veterans are exposed to numerous stressors (see Chapter 3), epidemiologic studies are not typically designed to address such types of exposures or the effects associated with specific stressors.

The committee adopted a policy of using only peer-reviewed published literature as the basis of its conclusions. Publications that were not peer-reviewed had no evidentiary value for the committee; that is, they were not used as evidence for arriving at conclusions about the degree of association between deployment to war zone and adverse health effects. The process of peer review by fellow professionals, which is one of the hallmarks of modern science, ensures high standards of quality but does not guarantee the validity of a study or that its results can be generalized, particularly with respect to questions that were not the objective of the original researchers. Accordingly, committee members read each study critically and considered its relevance and quality. In some instances, non-peer-reviewed publications provided background information for the committee and raised issues that required further literature searches. The committee did not collect original data, nor did it perform any secondary data analysis.

The committee chose not to adopt a formal meta-analysis approach because of practical concerns. First, there is a striking amount of heterogeneity in the epidemiologic studies in this report. They vary with respect to the nature, level, and measurement of exposure; the definition and measurement of outcomes; and study design. Further, many studies include multiple odds ratios or relative risk, corresponding to different study groups, control groups, outcomes, exposure measures, statistical models, etc. It is often not possible to choose a single measure from each study in a consistent way, and the meta-analysis would be subject to arbitrary decisions about data extraction. For many health effects the committee found only one or two primary papers and secondary papers. Although the committee used deployment as the exposure of interest for this report, the type, duration, and the nature of the deployment experience varied widely between the studies, again reducing the utility of a meta-analysis. The committee believed that a descriptive analysis would be more comprehensive and accurate given the varying quality and quantity of the studies for each health effect.

With that orientation to the committee’s task, the following sections provide a brief discussion of factors influencing the value of epidemiologic studies, the committee’s criteria for inclusion of studies in its review, considerations in evaluating the evidence or data provided by the studies, and the categories of association for the conclusions about the strength of the evidence presented in the studies.

TYPES OF EVIDENCE

The committee relied entirely on epidemiologic studies to draw its conclusions about the strength of the evidence for an association between deployment to a war zone (a stressor) and health effects (see Chapter 6). However, animal studies play a critical role in elucidating the mechanisms of the stress response (see Chapter 4) and provide a biological platform for many of the effects seen in humans, including that for posttraumatic stress disorder (PTSD) (see Chapter 5).

Animal Studies

Studies of laboratory animals are essential to understanding mechanisms of action, biologic plausibility, and providing response information about possible health effects when



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