evaluate it. In this chapter, the committee describes more fully how it has approached its task, in the hope that readers may then be in a better position to assess and interpret the committee's findings. By offering this information, the committee wishes to make the report useful to those who may seek to update its conclusions as new information is obtained. This chapter outlines the specific questions posed by the committee, the types of evidence it identified, its approaches to evaluating reports both singly and collectively, and the nature of the conclusions it felt that logic and evidence permitted. Against this background, details of the analysis and specific conclusions concerning each health effect appear in subsequent chapters. This chapter is based on a similar description in an Institute of Medicine report Adverse Effects of Pertussis and Rubella Vaccines (IOM, 1991), adapted to the current task.
Attributes of the diseases being considered, as well as the population exposed to herbicides, influenced the committee's analysis. The diseases can be characterized, for example, by their frequency, by the specificity of their symptoms, and by prior knowledge of their etiology and pathogenesis. Diseases, such as non-Hodgkin's lymphoma, that occur only rarely in exposed persons are more difficult to study than those that occur more frequently. Conditions such as soft tissue sarcoma that are ill defined, birth defects that are known to occur in the absence of herbicide exposure, or conditions that generally have unknown causes or mechanisms of development are also inherently difficult to investigate.
When the actual intensity or duration of exposure to a potential disease-causing agent is difficult to measure, as is generally true for herbicide exposure in Vietnam, comparisons between presumably exposed and presumably nonexposed persons become clouded. This is due to the misclassification of truly exposed individuals as unexposed or, more likely, the misclassification of truly unexposed people as exposed. For example, some studies compare veterans with experience in Vietnam to veterans who served during the same period of time, but not in Vietnam (Vietnam era veterans). If such a classification system is used as a surrogate for exposure to herbicide, it is likely that a substantial number of those presumed to be exposed on the basis of Vietnam service had either minimal or no actual exposure. The committee deemed the issue of exposure measurement to be so important that a separate chapter of this report is devoted to that topic. A section of Chapter 6 also addresses the assessment of exposure in epidemiologic studies.
It was because of the uncertain validity of exposure measurements in many of the studies of veterans that the committee decided to review studies of other groups potentially exposed to the herbicides contained in Agent Orange, to other herbicides, or to dioxin, the contaminant presumed by some to be the actual cause of the purported adverse effects of Agent Orange. These other groups include industrial and agricultural workers, Vietnamese