The following section briefly discusses types of evidence and the value of epidemiologic or clinical studies in determining whether an association exists. It is followed by a discussion of the committee’s specific inclusion criteria that were developed to help decide whether a particular study would be included and evaluated for this report. The committee also notes the numerous factors that it considered in evaluating the evidence in a study and, finally, presents the categories of association used in drawing conclusions about the strength of associations.
The committee relied entirely on clinical and human epidemiologic studies to draw its conclusions about the strength of evidence regarding associations between deployment to the Gulf War and health outcomes seen in Gulf War veterans. The committee acknowledges, however, that animal studies might prove helpful in providing biologic understanding of many of the effects seen in humans from specific exposures, such as pesticides, solvents, and nerve agents, which have been reported by troops deployed in the Gulf War. Furthermore, information from molecular and cellular biology, neuroimaging, and other types of human studies can be used to understand the biological mechanisms and identification of biomarkers for clinical outcomes. Such studies, however, are not, in general, included in this review.
In epidemiological research, analytical studies are designed to permit the examination of the association between two or more variables. Predictor variable or independent variable is a term used for an exposure to an agent of interest in a human population. Outcome variable or dependent variable is a term used to define a health or health-related event seen in a human population. Outcomes can also include a number of nonhealth results, such as use of medical services, social changes, and employment changes. One important goal of epidemiological research is to generate information that will help to understand whether exposure to a specific agent is associated with disease occurrence or other health outcomes. This goal is accomplished most straightforwardly in experimental studies in which the investigator controls the exposure (generally through random assignment) and the association between exposure and the subsequent occurrence of an outcome can be measured. Experimental studies are clearly not possible for studying the health effects of deployment to the Gulf War. Therefore, the studies included in this review are observational, not experimental, and compare health outcomes in those deployed to the Gulf with health outcomes seen in those who were in the military during the Gulf War but were not deployed to the Persian Gulf region. What is then assessed is the presence or absence of an association between the exposure and the outcomes.
Association is primarily a statistical concept referring to the quantification of the relationship (positive, negative, or none) between two variables (e.g., independent and dependent). In the presence of association, additional considerations are required for causality to be judged as the reason behind the observed association. Apart from arising from a causal relationship between exposure and outcome, there may be other possible reasons for finding associations in observational studies including random error (chance), systematic error (bias),