One method of developing an operational case definition is a statistical technique known as factor analysis (Ismail et al., 1999). Factor analysis is useful in identifying a small number of correlated variables from among a much larger number of observed variables, such as the symptoms that are reported in a survey of veterans. Factor analysis aggregates survey responses into statistical groupings of factors that may or may not have biological plausibility or clinical relevance. Several researchers have used factor analysis in their studies (described later in this chapter) on the health of Gulf War veterans. When factor analysis is employed in studies of veterans, the observed variables are measurements of veterans’ symptoms, and the fundamental factors are symptom groupings that may represent a potentially new syndrome. Any new syndrome (defined by factor analysis or other means) may have a distinct, albeit often unknown, etiology and pathogenesis (Taub et al., 1995). It is recognized that factor analysis has the potential to generate syndromes that may not be reproduced when a new population is examined.
When evidence is presented that the case definition—defined by factor analysis or other methods—successfully singles out a new patient population from comparison groups, the case definition may gain recognition by the medical establishment as a new syndrome (see Appendix D). There are many advantages to defining and classifying a new syndrome. The foremost advantage is to create a more homogeneous patient population, a crucial step for determining prevalence and ushering in diagnosis and treatment. A potential disadvantage is the mislabeling or misclassification of a condition, which can thwart progress for years, if not decades (Aronowitz, 1991). Classification of a new patient population also stimulates further understanding of the natural history of the disease, risk factors, and ultimately, etiology and pathogenesis. As more knowledge unfolds about etiology and pathogenesis, the classification of an established syndrome can rise to the level of a disease. The renaming of a syndrome as a disease8 implies that the etiology or pathology has been identified.
This section summarizes findings of population-based studies of Gulf War veterans. The next section summarizes findings from studies using other types of epidemiological designs. A population-based study is a methodologically robust type of epidemiologic study because its goal is to obtain information that is representative of the population of interest, in this case Gulf War veterans. The cohort may be the entire population of interest or a random selection from the population of interest. Population-based studies of Gulf War veterans sample a cohort of veterans by contacting them where they live, as opposed to where they seek treatment or where they serve in the military (e.g., a particular base, a particular branch such as the Air Force). Studies of military units or other military