ASSESSMENT OF VARIABILITY AND UNCERTAINTY DISTRIBUTIONS FOR PRACTICALRISK ANALYSES

In recent years the U.S. Environmental Protection Agency has been chal lenged both externally and internally to move beyond its traditional c onservative single-point treatment of various input parameters in risk assessments. In the first section, we assess when more involved distr ibution-based analyses might be indicated for such common types of ris k assessment applications as baseline assessments of Superfund sites. Then in two subsequent sections, we give an overview with some case st udies of technical analyses of (A) variability/heterogeneity and (B) u ncertainty. By ''interindividual variability'' is meant the real varia tion among individuals in exposure-producing behavior, in exposures, o r some other parameter (such as differences among individual municipal solid waste incinerators in emissions). In contrast, ''uncertainty'' is a description of the imperfection in knowledge of the true value of a particular parameter or its real variability in an individual or a group. In general uncertainty is reducible by additional information-g athering or analysis activities (better data, better models), whereas real variability will not change (although it may be more accurately k nown) as a result of better or more extensive measurements. The purpos e of the rather long-winded exposition of these two final sections is to show the differences between analyses of these two different things , both of which are described using the language of probability distri butions.