- or multi-specialty group
practice,’’ in which responses of solo
practice and single-specialty group were
combined. Physician specialty for solo
practices and group practices is also
presented (see ‘‘Technical Notes,’’
Table I for physician specialty
definitions). The physician specialty
categories grouped specific selfdesignated subspecialty codes provided
by the AMA and AOA on the sampling
frame. Information on physician
specialty was updated during the
NAMCS induction interview of the
physician.
Because estimates presented in this
report are based on a sample rather than
the universe of office-based physicians,
they are subject to sampling variability.
The standard errors are calculated using
Taylor series approximations in
SUDAAN, which take into account the
complex sample design of the NAMCS
(10). Estimates based on 20–29 cases
and/or estimates whose standard errors
represent more than 30 percent of the
estimate have an asterisk (*) to indicate
that they do not meet the reliability
standard set by NCHS. Chi-square tests

3

60

40

35.8
26.9
21.1

20

14.8

14.2
11.4 11.8
4.1

0

Solo

Partner

3–5

Practice size

6–10

10.7

8.4

1.2
11 or more

Solo and single- Multispecialty
specialty
group
group
Breadth of specialization

1

See Methods for details on estimating medical practices.

Figure 1. Percent distributions of office-based medical practices and physicians within
practices by size and breadth of specialization: United States, 2003–04

Advance Data No. 383 + March 12, 2007

and 78.9 and 21.1 percent of all
physicians worked in these practices,
respectively. About 79.4 and
20.6 percent of all visits, respectively,
were to solo and single-specialty
practices and multi-specialty group
practices. With the exception of visits in
the Northeast, the distribution of visits
by region was similar to the distribution
of medical practices. The Northeast
accounted for 23.8 percent of all
medical practices, but only 19.8 percent
of visits. The distributions of visits and
medical practices by metropolitan status
were similar.
The distribution of office-based
medical practices by financial and
process characteristics is shown
according to practice size in Table 2. In
general, the percent of revenue from
managed care contracts increased with
practice size, a pattern that reflects the
association between having any
managed care contracts and practice
size. Conversely, the percentage of
practices without managed care
contracts was inversely related to
practice size. A higher percentage of
small practices had some or a lot of
difficulty referring patients with private
insurance than larger practices.
Participation in a practice-based research
network also increased with practice
size, from 2.7 percent for solo practices
to 15.2 percent for practices with 11 or
more physicians. Use of electronic
billing records, electronic medical
records, and computerized prescription
order entry each increased with practice
size. Other characteristics, such as
percent of revenue from selected
payment sources, were not associated
with practice size (Table 2). On average,
medical practices received 45.1 percent
of revenues from private insurance,
36.3 percent from Medicare, and
17.1 percent from Medicaid.
With regard to the adoption of
information technology, 69.2 percent of
practices had electronic billing records,
which translates to 74.2 percent of
physicians using this technology
(Figure 2). The percentage of practices
adopting these systems is lower than
comparable percentages reported by
physicians because use of these
computerized clinical support systems
among physicians increases with

80

Practices1

74.2

Physicians

69.2

60

Percent

4

40

19.0
20

15.0
6.5

0

Uses electronic billing

Uses EMR

9.2

Uses CPOE

1See Methods for details on estimating medical practices.
NOTES: EMR is electronic medical records. CPOE is computerized prescription order entry.

Figure 2. Percent of office-based medical practices and physicians using computerized
administrative and clinical support systems: United States, 2003–04

practice size (11) and consequently
contributes more frequently to the
physician estimates than to practice
estimates. Similarly, the percentage of
practices that adopted electronic medical
records (15.0 percent) was lower than
the comparable percentage of physicians
(19.0 percent), and the percentage of
practices using computerized
prescription order entry systems
(6.5 percent) was lower than the
comparable percentage of physicians
(9.2 percent). This reflects the higher
likelihood of large practices to adopt
information technology and the fact that
the percentage of all physicians in these
practices is higher than the percentage
of small practices (11).
Finally, Table 3 presents solo and
group practices in terms of physician
specialty. In this table, the multispecialty group column represents the
residual after accounting for solo and
single-specialty group practices. Among
major specialties, psychiatric practices
(85.6 percent) were most likely to
operate as solo practices while pediatric
practices were least likely to operate as
solo practices (52.1 percent). Among the
69.2 percent of medical practices
involving solo physicians (Table 1), the
most frequent specialties were general
and family practice, internal medicine,
obstetrics and gynecology (data not

shown). Among the 22.4 percent of
practices organized as single-specialty
group practices, the top three specialties
were general and family practice
(17.0 percent), internal medicine
(13.9 percent), and pediatrics
(12.3 percent) (data not shown).

