Error Rates, Likelihood Ratios, and Jury Evaluation of Forensic Evidence

Abstract

Forensic examiners regularly testify in criminal cases, informing the jurors whether crime scene evidence likely came from a source. In this study, we examine the impact of providing jurors with testimony further qualified by error rates and likelihood ratios, for expert testimony concerning two forensic disciplines: commonly used fingerprint comparison evidence and a novel technique involving voice comparison. Our method involved surveying mock jurors in Amazon Mechanical Turk (N = 897 laypeople) using written testimony and judicial instructions. Participants were more skeptical of voice analysis and generated fewer “guilty” decisions than for fingerprint analysis (B = 2.00, OR = 7.06, p = <0.000). We found that error rate information most strongly decreased “guilty” votes relative to no qualifying information for participants who heard fingerprint evidence (but not those that heard voice analysis evidence; B = −1.16, OR = 0.32, p = 0.007). We also found that error rates and conclusion types led to a greater decrease on “guilty” votes for fingerprint evidence than voice evidence (B = 1.44, OR = 4.23, p = 0.021). We conclude that these results suggest jurors adjust the weight placed on forensic evidence depending on their prior views about its reliability. Future research should develop testimony and judicial instructions that can better inform jurors of the strengths and limitations of forensic evidence.

Judges and forensic science education: A national survey

“…We did not ask judges about other interventions designed to educate jurors, which themselves might require judicial education. For example, jury instructions might convey additional information to jurors about the strength and limitations of forensic evidence (Garrett, Crozier, & Grady, 2020). Or judges may instead approve funding for a defense expert, who might call attention to limitations of the relevant discipline.…”

An Assessment of methods and techniques used in Forensic Linguistics

“…It is important to note that this technology is not based on phonetics as discussed in literature review section however; there is a wide range of experts who have shown doubts and concerns about the validity and reliability. There is also research presented by business experts who used simulation tools in lab settings and concluded that VRA can be used to detect proxies for deception (Garrett, Crozier, and Grady, 2020).…”

Juror appraisals of forensic evidence: Effects of blind proficiency and cross-examination

“…Further, Mitchell and Garrett [3] found that laypeople assume fingerprint examiners are highly proficient, but it is possible that laypeople do not make this same assumption for other forensic disciplines, such that the effect of proficiency information may vary between forensic science disciplines and/or be amplified by effective cross-examination. Indeed, Garrett et al [15] provided laypeople with error-rate information regarding both fingerprint and voice comparison evidence, and they found that this information reduced guilty verdicts in the fingerprint condition, but it did not affect the weight given to voice comparison evidence, which was universally low. However, their study did not manipulate the individual examiner's error rate, but rather presented jurors with general error rates for each domain.…”

“…Prior research has found that jurors tend to place strong weight on fingerprint evidence, regardless of how the examiner phrases ultimate conclusions [20]. However, jurors' beliefs about the reliability of other forms of forensic evidence appear to vary between disciplines [15,21,22]. Unlike fingerprint evidence, few studies have been done examining how jurors evaluate bitemark evidence.…”