preferred over reports of medical symptoms or group of symptoms. For a study to be considered primary, the committee preferred studies that had an independent assessment of an outcome rather than self-reports of an outcome or reports by family members. It was preferable to have the health effect diagnosed or confirmed by a clinical evaluation, imaging, hospital record, or other medical record. For psychiatric outcomes, standardized interviews were preferred, such as the Structured Clinical Interview for the DSM-IV-TR (Diagnostic and Statistical Manual of Mental Disorders-IV-TR), the Diagnostic Interview Schedule, and the Composite International Diagnostic Interview. Similarly, for neurocognitive outcomes, standardized and validated tests were preferred. Additionally, the outcome had to be diagnosed after deployment. However, as self-reports of health outcomes and exposures account for the bulk of the Gulf War and health literature, the committee decided that it would not exclude such studies but rather considered them to be secondary. The committee recognized the potential for misclassification of a health outcome due to inaccurate recall in such studies.

CONSIDERATIONS IN ASSESSING THE STRENGTH OF EVIDENCE

The committee’s process for reaching conclusions about deployment during the Gulf War and its potential for adverse health outcomes was collective and interactive. Once a study was included in the review because it met the committee’s criteria, there were several considerations in assessing causality, including strength of the association, presence of a dose-response relationship, presence of a temporal relationship, consistency of the association, and biologic plausibility. The committee as a group reviewed the primary and secondary studies identified by the committee member responsible for each health outcome. The strengths and limitations of each study and its categorization as primary and secondary were discussed in plenary session and all committee members agreed on its contribution to the evidence base for each category of association for each health outcome. Because many of the studies were cited for more than one health outcome, committee members evaluated each study with equal vigor for every health outcome. The evidence tables were refined to include study limitations for the primary studies and to present the pertinent results; secondary studies were not included in the evidence tables. It should be noted that some of the larger cohort studies used a variety of methods and instruments to assess the health status of Gulf War veterans and it is for this reason that the committee discussed at some length the diagnostic approaches and use of self-reports for each paper. The assignment of a category of association was reached by committee consensus based on the weight of the evidence, including the studies cited in Volume 4, as well as any new studies. Those aspects of the committee’s review required thoughtful consideration of all the studies as well as expert judgment and could not be accomplished by adherence to a narrowly prescribed formula of what data would be required for each category of association or for a particular health outcome.

Categories of Association

The committee attempted to express its judgment of the available data clearly and precisely in the Summary and Conclusion section for each health outcome. It 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 committees that have evaluated vaccine safety, effects of herbicides used in Vietnam, and indoor pollutants related to asthma (IOM, 2000, 2003,



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