When Do Courts Think Base Rate Statistics are Relevant?
Jurimetrics Journal, Vol. 42, p. 373, 2002
30 Pages Posted: 12 Jul 2009 Last revised: 7 Feb 2011
Date Written: 2002
This paper identifies conditions under which appellate courts are more and less likely to treat background probabilities (i.e., base rates) as relevant. Courts are likely to view base rates as relevant when base rates (a) arise in cases that appear to have a statistical structure, (b) are offered to rebut an it-happened-by-chance theory, (c) are computed using reference classes that incorporate specific features of the focal case, or (d) are offered in cases when it is difficult or impossible to obtain evidence of a more individuating sort. irrelevant. Part I discusses the nature and logical relevance of base rates. Part II describes concerns that have been raised with regard to the admissibility and sufficiency of base rates. Part III identifies the mistrust that courts have long had for probabilistic evidence generally and base rates in particular. The early antagonism toward base rates, fear of inaccurate statistics, and resistance to inferences of guilt by trait association are documented. Part IV draws from the probabilistic reasoning literature to identify a series of conditions under which base rates may be viewed more favorably by the courts. Evidence from court rulings suggests that base rates are more likely to be treated as relevant (and admissible) when they are (a) offered to rebut an "it-happened-by-chance" theory, (b) computed using refined (as opposed to general) reference classes, or (c) offered in cases where evidence of a more individuating sort cannot (in principle) be obtained.
Keywords: Base rates, appellate courts, statistics
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