Discussion
This report provides descriptive
information on medical practices during
2003–04. Practice estimates provide new
perspectives on the organization and
delivery of office-based ambulatory
care. Because the physician sample
weight is directly modified by the
number of physicians in practice to
yield a medical practice weight, the
distribution of medical practices on
characteristics associated with practice
size varied distinctly from the
distribution of physicians on the same
characteristic. For example, the
one-third of office-based physicians who
are in solo practice contrasts with the
finding that two-thirds of medical
practices consist of solo practitioners.
On the other hand, group practices,
which account for one-fifth of medical
practices, contain about one-half of all
office-based physicians. Estimates of
practices and physicians are similar on
some but not all characteristics. For

Advance Data No. 383 + March 12, 2007
example, private insurance and Medicare
are the most frequent sources of revenue
for practices and physicians (2). The
percentage of practice revenue from
managed care contracts (44.7 percent) is
identical to the previously published
estimate for physicians (2). The
percentages of medical practices
adopting electronic medical records or
computerized prescription order entry
systems, however, were lower than the
comparable percentages reported by
physicians.
The overall estimate of group
practices (three or more physicians)
derived from the NAMCS is roughly
comparable to the estimate from the
Medical Group Management Association
(MGMA). The 2003–04 NAMCS
estimated there were 31,400 group
practices, while the MGMA study
estimated 34,490 group practices in
2004 (9). The MGMA estimate,
however, included radiology,
anesthesiology, and pathology group
practices, while the NAMCS estimate
did not. After adding the MGMA
estimate of anesthesia, radiology, and
pathology single specialty groups to the
2003–04 NAMCS estimate, the resulting
total was very close to the total in
MGMA’s universe of group practices
(12).
Although the overall NAMCS and
MGMA estimates are similar,
definitional differences exist between
the two data sets. The MGMA has a
greater percentage of large practices and
a lower percentage of small medical
groups than NAMCS (12). The NAMCS
percentage of large practices (11 or
more physicians) among all group
practices (6 percent) was smaller than
the comparable MGMA percentage
(15.7 percent) (12). Within practices
with 11 or more physicians, the
NAMCS averaged 17.7 physicians per
practice compared with 49.8 physicians
per practice reported by the MGMA
(12). Many of these differences stem
from the definition of practice size
reported in the MGMA. If a medical
group was ‘‘subordinate’’ to a larger
practice, MGMA listed only the size of
the ‘‘parent’’ organization in its data
base. In contrast, the NAMCS measured

the practice size at locations where the
physician saw patients; the size of the
‘‘parent’’ organization was not
measured. Finally, the MGMA estimate
included large Veterans Administration
hospital practices (12); such practices
were excluded from the NAMCS. Thus,
the NAMCS estimates are reasonable
based on the MGMA comparison as
long as the scope of the NAMCS survey
is taken into account.
This report has described how
estimates of medical practices were
derived from the NAMCS physician
data. These estimates are subject to
several limitations. First, practice
estimates are subject to variability in
how practice size is defined. For this
report, practice size was assumed to be
the size indicated in the first-listed
location. If a different measure of size
was used to estimate practices, for
example, the location where the majority
of patients were seen, the estimate of
practices would vary. Second, practice
characteristics were limited to
information collected during the
induction interview. The practice size
might be underestimated if the sampled
physician was an employee and worked
with different practices at different
locations. The induction interview
questionnaire did not include questions
that could identify this situation. Finally,
practice estimates derived from sampled
physician data are reasonable only for
characteristics of the overall practice
that do not vary by physician within a
practice, such as the use of electronic
medical records or number of managed
care contracts (Table 2). Practice
estimates from NAMCS data are not
reasonable for characteristics that vary
among physicians within a practice. For
example, variation in physician
characteristics and treatment practice
patterns can be collected only from
individual physicians within practices.
Policy makers interested in the
structure and policies of medical
practices may be interested in these
data. Practice estimates are an additional
way to monitor the dispersion of new
technologies and policies of medical

practices that affect care provided at
ambulatory medical visits.

