Sunday 15 October 2017

The Prosecutor's fallacy (1987)

Interpretation of statistical evidence in criminal trials: The prosecutor's fallacy and the defense attorney's fallacy.

Thompson, William C.,Schumann, Edward L.
Law and Human Behavior, Vol 11(3), Sep 1987, pp 167-187

Two experiments tested 217 undergraduates' ability to use statistical evidence on the incidence rate of a "matching" characteristic when judging the probable guilt of a criminal suspect, based on written descriptions of evidence. Exp I varied whether incidence rate statistics were presented as conditional probabilities or as percentages and found the former promoted inferential errors favoring the prosecution, while the latter produced more errors favoring the defense. Exp II exposed Ss to 2 fallacious arguments on how to interpret the statistical evidence. The majority of Ss failed to detect the error in one or both of the arguments. In both experiments, a comparison of Ss' judgments to Bayesian norms revealed a general tendency to underutilize statistical evidence. 

Image from HERE.