MilCohort Baseline Functional Health Nov07 BMC Public Health

BMC Public Health

BioMed Central

Open Access

Research article

The physical and mental health of a large military cohort: baseline
functional health status of the Millennium Cohort
Tyler C Smith*1, Mark Zamorski2, Besa Smith1, James R Riddle3,
Cynthia A LeardMann1, Timothy S Wells3, Charles C Engel4,
Charles W Hoge5, Joyce Adkins6, Dan Blaze7 for the Millennium Cohort
Study Team
Address: 1Department of Defense Center for Deployment Health Research, Naval Health Research Center, San Diego, CA, USA., 2Deployment
Health Section, Directorate of Medical Policy, Canadian Forces Health Services Group Headquarters and Department of Family Medicine,
University of Ottawa, Ottawa, Ontario, Canada., 3Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, USA., 4Deployment Health
Clinical Center, Walter Reed Army Medical Center, Silver Spring, MD, USA., 5Department of Psychiatry and Behavioral Sciences, Walter Reed Army
Institute of Research, Silver Spring, MD, USA., 6Office of the Assistant Secretary of Defense for Health Affairs, Force Health Protection, the
Pentagon, Washington, DC, USA. and 7Duke University Medical Center, Durham, NC, USA.
Email: Tyler C Smith* - [email protected]; Mark Zamorski - [email protected]; Besa Smith - [email protected];
James R Riddle - [email protected]; Cynthia A LeardMann - [email protected];
Timothy S Wells - [email protected]; Charles C Engel - [email protected];
Charles W Hoge - [email protected]; Joyce Adkins - [email protected];
Dan Blaze - [email protected]
* Corresponding author

Published: 26 November 2007
BMC Public Health 2007, 7:340

doi:10.1186/1471-2458-7-340

Received: 26 October 2006
Accepted: 26 November 2007

This article is available from: http://www.biomedcentral.com/1471-2458/7/340
© 2007 Smith et al; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract
Background: The US military is currently involved in large, lengthy, and complex combat operations around the world.
Effective military operations require optimal health of deployed service members, and both mental and physical health
can be affected by military operations.
Methods: Baseline data were collected from 77,047 US service members during 2001–2003 as part of a large,
longitudinal, population-based military health study (the Millennium Cohort Study). The authors calculated unadjusted,
adjusted, and weighted means for the Medical Outcomes Study Short Form 36-item Survey for Veterans physical (PCS)
and mental component summary (MCS) scores over a variety of demographic and military characteristics at baseline.
Results: The unadjusted mean PCS and MCS scores for this study were 53.4 (95% confidence interval: 53.3–53.4) and
52.8 (95% confidence interval: 52.7–52.9). Average PCS and MCS scores were slightly more favorable in this military
sample compared to those of the US general population of the same age and sex. Factors independently associated with
more favorable health status included male gender, being married, higher educational attainment, higher military rank,
and Air Force service. Combat specialists had similar health status compared to other military occupations. Having been
deployed to Southwest Asia, Bosnia, or Kosovo between 1998 and 2000 was not associated with diminished health
status.
Conclusion: The baseline health status of this large population-based military cohort is better than that of the US
general population of the same age and sex distribution over the same time period, especially in older age groups.
Deployment experiences during the period of 1998–2001 were not associated with decreased health status. These data
will serve as a useful reference for other military health studies and for future longitudinal analyses.

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Background
Military operations demand optimal physical and mental
health. Despite technological advances, operations continue to demand a high level of fitness and physical functioning. While serious physical health problems are
uncommon in young and middle-aged adults who make
up the bulk of the American military, mental health problems are prevalent in this age group in the general population. Common mental health problems such as mood
and anxiety disorders can interfere with the substantial
energy, concentration, motivation, and judgment
required for success of the military mission. Technological
advances and the complex nature of recent conflicts have,
if anything, increased demands in these areas. To the
extent that optimal physical and mental health are
required to defend the interests of a nation, the health of
service members is a matter of national security.
At least some military operations can result in mental and
physical health deterioration. A substantial burden of
mental health problems was identified in Vietnam War
veterans, and Persian Gulf War veterans reported a multitude of physical and psychological symptoms and illnesses at rates two to three times higher than
nondeployed veterans of the same era [1-6]. The healthrelated quality of life reported by these veterans has also
been shown to be significantly less favorable [2,7-9].
Increased risk of mental health problems and physical
symptom reporting by war veterans has been common
following other major conflicts [10]. While there is a consensus that war trauma can lead to measurable adverse
mental and physical health effects, emerging data suggest
that not all deployments have the same propensity to
cause these problems. For example, UK veterans from the
Bosnian conflict had much better health than UK Gulf
War veterans [7]. A recent survey of UK veterans of the current Iraq conflict showed surprisingly little difference in
their physical or mental health status relative to nondeployed controls [11,12]. However, a limitation in some of
the research following the 1991 Gulf War and more recent
conflicts was that comparison populations were unsuitable due to differences in health and composition of
deployed and nondeployed personnel [13,14]. While
researchers were diligent in documenting these limitations [15], there is an obvious and growing need for baseline data from which to answer the health concerns of
veterans.
The Millennium Cohort Study was launched in October
2000 [16], in response to the US Department of Defense
recommendation for a coordinated effort to study the
potential health effects of deployment-related exposures
[17], and the Institute of Medicine recommendation for a
systematic, longitudinal, population-based assessment of

