Gulf War and Health Volume 6: Physiologic, Psychologic, and Psychosocial Effects of Deployment-Related Stress (2008) / Chapter Skim
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2 CONSIDERATIONS IN IDENTIFYING AND EVALUATING THE LITERATURE
2 CONSIDERATIONS IN IDENTIFYING AND EVALUATING THE LITERATURE
Pages 17-30
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From page 17... ...
It began its work by overseeing extensive searches of the English-language, peer-reviewed medical and scientific literature. It identified epidemiologic studies of persistent health effects associated with veterans deployed during World War II, the Korean War, the Vietnam War, the 1991 Gulf War, and Operation Enduring Freedom (OEF)
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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)
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. Epidemiologic studies can establish statistical associations between exposures and health effects; associations are generally estimated by using relative risks or odds ratios.
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It can test hypotheses about whether an exposure to a specific stressor is related to the development of a health effect and can examine multiple health effects that may be associated with exposure to a given stressor. A cohort study starts by classifying study participants according to whether or not they have been exposed to the stressor under study, in this case deployment to a war zone.
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From page 21... ...
An advantage of a cohort study is that it is possible to calculate absolute rates of disease incidence.2 A final advantage, especially over cross-sectional studies (discussed below) , is that it may be possible to adjust each subject's followup health status for baseline health status so that the person acts as his or her own control vs defining a group as "diseasefree"; that may reduce a source of variation and increase the power to detect effects.
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In a cross-sectional study, effect size is measured as relative risk, prevalence ratio, or prevalence odds ratio. It might compare health effect or symptom rates between groups with and without exposure to the specific stressor.
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The health effect must have been diagnosed or confirmed by a clinical evaluation, a specific laboratory test, hospital record, or other medical record; for psychiatric outcomes, standardized interviews were necessary, such as the Structured Clinical Interview for the Diagnostic and Statistical Manual of Mental Disorders (DSM) -IV-TR, the Diagnostic Interview Schedule, and the Composite International Diagnostic Interview; for psychosocial effects, the effect needed to be obtained through the use of a validated or well-recognized instrument or the data obtained from databases maintained by a government agency or other appropriate organization.
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There are different types of bias, such as selection bias and information bias. Selection bias occurs when systematic error in obtaining study participants results in a potential distortion of the true association between exposure and outcome.
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Small increases in relative risks that are consistent among studies, however, might be evidence of an association, whereas some forms of extreme bias or confounding can produce a high relative risk. The statistical power of a study was important for it had to be able to detect effects of an unspecified magnitude, especially important for negative results.
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CATEGORIES OF ASSOCIATION The committee attempted to express its judgment about the available data as clearly and precisely as possible. The committee agreed to use the categories of association that have been established and used by previous Committees on Gulf War and Health and other Institute of Medicine (IOM)
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At least one high-quality3 study reports a positive association that is sufficiently free of bias, including adequate control for confounding, and corroborating studies provide support for the association but are not sufficiently free of bias, including confounding. Alternatively, several studies of lower quality might show a consistent association with results that are probably not due to bias, including confounding.4 Inadequate/Insufficient Evidence to Determine Whether an Association Exists Evidence is of insufficient quantity, quality, or consistency to permit a conclusion regarding the existence of an association between exposure to a specific agent and a specific health effect in humans.
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From page 28... ...
In addition, sufficient time might not have passed since deployment to detect the development of some health outcomes, for example, cancer or heart disease particularly in Gulf War, OEF, and OIF veterans. SUMMARY The committee reviewed and evaluated studies from the scientific and medical literature that were identified with searches of bibliographic databases and other methods.
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From page 29... ...
2007. Gulf War and Health, Volume 5: Infectious Diseases.
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From page 30... ...
1997. Invited commentary: How would we know a Gulf War syndrome if we saw one?
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