Lower health-related quality of life measures have been associated with increased health care utilization [20–22], posttraumatic stress disorder , disability , behavioral risk factors , 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 . To interpret differences in mean scores, a difference of five points in the scores is considered clinically and socially meaningful . 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 . 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, 46–48], 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 . 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 , irritable bowel syndrome , fibromyalgia , and chronic fatigue syndrome [53, 54]. Women serving in 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 . Common findings between the 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 .
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 . 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 . 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 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 . 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 36-item 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  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.