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service members' health [18]. The Cohort began enrollment for the 21-year longitudinal study in July 2001 and
completed enrolling Panel 1 participants in June 2003.
This report investigates the baseline mental and physical
health status of this large military cohort, as measured by
the Medical Outcomes Study Short Form 36-item Survey
for Veterans [19]. Health status as measured by the Medical Outcomes Study Short Form 36-item Survey for Veterans and its parent form (SF-36) have been associated with
increased health care utilization [20-22], posttraumatic
stress disorder [23], disability [22], behavioral risk factors
[22], and mortality [20,22] in US military veterans. Aside
from their value as a baseline for future analyses on this
cohort, these data are crucial to understanding the health
of US military personnel in several ways: These population-level data can be used to compare the health of the
military to other populations, such as the US general population. Baseline cross-sectional data presented in this
report will be useful as a reference for exploring the health
of particular subgroups within the military, such as those
in a particular military occupation or those who deployed
to a particular geographical area. Identification of risk
groups within the Millennium Cohort will also help target
those service groups for interventions; the identification
of these risk groups can also be used to generate hypotheses as to how military service influences health status.

Methods
Study population
The Millennium Cohort Study Panel 1 consists of 77,047
consenting military service members who were enrolled
using a modified Dillman approach [24] and offered both
Web and US postal-based submission options (36%
response rate of those invited to participate) [16]. The
invited Cohort was sampled from electronic personnel
records representing approximately 11.3 percent of the
2.2 million men and women in service as of October 1,
2000. Enrollment began in July 2001 and ended in June
2003. US military personnel serving in the Army, Navy,
Coast Guard, Air Force, and Marine Corps were selected
and oversampled for those with recent deployment experience to Southwest Asia, Bosnia, or Kosovo between 1998
and 2000, Reserve and National Guard members, and
female service members to ensure adequate power for statistical inferences over the 21-year follow-up period. The
Millennium Cohort baseline enrollment, designed to
invite sufficient numbers of women and recent deployers
has been shown to be representative in composition of
the US military [16]. Investigation of potential biases in
the Millennium Cohort Study have found no differences
in healthcare utilization, as measured by hospital encounters and outpatient care, prior to study invitation as a
determinant for enrollment (data not yet published).
Investigation of Millennium Cohort self-reported data

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have been found reliable in test re-test investigations and
when comparing to electronically maintained databases
and to have high internal consistency within standardized
instruments [16,25-31]. This research has been conducted
in compliance with all applicable federal regulations governing the protection of human subjects in research and
was reviewed by the Naval Health Research Center Institutional Review Board as Protocol NHRC.2000.0007.
Demographic data for the Millennium Cohort Study participants reflect status as of October 1, 2000. Data
included gender, date of birth (age in years: 17–24, 25–
34, 35–44, > 44), education level (no high school
diploma, high school diploma, some college, college
degree), marital status (never married, married, divorced),
race/ethnicity (White non-Hispanic, Black non-Hispanic,
Asian/Pacific Islander, Hispanic, and other), length of
service (in years: 0–3, 4–8, 9–15, ≥ 16), military rank
(enlisted, warrant officer, commissioned officer), service
component (Reserve/National Guard, active duty), service
branch (Army, Air Force, Navy/Coast Guard, Marines),
and US Department of Defense primary and duty occupations (combat specialist, other occupations) [32]. Additionally, Cohort members were identified with past
deployment experience to Southwest Asia, Bosnia, or Kosovo during the period of January 1, 1998, to September 1,
2000. For this study, missing demographic data for marital status, occupation, education, and rank were supplemented with self-reported data from the survey when
possible. This reduced those missing data for at least one
demographic characteristic from 1.8 percent to 0.7 percent of the Cohort.
Outcomes
The Millennium Cohort Study questionnaire consists of
more than 450 questions and components regarding diagnosed medical conditions, reported symptoms, psychosocial assessment, physical status, functional status, alcohol
use, tobacco use, occupation, alternative medicine use,
exposures, sleep patterns, deployment experience, and
basic demographic and contact data [16]. This paper
focuses on self-reported health status as measured by
Medical Outcomes Study Short Form 36-item Survey for
Veterans [19], a modified version of the Medical Outcomes Study 36-item Short Form Health Survey (SF-36)
[33]. Like the original SF-36, the Veterans SF-36 includes
eight health scales which can be summarized into two
summary scores, the mental component summary (MCS)
and the physical component summary (PCS) [34-37]. Differences in the Veterans SF-36 include using 5-level
response categories for the role emotional and role physical scales which provides more-precise estimates of role
functional impairment [38,39]. A validated approach was
used to create the MCS and PCS scores, that uses the same
factor weights, general population means, and standard

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deviations as the original SF-36 scoring mechanism,
which make the scores comparable to the original SF-36
version [38]. The PCS and MCS normative US scores have
a mean of 50 and a standard deviation of 10, allowing
comparison to other populations [34,35]. Higher MCS
and PCS scores reflect more favorable health status.
Statistical analyses
Descriptive investigation of Cohort characteristics compared with the invited participants and the 2000 US military were completed. Unadjusted aggregated Cohort
means were computed for the two summary components.
Multicollinearity was investigated among the variables
age, sex, education, marital status, race/ethnicity, recent
deployment to Southwest Asia, Bosnia, or Kosovo, length
of service, military rank, branch of service, and occupational category. These variables were included in an analysis of variance (ANOVA) to calculate adjusted means for
the two summary component scores with the Tukey
approach to adjust for multiplicity [40,41].

