ABSTRACT
To this day, some environmental risk assessors contend that risk assessment results must be probabilities. However, experience has clearly shown that risk assessments not only need not be probabilistic but also that probabilistic results are often unclear. Decision-makers do not want to make a decision based on a finding that the probability of impairment is 0.43, and in many regulatory contexts, they cannot. For example, fining an emitter for exceeding a criterion cannot be based on a probability that the criterion was exceeded. Decision making under uncertainty does not require that analytical results be expressed as probabilities. Most of the assessment results that are expressed as probabilities are frequencies and are more clearly expressed in those terms. Rescaling frequencies as probabilities aids some computations, but clear communication requires expressing a real-world property.
