Molecular epidemiology: principles and practices. Schulte PA, Perera FP, eds., San Diego, CA: Academic Press, 1993 Apr; :79-107
Biologic markers and the studies that include them need to be shown to have measurement, internal, and external validity before they can be used accurately in etiologic research and quantitative risk assessment. The use of invalid markers can result in nondifferential misclassifications of exposure or outcome, which can lead to underestimation of a true effect (Hogue and Brewster, 1988). Risk assessments based on studies that underestimate a true effect can lead to regulations that provide exposure limits that are thought to be "safe" but, in fact, are not. Conversely, a differential misclassification bias, depending on the direction of the bias, can lead to regulations that provide exposure limits that are either too high or too low. In quantitative risk assessment, the inferences derived from small study groups are generalized to larger populations. The strength of those inferences depends on the methodology of the study, including measurements and other design factors that lead to the results, Invalid measurements, inferences, or generalizations may lead to erroneous risk assessments. In this chapter, the three categories of validity are discussed in terms of how they apply to biologic markers for epidemiologic research and quantitative risk assessment.