ABSTRACT
Risk analysts emphasize the differences between variability and incertitude, which are fundamentally different kinds of uncertainty. Variability is heterogeneity and stochasticity, such as spatial variation in chemical concentration, temporal fluctuations in weather, and genetic differences in susceptibility among individuals. Incertitude, on the other hand, is incomplete knowledge such as that arising from measurement error, doubt about the model or abstraction that should be used, limited sample sizes, possible biases in empirical design, and use of surrogate data. Incertitude can generally be reduced by additional empirical effort, but this is not true for variability. Although variability can perhaps be better characterized by the collection of more data, its amount and patterns are usually objective facts of nature that are not diminished by empirical effort. Most analysts agree that it is essential to keep these 2 kinds of uncertainty separate in any assessment for the sake of planning effective remediation or management strategies.
