This paper proposes a subjective scale of scientific uncertainty that allows a source of scientific information to express to a lay audience the subjective level of certainty or uncertainty that it associates with a particular assertion of scientific fact, or to represent the range of expert opinion regarding that certainty or uncertainty. The scale is intended as a tool to help increase the precision and rationality of discourse in controversies in which generalists untrained in natural science must judge the merits of opposing arguments in disputes among scientific experts. It complements the quantitative scale of uncertainty, based on Bayesian statistics, used in the recent report of the Inter‐Governmental Panel on Climate Change. Both of these scales are designed for use in situations where the risk probabilities are not precisely known.

The scale takes advantage of the fact that there are many more standards of proof recognized in the US legal system beyond the familiar ‘criminal’ and ‘civil’ standards of ‘beyond a reasonable doubt’ and ‘preponderance of the evidence’, respectively, and that these standards correspond to levels of certainty or uncertainty that constitute acceptable bases for legal decisions in a variety of practical contexts. The levels of certainty or uncertainty corresponding to these standards of proof correspond rather well to the informal scale of certainty used by research scientists in the course of their everyday work, and indeed by ordinary people as they estimate the likelihood of one or another proposition.

Received 12 August 2002. Revised 27 October 2002. Accepted 5 November 2002.

Author notes

1Walsh School of Foreign Service, Georgetown University, 37th and O Streets, NW, Washington, D.C. 20057, USA