Although Cohort proportions and 2000 US military proportions of Reserve and National Guard members are similar, the initial sampling design of the Millennium Cohort
oversampled for female, recently deployed, and Reserve/
National Guard members. To account for designed oversampling for these characteristics, weighted means for
subgroups of the population were calculated based on the
inverse of the sampling fraction for the three characteristics oversampled: female, recently deployed, and Reserve/
National Guard member. Variance was estimated using
the Taylor series expansion theory to estimate sampling
errors based on the complex sampling [42]. Data management, ANOVA, weighted, and nonweighted analyses were
completed using SAS® software (Version 9.1, SAS Institute,
Inc., Cary, North Carolina) [43].

Results
Of the 77,047 Cohort participants, complete demographic and questionnaire data for the MCS and PCS
scores were available for 75,413 (97.9%). The study population consisted of US military personnel proportionately more likely to be: 35 or older, college educated,
married, White non-Hispanic, in service more than 8
years, and in the officer ranks (Table 1). Chi-square tests
of association suggest statistically significant differences
in composition between the 2000 US military, the invited
Cohort and the Cohort members, although much of the
difference is due to the sampling strategy employed [16].
Table 2 reports the unadjusted means for the MCS and
PCS scores stratified by gender. All scores are above the US
population mean of 50, with the exception of mean MCS
scores for women in a few subgroups including those who
are younger, less educated, serving in the Marine Corps

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Table 1: Characteristics of Millennium Cohort Study Members at Baseline and the US Military in October 2000

Characteristic*
Sex
Male
Female
Age, years
17–24
25–34
35–44
> 44
Education
No high school diploma
High school diploma diploma/equivalent
Some college
College degree
Marital status
Never married
Married
Divorced
Race/ethnicity
White non-Hispanic
Black non-Hispanic
Asian/Pacific Islander
Hispanic
Other
1998–2000 deployment to Bosnia, Kosovo, or
Southwest Asia
No
Yes
Length of service, years
0–3
4–8
9–15
≥ 16
Military rank
Enlisted
Warrant officer
Commissioned officer
Service component
Reserve/National Guard
Active duty
Branch of service
Army
Air Force
Navy and Coast Guard
Marines
Occupational category
Combat specialists
Other occupations

Cohort† N = 75,413 n (%)

Invited Cohort‡ N = 207,683 (%)

US Military§ N = 2,140,959 (%)

55,307
20,106

(73.3)
(26.7)

(76.3)
(23.7)

(84.8)
(15.2)

14,249
26,479
24,891
9,794

(18.9)
(35.1)
(33.0)
(13.0)

(27.8)
(35.8)
(27.0)
(9.5)

(32.1)
(34.0)
(25.3)
(8.6)

4,620
32,329
19,333
19,131

(6.1)
(42.9)
(25.6)
(25.4)

(7.5)
(49.1)
(25.0)
(18.5)

(8.2)
(53.3)
(21.0)
(17.4)

22,568
47,680
5,165

(29.9)
(63.2)
(6.9)

(38.3)
(55.6)
(6.1)

(41.0)
(54.0)
(5.1)

52,613
10,294
5,983
4,838
1,685

(69.8)
(13.7)
(7.9)
(6.4)
(2.2)

(66.0)
(18.3)
(6.3)
(7.1)
(2.3)

(67.8)
(18.9)
(3.3)
(7.9)
(2.1)

52,590
22,823

(69.7)
(30.3)

(70.0)
(30.0)

(90.0)
(10.0)

13,661
17,042
19,530
25,180

(18.1)
(22.6)
(25.9)
(33.4)

(21.9)
(26.7)
(23.8)
(27.5)

(31.0)
(21.0)
(21.0)
(27.1)

58,228
1,345
15,840

(77.2)
(1.8)
(21.0)

(83.9)
(1.2)
(15.0)

(84.6)
(1.1)
(14.2)

32,418
42,995

(43.0)
(57.0)

(47.3)
(52.7)

(41.5)
(58.5)

35,773
22,074
13,696
3,870

(47.4)
(29.3)
(18.2)
(5.1)

(44.9)
(29.6)
(19.3)
(6.2)

(46.5)
(24.3)
(20.5)
(8.6)

15,083
60,330

(20.0)
(80.0)

(20.8)
(79.2)

(22.3)
(77.7)

* All characteristics are significantly different between the Cohort, Invited Cohort, and US Military (p < .05).
† Only participants with complete demographic data, mental component summary and physical component summary scores were included in this
study.
‡ Includes invited members who were contacted by US Postal Service mail at least one time and had complete demographic data, with the exception
of length of service.
§ Based on US military service rosters of October 2000 with complete demographic data.

