Error vs. accuracy rates: Evaluating forensic expert credibility

Abstract

In a criminal trial, judges and jurors need to be able to quickly, easily, and confidently interpret forensic expert data. It is also important for them to determine the credibility of forensic experts in order to properly evaluate their opinion on legal evidence. Communicating forensic experts’ opinions on legal evidence has been in the spotlight for researchers. More recently, focus has begun to shift towards the importance of understanding and communicating forensic expert performance data. In a 2 x 2 x 2 fully between-subjects design three fictitious expert reports (extracted from Martire, et al., 2020) were used to communicate forensic expert performance data to mock jurors (N = 143). The framing of the data - (error rates vs. accuracy rates), presentation format (data presented as a mean value vs. individual data points on a scatterplot) and colour of the data (colour vs. greyscale) - were manipulated. To measure judgements of credibility in forensic experts the traits ‘reliable’, ‘accurate’, and ‘trustworthy’ were used. Negatively framed forensic expert performance data (i.e. error rates) lead to lower credibility ratings compared to positively framed forensic expert performance data (i.e. accuracy rates). Presentation format had no significant effect on participants’ credibility ratings. Finally, participants’ ratings of expert credibility were significantly greater when data was presented in colour compared with greyscale. The outcome of this study extends research on message framing to a legal decision making context looking at the communication of forensic data.