References
1. Schappert SM, Burt CW. Ambulatory
care visits to physician offices,
hospital outpatient departments and
emergency departments: United
States, 2001–02. National Center for
Health Statistics. Vital Health Stat
13(159). 2006.
2. Hing E, Burt CW. Characteristics of
office-based physicians and their
practices, 2003–04. Vital Health Stat
13(164). 2007.
3. Sirken MG. Network Sampling. In:
Armitage P, Colton T, eds.
Encyclopedia of Biostatistics. p.
2977–86. West Sussex, UK: John
Wiley and Sons Ltd. 1998.
4. Sirken MG, Levy PS. Multiplicity
estimation of proportions based on
ratios of random variables. J Am Stat
Assoc 69:68–73. 1974.
5. Sirken M, Shimizu I, Judkins D. The
population based establishment
survey. Proceedings of the Survey
Research Section. Am Stat Assoc. p.
470–3. 1995.
6. Burt CW, Hing E. Making patientlevel estimates from medical
encounter records using a multiplicity
estimator. Stat Med. 2007. [Epub
ahead of print]
7. National Center for Health Statistics.
Public-use data file documentation.
2003 National Ambulatory Medical
Care Survey. Hyattsville, MD. 2005.
8. National Center for Health Statistics.
Public-use data file documentation.
2004 National Ambulatory Medical
Care Survey. Hyattsville, MD. 2006.
9. Gans D, Kralewski J, Hammons T,
Dowd B. Medical groups’ adoption
of electronic health records and
information systems. Health Aff
24(5):1323–33. 2005.
10. Research Triangle Institute.
SUDAAN user’s manual, release 8.0.
Research Triangle Park, NC:
Research Triangle Institute. 2002.
11. Burt CW, Sisk JE. Which physicians
and practices are using electronic
medical records? Health Aff
24(5):1334–43. 2005.
12. Personal communication with Dan
Gans of Medical Group Management
Association. May 6, 2006.

5

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Advance Data No. 383 + March 12, 2007

Table 1. Number and percent distribution of office-based medical practices, physicians within practices, and office visits with
corresponding standard errors, by selected practice characteristics: United States, 2003–04
Medical practices1

Physicians within practices2

Characteristic

Number

Standard
error

Number

Total . . . . . . . . . . . . . . . . . . . . . . . . . .

161,200

5,300

311,200

Total . . . . . . . . . . . . . . . . . . . . . . . . . .

100.0

...

100.0

88.3
11.7

1.1
1.1

.
.
.
.
.

69.2
11.4
14.2
4.1
1.2

Solo and single-specialty group . . . . . . . . .
Multi-specialty group . . . . . . . . . . . . . . . .

Standard
error

Office visits within practices

Number

Standard
error

908,440,000

27,969,000

...

100.0

...

85.6
14.4

1.0
1.0

85.0
15.0

1.2
1.2

1.4
1.0
0.9
0.3
0.1

35.8
11.8
26.9
14.8
10.7

1.4
0.9
1.4
0.9
0.9

36.8
12.3
27.1
14.0
9.8

1.7
1.0
1.6
1.0
0.9

91.6
8.4

0.7
0.7

78.9
21.1

1.3
1.3

79.4
20.6

1.4
1.4

92.0
0.5
7.4

0.8
0.1
0.8

85.7
2.0
12.3

1.2
0.4
1.1

86.9
1.7
11.4

1.3
0.4
1.2

.
.
.
.

23.8
18.1
35.7
22.3

1.3
1.0
1.8
1.3

21.4
22.3
33.7
22.6

1.1
1.0
1.3
0.8

19.8
20.9
38.1
21.2

1.3
1.1
1.6
1.1

MSA . . . . . . . . . . . . . . . . . . . . . . . . .
Non-MSA . . . . . . . . . . . . . . . . . . . . . .

