Scientific Evidence

Reasonable Certainty: A Term It Is Certainly Reasonable to Repudiate

By Jules M. Epstein

Virtually every expert witness examination concludes with the question “and do you hold those beliefs/opinions/conclusions to a reasonable degree of [discipline] certainty.” The phrase, like the term “insane,” is a legal construct; but in science and elsewhere the words have no meaning, as science may include a measure of uncertainty, scientific knowledge evolves, and there is no agreed metric within or across disciplines for what degree of certainty—32 percent, 45 percent, 97 percent—is “reasonable.” The standard is subjective to the declarant.

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