The Law Doesn't Say Much About Base Rates

16 Pages Posted: 23 Sep 2013

Date Written: March 15, 2013


In his recent contribution “Neglect the Base Rate: It’s the law!” (, Prof. Engel makes the point that the law forbids making group-to-individual inferences based on the relative frequency of a property of interest in a reference class (the “base rate”), unless the members of the reference class were “acting in concert” or the base rate information serves as exculpating, rather than inculpating, evidence in criminal cases. While making his argument, Prof. Engel also criticizes a “Bayesian” approach to evidence evaluation, i.e. an approach that posits that the partial beliefs of judges should conform to the axioms of probability theory, suggesting that it leads to conclusions which are incompatible with substantive law. Prof. Engel develops his arguments based on a series of examples. In this response, I purport to show that, i) subjective probability theory does not lead to solutions of the problems presented by Prof. Engel that are contrary to substantive law; and, ii) the law does not generally prohibit the use of base rate information where it admits other relevant evidence. In fact, the law does not say much about the use of base rate information at all, because it is hardly ever directly relevant outside the world of hypothetical cases of legal scholars. On the other hand, base rate information can be epistemically useful, and a blanket prohibition cannot be read from the law.

Keywords: base rate, Bayes’ rule, probabilistic inference

Suggested Citation

Schweizer, Mark, The Law Doesn't Say Much About Base Rates (March 15, 2013). Available at SSRN: or

Mark Schweizer (Contact Author)

Swiss Federal Patent Court ( email )

St. Leonhardt-Strasse 49
St. Gallen, 9023


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