88.3
11.7

1.1
1.1

74.2
19.0

1.2
1.3

87.4
12.6

1.6
1.6

Average number of physicians
8,000
Percent distribution

Number of in-scope office locations
One . . . . . . . . . . . . . . . . . . . . . . . . . .
More than one . . . . . . . . . . . . . . . . . . . .
Practice size3

Solo . . . .
Partner . .
3–5. . . . .
6–10 . . . .
11 or more

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Breadth of specialization

Ownership
Physician or group . . . . . . . . . . . . . . . . .
HMO4 . . . . . . . . . . . . . . . . . . . . . . . . .
Other . . . . . . . . . . . . . . . . . . . . . . . . .
Geographic region

Northeast .
Midwest . .
South . . .
West . . . .

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Metropolitan status5

1

See ‘‘Methods’’ for details on estimating practices.

Includes nonfederal physicians who see patients in offices. Excludes radiologists, pathologists, and anesthesiologists.

Practice size is number of physicians in the practice.

4
HMO is health maintenance organization.

5
MSA is metropolitan statistical area.

2
3

NOTE: Numbers may not add to totals because of rounding.

Table 2. Selected characteristics office-based medical practices by practice size with corresponding standard errors: United States, 2003–04
Practice size1
All
practices

Characteristic

Solo

Partner

Practice size1

3–5

6–10

11 or
more

All
practices

Solo

Percent distribution

Partner

3–5

6–10

11 or
more

Standard error

Number of managed care contracts
Total . . . . .
None2 . . . .
Less than 3 .
3–102 . . . . .
11 or more2 .
Unknown . .

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100.0
13.3
10.8
40.9
31.7
3.2

100.0
15.9
11.3
41.9
27.9
3.0

100.0
9.3
8.0
36.4
42.1
4.1

100.0
6.9
10.3
41.8
37.9
3.1

100.0
5.8
9.7
37.0
43.2
4.4

100.0
3.9
16.8
28.7
43.4
7.1

...
1.3
1.1
1.7
1.8
0.9

...
1.7
1.5
2.2
2.2
1.1

...
2.5
1.9
4.3
4.7
1.6

...
1.3
1.7
2.8
2.9
0.9

...
1.7
2.2
3.9
4.1
1.2

...
1.8
4.6
4.5
5.4
2.2

Percent of revenue from managed care contracts2,3

. . . . . . . . . .

44.7

43.7

44.5

46.4

52.4

47.5

1.2

1.4

2.9

1.9

2.4

3.1

43.7
32.2
13.5
10.6

41.0
33.5
13.5
11.5

48.9
28.8
13.8
9.4

50.9
29.1
12.7
8.6

49.5
29.2
15.7
7.1

48.3
33.4
9.8
8.5

0.9
0.9
0.6
0.8

1.1
1.1
0.8
1.0

1.9
1.7
1.5
1.3

1.7
1.5
1.0
1.3

1.9
1.8
1.5
0.9

2.2
2.6
1.6
1.5

Percent of revenue from selected sources4
Private insurance2 .
Medicare. . . . . . .
Medicaid . . . . . . .
Other sources2 . . .

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Any difficulty referring certain types
of patients for specialty consultation5
Medicaid . . . . . .
Medicare. . . . . . .
Private insurance2 .
Uninsured . . . . . .

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Percent
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32.2
11.0
16.2
33.1

32.3
11.5
17.4
33.7

27.4
8.3
14.0
29.2

36.1
11.0
13.6
34.1

32.6
9.4
12.5
31.2

28.5
9.9
7.2
28.8

1.8
1.0
1.3
1.6

2.3
1.3
1.8
2.3

3.5
2.2
2.3
3.3

2.7
2.0
2.1
2.7

3.4
2.1
2.8
3.7

4.3
3.2
2.8
4.2

Participates in practice-based research network2,6 . . . . . . . . . . . .

