THE USE OF DEFAULTS
Many U.S. Environmental Protection Agency (EPA) decisions consider only health factors. EPA’s primary general approach for considering uncertainty in this class of problems has been to use safety or default adjustment factors. (See Chapter 2 for a discussion of the use of default adjustment factors, or defaults.) The decision rule for these approaches is to set a standard or regulation that is highly protective by applying defaults. These approaches are health protective in nature, widely used, and sometimes embodied in statutes. As discussed in Science and Decisions (NRC, 2009), many of the defaults that EPA uses were developed on a scientific basis and can be adequate and acceptable to use in some risk assessments. For example, in instances when there is not adequate information or when the potential uncertainties are such that the use of defaults compared to quantitative uncertainty analyses is unlikely to affect a decision, defaults can be used.
One of the main objections that decision analysts have to using default factors is that they incorporate implicit judgments by analysts or scientists who do not make the regulatory decision. Furthermore, those judgments and their implications are not always independent and are not always explained to decision makers, which makes it difficult for the decision makers to properly interpret the assessment in the context of other factors.
Using health-protective (called conservative) analytic or default approaches to account for multiple uncertainties can result in an overestimation of health risks and a level of precaution in excess of one based on expected values (Nichols and Zeckhauser, 1988; Viscusi et al., 1997). With this happens—a situation sometimes referred to as compounding conservatism—the precaution level for each individual analysis might be such that the marginal cost of precaution equals or slightly outweighs the marginal health benefit, but when multiple analyses use that level of precaution and are combined, the precaution level becomes such that the overall marginal cost far exceeds the overall marginal benefit. It is unclear, however, how extensive that problem is in reality, and, as discussed by the Government Accountability Office, EPA has taken steps to improve such analyses and avoid some of the problems of compounding conservatism (GAO, 2006). Cullen (1994) evaluated the effects of potential compounding conservatism and found that “there exist cases in which conservatism compounds dramatically, as well as those for which the effect is less notable” (p. 392).
To the extent that the probability distribution function is flat and wide (that is, it has “fat tails”; see Farber, 2007, for discussion) rather than being tall and single-peaked, the safety factor will be high relative to the expected value. Conversely, if the probabilities of adverse outcomes are very