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Table 2: Unadjusted Mental and Physical Component Summary Scores* for Millennium Cohort Study Participants (N = 75,413)

MCS
Characteristic
Full Cohort
Age, years
17–24
25–34
35–44
≥ 44
Education
No high school diploma
High school diploma diploma/equivalent
Some college
College degree
Marital status
Never married
Married
Divorced
Race/ethnicity
White non-Hispanic
Black non-Hispanic
Asian/Pacific Islander
Hispanic
Other
1998–2000 deployment to Bosnia, Kosovo, or Southwest Asia
No
Yes
Length of service, years
0–3
4–8
9–15
≥ 16
Military rank
Enlisted
Warrant officer
Commissioned officer
Service component
Reserve/National Guard
Active duty
Branch of service
Army
Air Force
Navy and Coast Guard
Marines
Occupational category
Combat specialists
Other occupations

PCS

Males

Females

Males

Females

53.4 (53.3, 53.5)

51.1 (50.9, 51.2)

53.6 (53.5, 53.6)

52.9 (52.8,53.0)

50.8 (50.5, 51.0)
53.2 (53.0, 53.3)
54.2 (54.1, 54.3)
55.2 (55.0, 55.4)

48.9 (48.6, 49.2)
51.0 (50.7, 51.2)
52.3 (52.0, 52.6)
53.6 (53.2, 54.0)

54.2 (54.1, 54.4)
54.2 (54.1, 54.3)
53.0 (52.9, 53.1)
52.7 (52.5, 52.8)

53.1 (52.8, 53.3)
53.5 (53.3, 53.7)
52.4 (52.2, 52.7)
52.1 (51.7, 52.4)

51.9 (51.5, 52.3)
52.3 (52.2, 52.5)
54.0 (53.9, 54.2)
55.0 (54.9, 55.2)

49.9 (49.4, 50.5)
49.7 (49.4, 49.9)
51.8 (51.5, 52.0)
53.0 (52.8, 53.3)

52.9 (52.6, 53.2)
52.9 (52.8, 53.0)
53.4 (53.3, 53.5)
55.1 (55.0, 55.2)

52.6 (52.2, 53.1)
52.0 (51.8, 52.2)
53.2 (53.0, 53.4)
54.3 (54.1, 54.5)

51.6 (51.5, 51.8)
54.1 (54.0, 54.2)
53.4 (53.1, 53.8)

50.3 (50.1, 50.6)
51.7 (51.5, 51.9)
51.2 (50.7, 51.6)

54.4 (54.3, 54.6)
53.3 (53.2, 53.3)
53.2 (52.9, 53.5)

53.2 (53.1, 53.4)
52.7 (52.6, 52.9)
52.5 (52.1, 52.8)

53.2 (53.2, 53.3)
54.2 (54.0, 54.4)
54.3 (54.1, 54.6)
53.1 (52.8, 53.4)
53.0 (52.4, 53.5)

50.8 (50.6, 51.0)
51.8 (51.5, 52.2)
52.3 (51.8, 52.8)
50.3 (49.7, 50.9)
50.2 (49.3, 51.2)

53.6 (53.5, 53.7)
52.8 (52.6, 53.0)
54.3 (54.1, 54.5)
53.6 (53.3, 53.8)
53.0 (52.6, 53.5)

53.2 (53.1, 53.4)
51.8 (51.6, 52.1)
53.8 (53.4, 54.2)
52.9 (52.5, 53.4)
52.0 (51.3, 52.8)

53.4 (53.3, 53.5)
53.5 (53.4, 53.7)

51.1 (50.9, 51.2)
51.3 (50.9, 51.7)

53.6 (53.5, 53.7)
53.5 (53.4, 53.6)

52.9 (52.8, 53.1)
52.8 (52.5, 53.1)

51.2 (50.9, 51.4)
52.6 (52.4, 52.8)
53.9 (53.7, 54.0)
54.5 (54.4, 54.6)

49.4 (49.1, 49.7)
50.5 (50.2, 50.8)
52.1 (51.8, 52.4)
52.6 (52.3, 52.9)

54.3 (54.2, 54.5)
54.3 (54.1, 54.4)
54.0 (53.9, 54.1)
52.5 (52.4, 52.6)

53.1 (52.9, 53.3)
53.4 (53.2, 53.6)
53.4 (53.1, 53.6)
51.8 (51.6, 52.1)

52.9 (52.8, 53.0)
55.4 (55.0, 55.9)
55.3 (55.1, 55.4)

50.4 (50.3, 50.6)
54.0 (52.6, 55.4)
53.4 (53.1, 53.6)

53.1 (53.0, 53.1)
52.9 (52.5, 53.3)
55.5 (55.4, 55.6)

52.5 (52.3, 52.6)
51.6 (50.3, 52.9)
54.6 (54.4, 54.8)

53.8 (53.7, 53.9)
53.1 (53.0, 53.2)

51.8 (51.5, 52.0)
50.5 (50.3, 50.7)

54.1 (54.0, 54.2)
53.2 (53.1, 53.3)

53.7 (53.5, 53.8)
52.2 (52.0, 52.4)

53.0 (52.9, 53.1)
54.5 (54.3, 54.6)
53.1 (52.9, 53.3)
52.7 (52.4, 53.1)

50.7 (50.5, 51.0)
52.2 (52.0, 52.5)
50.4 (50.0, 50.8)
49.6 (48.6, 50.6)

53.1 (53.0, 53.2)
54.2 (54.0, 54.3)
53.8 (53.7, 54.0)
53.8 (53.6, 54.1)

52.1 (52.0, 52.3)
54.1 (53.9, 54.3)
53.2 (52.9, 53.4)
52.6 (51.8, 53.4)

53.8 (53.7, 53.9)
53.3 (53.2, 53.4)

51.0 (50.4, 51.6)
51.1 (50.9, 51.3)

54.1 (54.0, 54.2)
53.4 (53.3, 53.5)

53.3 (52.9, 53.8)
52.9 (52.8, 53.0)

* Medical Outcomes Study 36-Item Short Form Health Survey for Veterans. 1998 general US population means used to calculate mental and
physical component summary scores (MCS, PCS).

and who have 3 years or less of service. The weighted and
adjusted subgroup means for the MCS and PCS scores are
reported in Table 3. The weighted subgroup means are
useful for identifying strata at increased risk for decreased
mental and physical health status, while the adjusted subgroup means better reflect the independent contribution

of the risk factors. The adjusted and weighted subgroup
means showed virtually identical patterns of risk factors,
though, as expected, the adjusted means showed less dramatic differences.