3.8

2.7

4.8

5.5

9.7

15.2

0.7

0.7

1.8

1.8

2.7

3.5

69.2
15.0
6.5

64.4
12.9
4.9

77.9
17.4
6.7

81.8
19.5
11.9

78.8
23.5
14.4

79.5
30.2
10.5

1.5
1.2
0.9

2.0
1.3
1.1

3.0
3.0
1.8

2.1
2.6
2.3

3.0
3.2
2.6

4.1
4.3
2.8

81.2

80.9

68.6

3.0

5.2

...

4.1

4.9

10.6

7

Computerized adminstrative and clinical support systems

Uses electronic billing records2 . . . . . . . . . . . . . . . . . . . . . . . .
Uses electronic medical records2 . . . . . . . . . . . . . . . . . . . . . . .
Uses computerized prescription order entry system (CPOE)2 . . . . .

Advance Data No. 383 + March 12, 2007

Mean percent

Mean percent
Percent of prescriptions written using CPOE8 . . . . . . . . . . . . . . .

80.9

78.7

*

* Figure does not meet standards of reliablity.
. . . Data not applicable.
1
Practice size is number of physicians in practice. See ‘‘Methods’’ for details on estimating practices.
2
Significant weighted linear trend with practice size (p<0.05).
3
Mean percent among practices with any managed care revenue. The missing value for managed care revenue is 12 percent.
4
Mean percent of revenue among practices. Sum will approximate a percent distribution but responses were provided as a percentage for each source of revenue. Cases with missing data were excluded (6–15 percent depending on type of payment
source).
5
Missing data ranged from 12–22 percent depending on type of payment source.
6
Missing data for practice-based research network is 9 percent.
7
Missing values for electronic billing records is 7 percent, 1 percent each for electronic medical records and 2 percent for CPOE.
8
Mean percent of prescriptions written among practices using CPOE. The missing value for prescriptions written using CPOE is 12 percent.
NOTE: Numbers may not add to totals because of rounding.

7

8

Advance Data No. 383 + March 12, 2007

Table 3. Percent distribution of solo and group office-based practices with corresponding standard errors, by specialty type:
United States, 2003–04
Group

Physician specialty1

Total

Solo

Singlespecialty

Group
Multispecialty

Total

Solo

Percent distribution

Singlespecialty

Multispecialty

Standard error

Total . . . . . . . . . . . . . . . . . . . . . . . . . .

100.0

69.2

22.4

8.4

...

1.4

1.3

0.7

General and family practice .
Internal medicine . . . . . . .
Pediatrics . . . . . . . . . . . .
Obstetrics and gynecology .
Psychiatry . . . . . . . . . . .
Orthopedic surgery . . . . . .
Cardiovascular diseases. . .
Opthalmology . . . . . . . . .
General surgery . . . . . . . .
Dermatology . . . . . . . . . .
Urology . . . . . . . . . . . . .
Otolaryngology. . . . . . . . .
Neurology. . . . . . . . . . . .
All other specialties . . . . . .

100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0

68.5
70.0
52.1
68.4
85.6
59.9
64.7
72.9
67.3
74.4
64.5
67.1
73.3
70.6

21.6
18.9
37.4
23.4
11.5
32.4
28.7
19.4
25.4
18.6
32.9
26.7
23.5
20.3

9.9
11.1
*10.5
*
*
*7.7
*6.5
*7.6
7.3
*7.0
*
*6.2
*3.1
9.1

...
...
...
...
...
...
...
...
...
...
...
...
...
...

2.9
4.2
6.0
5.0
2.2
5.8
5.4
3.8
4.4
3.9
5.0
4.7
3.7
3.2

2.4
3.3
5.1
4.1
1.9
5.1
5.0
3.7
3.9
3.1
4.6
4.2
3.5
2.8

1.4
2.2
2.4
...
...
2.5
1.5
2.0
1.7
2.3
...
1.9
1.0
1.7

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* Figure does not meet standards of reliability.
. . . Data not applicable.
1
Physician specialty is defined in ‘‘Technical Notes,’’ Table I.
NOTE: Numbers may not add to totals because of rounding.

