Why Courts Should Use Statistics

Guest: Daniel J. Denis, PhD, Associate Professor of Quantitative Psychology, University of Montana A number of years ago, Julie Rose had a very frustrating experience as a jury member on a federal criminal case. After they’d heard all the evidence, jurors were told that deciding the defendant was guilty meant we needed to be confident – “beyond a reasonable doubt” – that he’d committed the crime. The frustrating part was that none of the jury members seemed to agree on just what “beyond a reasonable doubt” meant. Some of them felt the evidence was so strong there was maybe an 85 to 90 percent chance of guilt. But a few jurors insisted that even one inkling of doubt, one weak spot in the prosecutor’s case meant “reasonable doubt.” In the end, those few jurors prevented the jury from getting the unanimous vote they needed for a guilty verdict and the guy got off.  Here’s the core problem: we’re asking jurors to make a decision based on statistical probabilities without giving them even a basic understanding of how statistics work.