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Table 3: Adjusted and Weighted Mental and Physical Component Summary Score Means for Millennium Cohort Study Participants (N
= 75,413)

MCS
Characteristic
Sex
Male
Female
Age, years
17–24
25–34
35–44
≥ 44
Education
No high school diploma
High school diploma diploma/equivalent
Some college
College degree
Marital status
Never married
Married
Divorced
Race/ethnicity
White non-Hispanic
Black non-Hispanic
Asian/Pacific Islander
Hispanic
Other
1998–2000 deployment to Bosnia, Kosovo, or Southwest Asia
No
Yes
Length of service, years
0–3
4–8
9–15
≥ 16
Military rank
Enlisted
Warrant officer
Commissioned officer
Service component
Reserve/National Guard
Active duty
Branch of service
Army
Air Force
Navy and Coast Guard
Marines
Occupational category
Combat specialists
Other occupations

PCS

Adjusted Means†

Weighted Means‡

Adjusted Means

Weighted Means

54.2a
52.2b

53.3a
51.0b

54.5a
53.4b

53.5a
52.8b

52.1a
53.0b
53.4c
54.1d

50.2a
52.7b
53.9c
55.1d

55.2a
54.7b
53.4c
52.4d

54.0a
54.1a
52.8b
52.6c

52.6a
53.0a
53.5b
53.6b

51.6a
51.8a
53.5b
54.7c

53.4a
53.5a
54.1b
54.8c

52.8a
52.7a
53.3b
54.9c

53.1a
53.7b
52.7a

51.1a
53.8b
52.7c

54.3a
53.8b
53.8b

54.3a
53.1b
52.9b

52.6a
54.2b
53.3c
53.1c
52.7a, c

52.8a
53.5b
54.1c
52.6a
52.3a

53.8a, c
53.7a
54.5b
54.1b, c
53.5a

53.5a
52.5b
54.1c
53.4a
52.7b

53.1a
53.3b

52.9a
53.4b

54.0a
53.9a

53.4a
53.6b

52.6a
53.0b
53.5c
53.6c

50.6a
52.2b
53.5c
54.3d

54.0a
54.1a
54.2a
53.4b

54.1a
54.1a
53.9b
52.4c

52.1a
53.8b
53.7b

52.3a
55.2b
54.9b

53.1a
53.8b
54.9c

52.9a
52.6a
55.3b

53.5a
52.8b

53.5a
52.6b

55.0a
52.9b

54.0a
53.0b

52.7a
53.8b
52.9a, c
53.3c

52.7a
54.0b
52.6a
52.4a

52.9a
54.5b
54.4b
54.0c

52.9a
54.2b
53.8c
53.5d

53.3a
53.1b

53.6a
52.8b

54.0a
53.9a

53.9a
53.3b

* Medical Outcomes Study 36-Item Short Form Health Survey for Veterans. 1998 general US population means used to calculate mental and physical
component summary scores (MCS, PCS).
† Means are adjusted for all variables in the table.
‡ Weighting based on the inverse of the sampling scheme.
a, b, c Letters that are different indicate statistically significant differences (p < 0.05) of adjusted and weighted means. Same letters indicate no
statistically significant differences in means. Tukey's method was used to adjust for multiple comparisons.

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Table 4: Adjusted Mental and Physical Component Summary Score* Means Stratified by Active Duty and Reserve/National Guard
Military Women†

Active Duty (n = 10,469)
Characteristic
Age, years
17–24
25–34
35–44
≥ 44
Education
No high school diploma
High school diploma diploma/equivalent
Some college
College degree
Marital status
Never married
Married
Divorced
Race/ethnicity
White non-Hispanic
Black non-Hispanic
Asian/Pacific Islander
Hispanic
Other
1998–2000 deployment to Bosnia, Kosovo, or Southwest Asia
No
Yes
Length of service, years
0–3
4–8
9–15
≥ 16
Military rank
Enlisted
Warrant officer
Commissioned officer
Branch of service
Army
Air Force
Navy and Coast Guard
Marines
Occupational category
Combat specialists
Other occupations

Reserve/National Guard (n = 9,637)