Advance Data No. 383 + March 12, 2007

Technical Notes
Physician specialty groups
Physician specialty is based on the
15 strata of physician specialties used
for sampling purposes in the NAMCS
survey design. One stratum, doctors of
osteopathy, was based on information
from the AOA. The ‘‘physician
specialty’’ classification presented in this
report includes the same physician
specialty strata used for sampling
purposes with the exception of the
doctors of osteopathy stratum, which is

combined with doctors of medicine in
the following 14 categories: general and
family practice, internal medicine,
pediatrics, general surgery, obstetrics
and gynecology, orthopedic surgery,
cardiovascular diseases, dermatology,
urology, psychiatry, neurology,
ophthalmology, otolaryngology, and a
residual category of other specialties.
Table I defines the 14 ‘‘physician
specialty’’ categories in terms of
self-designated subspecialty provided by
the AMA and AOA. The ‘‘physician
specialty’’ classification is updated with
information provided by sampled

9

physicians at the time of the survey. In
this classification, for example, a
pediatric cardiologist is grouped with
other pediatricians.
It should be noted that although
emergency medicine physicians made up
2.5 percent of sampled physicians in
2003–04 and are included in the
physician specialty category ‘‘all other
specialties,’’ few of these physicians are
included in the NAMCS. They often fall
outside the scope for the survey because
they rarely see patients in an office
setting.

Table I. Reclassification of physician specialty based on American Medical Association subspecialty designations for use in the National
Ambulatory Medical Care Survey
Physician specialty

Subspecialty designation

General and family practice . . . . . . FP - Family practice
FPG - Family practice, geriatric medicine
FSM - Sports medicine (family practice)
GP - General practice
Internal medicine . . . . . . . . . . . . . IM - Internal medicine
Pediatrics . . . . . . . . . . . . . . . . . ADL - Adolescent medicine
CCP - Critical care pediatrics
DBP - Developmental-behavioral pediatrics
MPD - Internal medicine (pediatrics)
NDN - Neurodevelopmental disabilities
NPM - Neonatal-perinatal medicine
PD - Pediatrics
PDA - Pediatric allergy
PDC - Pediatric cardiology
PDE - Pediatric endocrinology
PDI– Pediatric infectious diseases
PDP - Pediatric pulmonology
PDT - Medical toxicology (pediatrics)
PEM - Pediatric emergency medicine
PG - Pediatric gastroenterology
PHO - Pediatric hematology or oncology
PN - Pediatric nephrology
PPR - Pediatric rheumatology
PSM - Sports medicine (pediatrics)
General surgery . . . . . . . . . . . . . GS - General surgery
Obstetrics and gynecology . . . . . . . GO - Gynecological oncology
GYN - Gynecology
MFM - Maternal and fetal medicine
OBG - Obstetrics and gynecology
OBS - Obstetrics
OCC -Critical care medicine (obstetrics and gynecology)
REN - Reproductive endocrinology
Orthopedic surgery. . . . . . . . . . . . OAR - Adult reconstructive orthopedics
OFA - Foot and ankle orthopedics
OMO - Musculoskeletal oncology
OP - Pediatric orthopedics
ORS - Orthopedic surgery
OSM - Sports medicine (orthopedic surgery)
OSS - Orthopedic surgery of the spine
OTR - Orthopedic trauma
Cardiovascular diseases . . . . . . . . CD - Cardiovascular diseases
Dermatology. . . . . . . . . . . . . . . . D - Dermatology
Urology . . . . . . . . . . . . . . . . . . . U - Urology
UP - Pediatric urology

10

Advance Data No. 383 + March 12, 2007

Table I. Reclassification of physician specialty based on American Medical Association subspecialty designations for use in the National
Ambulatory Medical Care Survey—Con.
Physician specialty

