Understanding of forensic expert reports by judges, defense lawyers and forensic professionals

Abstract

Forensic scientists strongly advocate the use of likelihood ratios for expressing the diagnostic value of evidence in technical forensic reports. They call this the logically correct approach. The correct comprehension of such likelihood ratios by jurists, however, appears to be particularly problematic. The present research has empirically investigated this issue for defense lawyers and criminal judges. For comparative purposes forensic professionals, many of whom use the logically correct approach, were included in the study as well.

Using fictitious forensic reports, it was shown that proper understanding of likelihood ratios by jurists is quite poor, due mainly, but not exclusively, to the prosecutor's fallacy. Forensic professionals outperformed jurists to a large extent but made many mistakes themselves. It is further shown that participants’ self-expressed supposed level of understanding of logically correct conclusions is quite high and thus not justified by their levels of proper understanding. Suggestions for how to improve the situation are presented.

Keywords:

• technical forensic evidence
• likelihood ratio
• diagnostic value
• understanding
• judges
• lawyers
• experts

Acknowledgements

This research has been supported by funding from the Dutch Forensic Institute (NFI). This article is based on the Dutch report Bijkans Begrepen? (De Keijser, Elffers, Kok, & Sjerps, 2009). The authors are grateful to those who assisted in making it possible to collect data in hard to reach populations, to all participants in the study, and to two anonymous reviewers for their careful and useful comments.