MCS

PCS

MCS

PCS

50.0a
50.9b
51.1a, b
51.7a, b

53.5a
53.2a
51.7b
49.9c

51.8a
52.6a
53.7b
54.7c

55.0a
54.3b
53.2c
52.1d

48.2a
51.2b
52.2c
52.1b, c

51.3a, b
51.6b
52.5a
53.0a

52.8a
52.9a
53.6a
53.5a

53.1a
53.3a
53.8b
54.4b

51.1a
51.6a
50.1b

52.5a
51.9b
51.8a, b

53.9a
53.3b
52.5c

53.8a
53.6a
53.6a

50.4a
52.2b
51.2a, b
50.2a
50.6a, b

51.9a
51.6a
52.9b
52.3a, b
51.7a, b

52.6a
54.1b
53.6a, b
53.5a, b
52.3a

53.9a
53.5a, b
54.6a
53.9a, b
52.4b

50.6a
51.3b

52.0a
52.2a

53.4a
53.0a

53.9a
53.4a

50.0a
50.7a, b
51.4b
51.7b

51.9a
52.3a, b
52.8b
51.3a

53.3a
52.8a
53.4a
53.3a

53.8a
53.9a
53.5a
53.4a

49.5a
51.9a, b
51.4b

51.2a
51.8a, b
53.3b

51.8a
54.2a, b
53.6b

53.2a
53.1a, b
54.6b

50.9a
52.0b
50.2a
50.6a, b

50.6a
53.0b
52.5b
52.1b

52.1a
53.6b
52.9b
54.2a, b

52.7a
54.4b
54.0b
53.5a, b

51.1a
50.8a

52.0a
52.2a

53.2a
53.2a

53.5a
53.8a

* Medical Outcomes Study 36-Item Short Form Health Survey for Veterans. 1998 general US population means used to calculate mental and
physical component summary scores (MCS, PCS).
† Only women with MCS and PCS scores and complete demographic data are reported in this table.
a, b, c Letters that are different indicate statistically significant differences (p < 0.05) of adjusted and weighted means. Same letters indicate no
statistically significant differences in means. Tukey's method was used to adjust for multiple comparisons.

Females had significantly less favorable mental and physical health status. Older participants and those with
longer lengths of service had more favorable mental
health but less favorable physical health. Participants with
lower levels of educational attainment had less favorable
mental and physical health. Overall, married participants,
officers, Reserve/National Guard members, and Air Force

members had significantly higher mental and physical
summary scores. With the exception of adjusted PCS
scores, combat specialists had slightly more favorable
MCS and PCS scores. Although the difference was small,
participants who had deployed to Southwest Asia, Kosovo, and Bosnia between 1998 and 2001 had slightly

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more favorable weighted MCS and PCS scores; this difference persisted in the adjusted MCS but not PCS scores.
To better understand gender-specific mental and physical
health in the context of professional versus Reserve/
National Guard or "citizen-soldiers," Table 4 presents
female-only data stratified by active-duty or Reserve/
National Guard status. Reserve/National Guard women
had higher overall MCS and PCS scores when compared
with active-duty personnel. Increasing age suggested more
favorable MCS scores and less favorable PCS scores in
both active-duty and Reserve/National Guard women.
Increased education suggested more favorable mental
health among active-duty women and more favorable
physical health among Reserve/National Guard women.
Reserve/National Guard men had slightly more favorable
mental and physical health than active-duty men (Table
5). Trends with age seen previously in women were also
found in men. More favorable mental health and less
favorable physical health for both active-duty and
Reserve/National Guard men were found with increasing
age. Higher education and being an officer were associated with more favorable mental and physical health for
both active duty and Reserve/National Guard men. Combat specialty occupations were associated with more favorable mental health and physical health for Reserve/
National Guard men, but only more favorable mental
health for active-duty men.

Discussion
Lower health-related quality of life measures have been
associated with increased health care utilization [20-22],
posttraumatic stress disorder [23], disability [22], behavioral risk factors [22], and mortality [20,22]. This report
highlights relatively good health in a large military cohort.
Additionally, we have identified a number of sociodemographic and military characteristics that were independently associated with physical and mental health status in
service members on active duty and in the US National
Guard and Reserves. These included sex, age, rank, educational attainment, marital status, race/ethnicity, duration
of military service, component of service, branch of service, and combat occupation specialties. Interestingly,
those having recent deployment experience to Southwest
Asia, Kosovo, or Bosnia were independently associated
with slightly more favorable mental or physical health status as measured by the Medical Outcomes Study Short
Form 36-item Survey for Veterans.
Results from this study may be compared to published US
general population norms. The PCS and MCS normative
US scores have a mean of 50 and a standard deviation of
10, allowing comparison between populations [34]. To
interpret differences in mean scores, a difference of five

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points in the scores is considered clinically and socially
meaningful [44]. Age-comparable unadjusted MCS and
PCS scores for the Millennium Cohort were higher in
comparison with data for the 1998 US general population
norms for most age categories [34]. The mean PCS scores
for males and females aged 18–34 years for the general
population are nearly identical to the mean PCS scores for
Millennium Cohort males and females aged 17–24 and
25–34 years of age. However, as age increases the Millennium Cohort mean PCS scores get proportionately higher
compared to those of the US population. For example,
PCS scores of those aged over 44 years in the Millennium
Cohort are about 2 and 4 points higher in men and
women, respectively, compared to those aged 44 to 54
years in the US general population. Mean MCS scores are
higher in the Millennium Cohort at all age-comparable
groups compared to the general US general population.
Similar to the PCS scores, the largest differences are seen
in the oldest age groups. The youngest Millennium
Cohort age groups (17–24 years) have mean MCS scores
that are about 2 points higher than the general US population while the scores for the oldest groups (older than
44 years) are 3 points higher in the women and more than
5 points higher in the men. These higher scores, especially
in the older age groups, may be due to healthier people
entering and remaining in the US military. There are certain physical and mental criteria that must be met to continue service in the US military, which may explain the
higher scores when compared to the general US population. Higher MCS and PCS scores in military populations
in comparison with US norms have been replicated in
other studies where select US military populations were
compared with US normative scores. However, as noted
above, the statistical significance found comparing
Cohort participants to normative values may not indicate
clinically significant differences in health status [8,9,45].
Mean PCS and MCS score trends among Millennium
Cohort members were similar to those observed in civilian populations. As reported by other researchers [44,4648], we observed increasing mean MCS scores and
decreasing mean PCS scores with increasing age. In this
cohort, women reported lower mental and physical functioning than men, similar to their civilian counterparts. A
2002 cross-sectional survey of 4,506 Swedes found that
SF-36 scores differed by gender. The authors hypothesize
that these differences were due, in part, to gender disparities in work, income, daily living, social life, and expectations between men and women [49]. The authors of this
study also noted that there were gender differences in the
prevalence and severity of self-reported pain associated
with headaches and musculoskeletal disorders, which has
also been observed by others for rheumatoid arthritis
[50], irritable bowel syndrome [51], fibromyalgia [52],
and chronic fatigue syndrome [53,54]. Women serving in