Subspecialty designation

Psychiatry . . . . . . . . . . . . . . . . . ADP - Addiction psychiatry

CHP - Child psychiatry
NUP - Neuropsychiatry
P - Psychiatry
PFP - Forensic psychiatry
PYA - Psychoanalysis
PYG - Geriatric psychiatry
Neurology . . . . . . . . . . . . . . . . . CHN - Child neurology
CN - Clinical neurophysiology
ESN - Endovascular surgical neuroradiology
N - Neurology
NRN - Neurology (diagnostic radiology)
Ophthalmology . . . . . . . . . . . . . . OPH - Ophthalmology
PO - Pediatric ophthalmology
Otolaryngology . . . . . . . . . . . . . . NO - Otology-neurotology
OTO - Otolaryngology
PDO - Pediatric otolaryngology
All other . . . . . . . . . . . . . . . . . . A - Allergy
ADM - Addiction medicine
AI - Allergy and immunology
ALI - Allergy and immunology or diagnostic laboratory immunology
AM - Aerospace medicine
AMI - Adolescent medicine (internal medicine)
AS - Abdominal surgery
CBG - Clinical biochemical genetics
CCG - Clinical cytogenetics
CCM - Critical care medicine
CCS - Critical care surgery
CFS - Craniofacial surgery
CG - Clinical genetics
CMG - Clinical molecular genetics
CRS - Colon and rectal surgery
CS - Cosmetic surgery
DDL - Dermatological immunology or
diagnostic laboratory immunology
DIA - Diabetes
DS - Dermatologic surgery
EM - Emergency medicine
END - Endocrinology
EP - Epidemiology
ESM - Sports medicine (emergency medicine)
ETX - Medical toxicology (emergency medicine)
FPS - Facial plastic surgery
GE - Gastroenterology
GPM - General preventive medicine
HEM - Hematology
HEP - Hepatology
HNS - Head and neck surgery
HO - Hematology or oncology
HS - Hand surgery
HSP - Hand surgery (plastic surgery)
HSS - Hand surgery (surgery)
IC - Interventional cardiology
ICE - Cardiac electrophysiology
ID - Infectious diseases
IG - Immunology
ILI - Internal medicine or diagnostic laboratory immunology
IMG - Geriatric medicine (internal medicine)
ISM - Sports medicine (internal medicine)
LM - Legal medicine
MDM - Medical management
MG - Medical genetics
NEP - Nephrology
NS - Neurological surgery
NSP - Pediatric surgery (neurology)
NTR - Nutrition
OM - Occupational medicine
OMF - Oral and maxillofacial surgery
OMM - Osteopathic manipulative medicine

Advance Data No. 383 + March 12, 2007

11

Table I. Reclassification of physician specialty based on American Medical Association subspecialty designations for use in the National
Ambulatory Medical Care Survey—Con.
Physician specialty
All other—Con.

Subspecialty designation
ON - Medical oncology
PA - Clinical pharmacology
PCC - Pulmonary critical care medicine
PCS - Pediatric cardiothoracic surgery
PDS - Pediatric surgery
PE - Pediatric emergency medicine (emergency medicine)
PHM - Pharmaceutical medicine
PHP - Public health or general preventive medicine
PLI - Pediatric diagnostic laboratory immunology
PLM - Palliative medicine
PM - Physical medicine and rehabilitation
PMD - Pain medicine
PMM - Sports medicine (physical medicine and rehabilitation)
PRM - Pediatric rehabilitation medicine
PRO - Proctology
PS - Plastic surgery
PSH - Plastic surgery within the head and neck
PTX - Medical toxicology (preventive medicine)
PUD - Pulmonary diseases
RHU - Rheumatology
SCI - Spinal cord injury
SM - Sleep medicine
SO - Surgical oncology
TRS - Traumatic surgery
TS - Thoracic surgery
TTS - Transplant surgery
UCM - Urgent care medicine
UM - Undersea medicine
VM - Vascular medicine
VS - Vascular surgery
OS - Other specialty
US - Unspecified

12

Advance Data No. 383 + March 12, 2007

Figure I. Excerpts from the 2004 Physician Induction Interview (PII) form

Advance Data No. 383 + March 12, 2007

13

14

Advance Data No. 383 + March 12, 2007

Advance Data No. 383 + March 12, 2007

15

16

Advance Data No. 383 + March 12, 2007

Suggested citation

Copyright information

Hing E, Burt CW. Office-based medical
practices: Methods and estimates from the
National Ambulatory Medical Care Survey
Advance data from vital and health statistics;
no 383. Hyattsville, MD: National Center for
Health Statistics. 2007.

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.

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HEALTH & HUMAN SERVICES
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National Center for Health Statistics
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07-0023 (3/07)
CS108823
T27458
DHHS Publication No. (PHS) 2007-1250

National Center for Health Statistics
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