references found in of the initial searches, the committee focused on 850 potentially relevant epidemiologic studies for its review and evaluation.

The committee limited its review of the literature primarily to epidemiologic studies of Gulf War veterans to determine the prevalence of diseases and symptoms in that population. Those studies typically examine veterans’ health outcomes in comparison with outcomes in their nondeployed counterparts.

The committee decided to use only peer-reviewed published literature on which to base its conclusions. The process of peer review by fellow professionals increases the likelihood of a high-quality study but does not guarantee its validity or the generalizability of its findings to the entire group of subjects under review. Accordingly, committee members read each study critically and considered its relevance and quality. The committee did not collect original data, nor did it perform any secondary data analysis (exception to calculate response rates for consistency among studies).

After securing the full text of the peer-reviewed epidemiologic studies it would review, the committee determined which studies would be considered primary or secondary studies. Primary studies provide the basis of the committee’s findings. To be included in the committee’s review as a primary study, a study had to meet specified criteria. The criteria include studies that provide information about specific health outcomes, demonstrate rigorous methods, describe its methods in sufficient detail, include a control or reference group, have the statistical power to detect effects, and include reasonable adjustments for confounders. Other studies were considered secondary for the purpose of this review and provided background information or “context” for the report. Another step that the committee took in organizing its literature was to determine how all the studies were related to one another. Numerous Gulf War cohorts have been assembled, from several different countries; from those original cohorts many derivative studies have been conducted. The committee organized the literature into the major cohorts and derivative studies because they didn’t want to interpret the findings of the same cohorts as though they were results from unique groups (Chapter 4).

LIMITATIONS OF THE GULF WAR STUDIES

Overall, the studies of Gulf War veterans’ health are of varied quality. Although, they have provided valuable information, many of them have limitations that hinder accurate assessment of the veterans’ health status. Common study limitations include use of a population that was not representative of the entire Gulf War population, reliance on self-reports rather than objective measures of symptoms, low participation rates, and a period of investigation that was too brief to detect health outcomes with long latency such as, cancer. In addition, many of the US studies are cross-sectional, and this limits the opportunity to learn about symptom duration, long-term health effects, latency of onset, and prognosis.

OVERVIEW OF HEALTH OUTCOMES

While examining health outcomes in Gulf War-deployed veterans, numerous researchers have attempted to determine whether a set of symptoms reported by veterans could be defined as a unique syndrome or illness. Investigators have attempted, by using factor or cluster analysis, to define a unique health outcome, but none has been identified. Every study reviewed by this



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