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Table 5: Adjusted Mental and Physical Component Summary Score* Means Stratified by Active Duty and Reserve/National Guard
Military Men†

Active Duty (n = 32,526)
Characteristic
Age, years
17–24
25–34
35–44
≥ 44
Education
No high school diploma
High school diploma
diploma/equivalent
Some college
College degree
Marital status
Not married
Married
Divorced
Race/ethnicity
White non-Hispanic
Black non-Hispanic
Asian/Pacific Islander
Hispanic
Other
1998–2000 deployment to
Bosnia, Kosovo, or
Southwest Asia
experience‡
No
Yes
Length of service, years
0–3
4–8
9–15
≥ 16
Military rank
Enlisted
Warrant officer
Commissioned officer
Branch of service
Army
Air Force
Navy and Coast Guard
Marines
Occupational category
Combat specialists
Other occupations

Reserve/National Guard (n = 22,781)

MCS

PCS

MCS

PCS

52.4a
53.4b
53.8b
54.5c

54.6a
54.3a
53.2b
52.1c

54.1a
54.1a
54.4a
55.2b

56.6a
55.8b
54.7c
53.6d

52.1a
53.6b

52.7a
53.4b

53.9a
54.3a

54.6a
54.8a

54.3c
54.1b, c

53.7b
54.4c

54.7b
54.9b

55.3b
56.1c

53.2a
54.2b
53.2a

54.0a
53.5b
53.1b

54.1a
54.9b
54.3a

55.5a
54.8b
55.2a, b

53.0a
54.7b
53.4a
53.5a
53.2a

53.3a
53.6a, b
53.9b
53.9b
53.2a, b

54.1a
55.0b
54.2a, b
54.7a, b
54.2a, b

55.3a
54.9a
55.6a
55.0a
55.1a

53.4a
53.6a

53.5a
53.6a

54.5a
54.4a

55.4a
54.9b

52.4a
53.5b
54.1c
54.0b, c

53.8a
53.9a
53.9a
52.6b

54.8a
54.1b
54.3a, b
54.6a

55.5a
55.1a
55.2a
54.9a

52.5a
53.9b
54.2b

52.4a
53.5b
54.7c

53.4a
55.3b
54.6b

54.5a
55.1a, b
55.9b

53.0a
54.0b
53.3a, c
53.8b, c

52.5a
54.2b
54.0b
53.5c

54.0a
55.3b
54.6c
53.9a, c

54.2a
55.6b
55.5b
55.3b

53.7a
53.4b

53.6a
53.5a

54.6a
54.3b

55.3a
55.0b

* Medical Outcomes Study 36-Item Short Form Health Survey for Veterans. 1998 general US population means used to calculate mental and
physical component summary scores (MCS, PCS)
† Only men with MCS and PCS scores and complete demographic data are reported in this table.
a, b, c Letters that are different indicate statistically significant differences (p < 0.05) of adjusted and weighted means. Same letters indicate no
statistically significant differences in means. Tukey's method was used to adjust for multiple comparisons.

the military have reported that they suffer from psychosocial and interpersonal stress associated with being female
in the military and that this generally had a stronger
impact on women's than on men's mental health [55,56].
This may be supported by the notable lower adjusted

mean MCS scores among active-duty women with no high
school diploma when compared to their Reserve/National
Guard counterparts. We observed similar results to those
reported by Voelker et al., who studied 1991 Gulf War-era
military personnel [8]. Common findings between the

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current study and Voelker et al. include decreased mean
PCS scores for those who were married or in the Army,
and decreased mean MCS scores for service members who
were in the Army, divorced, and had shorter lengths of
service. In this study, we report increased mean PCS scores
with increasing education, which has also been observed
in a study of health-related quality of life among a cohort
of 1991 Gulf War and Germany-deployed veterans [9].
The Millennium Cohort unadjusted means of the mental
and physical component summary scores were much
higher than that of Department of Veterans Affairs (VA)
populations presenting for care [21,57,58]. The mean
MCS and PCS scores for VA enrollees who filled out a
questionnaire in 1999 or 2000 were 42.8 and 40.7 for
women aged 18 to 44 years and 43.4 and 40.2 for men
aged 18 to 44. The MCS scores were about 6 to 10 points
higher, while the PCS scores were 11 to 14 points higher
among similar Millennium Cohort groups of men and
women [58]. This difference is likely due to dissimilarities
in VA eligibility criteria that emphasize service-related
injury and illness and unmet health service need [21,57].
As the Millennium Cohort ages and members separate or
retire from military service and begin to use the VA health
care system, comparisons of baseline functional health of
these members will enhance the growing knowledge of
predictors of mental and physical impairment after military service.
There are notable limitations to these analyses that should
be discussed. The Millennium Cohort Study baseline
enrollment ended with 36% of those invited consenting
to participate in the 21-year study. As with any survey
study, response bias and generalizability is a concern and
should be investigated when possible. Although participants self-selected in accepting the invitation to become
part of the cohort reports of Millennium Cohort baseline
data suggest a representative sample of military personnel
measured by demographic and health characteristics and
reliable health, vaccination, and deployment reporting
[16,25-31]. Due to the Cohort being constructed to sample more women, those with recent deployment experience to Southwest Asia, Bosnia, or Kosovo, and Reserve/
National Guard, there are compositional differences
between the target population and those in military service in October 2000 [16]. However, as demonstrated by
the slight difference in weighted means and nonweighted
means, these proportional differences have minimal
impact when generalizing to the US military. Investigation of a health bias for enrolling in the Millennium
Cohort suggested little health differences in responders
and nonresponders with respect to hospitalization and
outpatient encounters in the year prior to enrollment
(data not yet published). Further, reporting bias may have
been introduced to these functional health estimates

