Bayes' theorem in the trial process: Instructing jurors on the value of statistical evidence.

Abstract

The use of statistics and probabilities as legal evidence has recently come under increased scrutiny. Judges' and jurors' ability to understand and use this type of evidence has been of special concern. Finkelstein and Fairley (1970) proposed introducing Bayes' theorem into the courtroom to aid the fact-finder evaluate this type of evidence. The present study addressed individuals' ability to use statistical information as well as their ability to understand and use an expert's Bayesian explanation of that evidence. One hundred and eighty continuing education students were presented with a transcript purportedly taken from an actual trial and were asked to make several subjective probabiliy judgments regarding blood-grouping evidence. The results extend to the trial process previous psychological research suggesting that individuals generally underutilize statistical information, as compared to a Bayesian model. In addition, subjects in this study generally ignored the expert's Bayesian explanation of the statistical evidence.

Lay understanding of forensic statistics: Evaluation of random match probabilities, likelihood ratios, and verbal equivalents.

“…These studies have consistently found that judgments of guilt varied appropriately (Faigman & Baglioni, 1988;Goodman, 1992;Smith et al, 1996). In every case people gave more weight to the forensic evidence (greater shifts in estimated chances of guilt) when the RMP was low than when it was higher.…”

“…Research has generally found that simulated jurors are less responsive to forensic evidence than Bayesian models indicate they should be (Thompson & Schumann, 1987;Faigman & Baglioni, 1988;Goodman, 1992;Smith et al, 1996;Schklar & Diamond, 1999;Nance & Morris, 2002, 2005Martire et al, 2013Martire et al, , 2014; for reviews of the early studies see Koehler, 2001;Kaye & Koehler, 1991;Thompson, 1989). But Thompson, Kaasa, and Peterson (2013) recently questioned whether jurors always underutilize forensic evidence relative to Bayesian norms.…”

Errors in Judicial Decisions

“…They got the same result when they gave participants the prior assessment of guilt. Faigman and Baglioni (1988) and, in a different setting of liability for personal injury, Bornstein (2004) got this result as well. Other references are Edwards (1968) and Saks and Kid (1980).…”

The psychology of interpreting expert evaluative opinions

“…Instead, the evidence suggests that people have a generally poor understanding of probabilities and statistics; tend to confuse likelihoods and posterior beliefs (as in the prosecutor and defence attorney fallacies) 20 ; and tend toward an under-valuing of the evidence as compared to normative (i.e. Bayesian) models [21][22][23][24] . Moreover, meta-analyses of risk communication literature 25 in addition to numerous studies focusing specifically on probabilistic expressions in court [26][27][28][29] suggest that decision-makers are sensitive to the form in which statistics are presented and have a preference for natural frequencies or event rates.…”