A STRATEGY FOR COMPARING THE CONTRIBUTIONS OF ENVIRONMENTAL CHEMICALS
Assessments of the import of an association observed between an environmental chemical exposure and child neurodevelopment often focus solely on the magnitude of the effect size and its associated p-value (i.e., whether it is <0.05). Effect size is expressed in various forms, as the difference between the mean scores of “exposed” and “unexposed” groups, the change in score per unit change in an exposure biomarker, or the change in risk (relative risk, odds ratio) associated with a particular value of the biomarker. Among the reasons cited to dismiss an effect size is that is it is “clinically unimportant” (e.g., Kaufman, 2001). This perspective fails to place the effect estimate in a public health context, however. Estimating the population burden attributable to a factor requires a metric that reflects not only the magnitude of the risk associated with the factor but the frequency with which the factor occurs in the population (Steenland and Armstrong 2006), a concept embodied in the “environmentally attributable fraction” model (IOM, 1981). Although a factor that is associated with a large impact would be a significant burden to a patient, it might not be a major contributor to population burden if it rarely occurs. Conversely, a factor that is associated with a modest impact, but which occurs frequently, could contribute substantially to population burden. The objective of this paper is to describe a population-oriented approach to estimating risk factor burden as an alternative to the usual disease-oriented approach. This approach is then used to compare the population burdens associated with major medical, social, and chemical risks to child development.
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