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based on an investigation that reported military personnel
enrolling soon after the tragic events of September 11,
2001, reported significantly better mental and physical
health during the first few months after the attacks than in
months prior to the attacks [59]. The finding that those
having past deployment experience to Southwest Asia,
Kosovo, or Bosnia had slightly more favorable mental or
physical health status may simply be due to a selection
process where more healthy individuals are deployed.
Lastly, although the SF-36 and Veterans SF-36 have undergone reliability investigations and are thought to be reasonable instruments for measuring health perception
[33,38,60,61], the use of standardized instruments and
self-reported data as a surrogate for clinical health assessment is imperfect.
Despite limitations, our study has a number of strengths.
This report documents a very large, population-based
investigation of health of current US military members as
measured by the Medical Outcomes Study Short Form 36item Survey for Veterans. The large study population with
many demographic characteristics allowed for robust estimates of the two summary scores while adjusting for differences in populations using ANOVA techniques. In
addition, the use of standardized instruments allows for
the comparison with other populations, such as the US
population in general [62] or other military populations
[57,63]. Most importantly, the future strength of these
data will be in the longitudinal comparison with baseline
health during and after deployment as well as in comparison with civilian and other veteran organizations.

Conclusion
Recent reports have suggested significant mental health
morbidity in US military personnel returning from military deployment to Iraq and Afghanistan [64,65], as well
as increased risk of neuropsychological compromise after
deployment in a cohort of Army personnel [66]. These
reports have added to the mounting concern over the
physical and mental health of returning deployed personnel as well as the effect this may have on family members,
health care utilization, and diminished military readiness
for future deployments. In this report, we described the
baseline functional health of a large US military cohort as
measured by the Medical Outcomes Study Short Form 36item Survey for Veterans. Our findings suggest, on average, a mentally and physically healthier population than
other comparison populations, and will be instrumental
in prospectively evaluating health after deployment in a
large, population-based military cohort.

Abbreviations
ANOVA, analysis of variance; MCS, mental component
summary score; PCS, physical component summary
score; VA, Department of Veterans Affairs

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Competing interests
The author(s) declare that they have no competing interests.

4.

Authors' contributions

5.

TS, BS, and CL performed the statistical analysis. All
authors helped conceive the study, participated in its
design and coordination, and helped to draft the manuscript. All authors read and approved the final manuscript.

6.

7.

Acknowledgements
We thank Scott L. Seggerman from the Management Information Division,
Defense Manpower Data Center, Seaside, California. Additionally, we
thank Lacy Farnell; Isabel Jacobson; Travis Leleu; Robb Reed; Steven Spiegel;
Kari Welch, Charlene Wong, and Jim Whitmer from the Department of
Defense Center for Deployment Health Research, Naval Health Research
Center; and Michelle Stoia, also from the Naval Health Research Center.
We also thank COL Karl E. Friedl and all the professionals from the US
Army Medical Research and Materiel Command, especially those from the
Military Operational Medicine Research Program, Fort Detrick, Maryland.
We appreciate the support of the Henry M. Jackson Foundation for the
Advancement of Military Medicine, Rockville, Maryland, USA.
In addition to the authors, the Millennium Cohort Study Team is composed
of Margaret AK Ryan1; Tomoko I. Hooper2; Gregory C. Gray3; Gary D.
Gackstetter2,4; Edward J. Boyko5; and Paul J. Amoroso.6

8.

9.
10.
11.

12.

1 Department of Defense Center for Deployment Health Research at the
Naval Health Research Center, San Diego, CA, USA.

13.

2 Uniformed Services University of the Health Sciences, Bethesda, MD,
USA.

14.

3 College

of Public Health, University of Iowa, Iowa City, IA, USA.

4Analytic

Services, Inc. (ANSER), Arlington, VA, USA.

15.

5 Seattle Epidemiologic Research and Information Center, Veterans Affairs
Medical Center, Seattle, WA, USA.
6 Madigan

16.

Army Medical Center, Tacoma, WA, USA.
17.

This represents report 06–26, supported by the US Department of
Defense, under Work Unit no. 60002. The views expressed in this article
are those of the authors and do not reflect the official policy or position of
the US Department of the Navy, US Department of the Army, US Department of the Air Force, US Department of Defense, US Department of Veterans Affairs, or the US Government. This research has been conducted in
compliance with all applicable federal regulations governing the protection
of human subjects in research (Protocol NHRC.2000.007).

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