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14 EXPOSURE OF VETERANS TO AGENT ORANGE AND OTHER HERBICIDES units maintain detailed histories of their Vietnam experience and active contact with their former members, and they may also present opportuni- ties for epidemiologic research. The E4 EOI does not measure dose or actual exposure of individuals to herbicides. It is instead a systematic means of classifying potential exposure. Individual dose or exposure measures are the most desirable inputs to epidemiologic studies, but it is often impossible or impractical to obtain them, so researchers routinely assign exposure estimates that are based on indirect or aggregate indexes. That strategy is the norm, for example, in occupational-cohort studies: because individual exposure data are seldom available, workers' exposures are typically assigned from estimates of the average exposure of groups defined by job or work area. Because of individual variations in behavior, biology, and exposure con- ditions, individuals with identical exposure indexes may have different actual exposures. That is a form of exposure-measurement error. Such errors are typically random with respect to disease status, and in that situation they are likely to cause epidemiologic studies to underestimate the magnitude of any association between exposure and outcome. That potential for bias does not invalidate the use of indexes but instead means that care must be exercised in constructing studies based on such mea- sures and interpreting their results. It is also important to recognize that an exposure index like the E4 EOI cannot be directly validated or invalidated through an epidemiologic study. Studies based on well-formed hypotheses about outcomes associ- ated with exposure would provide the strongest evidence regarding the validity of exposure estimates. A positive association with an end point strongly believed to be related to herbicides or dioxin in particular would tend to confirm that the E4 EOI measures relevant exposures. However, given the uncertainties inherent in all epidemiologic studies, it is unlikely that any single study could constitute a strong test of validity. Rather, evaluation of the patterns of results from multiple studies would provide guidance as to the performance of the E4 EOI as an indicator of any particular exposure. CONCLUSIONS AND RECOMMENDATIONS On the basis of the findings discussed above, the committee con- cludes that a valid exposure reconstruction model for wartime herbicide exposures of US veterans of Vietnam is feasible. The committee therefore recommends that the Department of Veter- ans Affairs and other government agencies facilitate additional epidemio- logic studies of veterans by nongovernment organizations and indepen- dent researchers. The committee responsible for the 1994 report, in making