NIST urges caution in use of courtroom evidence presentation method


The method involves the use of Likelihood Ratio (LR), a statistical tool that gives experts a shorthand way to communicate their assessment of how strongly forensic evidence, such as a fingerprint or DNA sample, can be tied to a suspect. In essence, LR allows a forensics to boil down a potentially complicated set of circumstances into a number—providing a pathway for experts to concisely express their conclusions based on a logical and coherent framework. LR's proponents say it is appropriate for courtroom use; some even argue that it is the only appropriate method by which an expert should explain evidence to jurors or attorneys.




"We are not suggesting that LR should never be used in court, but its envisioned role as the default or exclusive way to transfer information is unjustified," Lund said. "Bayesian theory does not support using an expert's opinion, even when expressed numerically, as a universal weight of evidence. Among different ways of presenting information, it has not been shown that LR is most appropriate."


The trouble arises when other people—such as jurors—are instructed to incorporate the expert's LR into their own decision-making. An expert's judgment often involves complicated statistical techniques that can give different LRs depending on which expert is making the judgment. As a result, one expert's specific LR number can differ substantially from another's.

"Two people can employ Bayesian reasoning correctly and come up with two substantially different answers," Lund said. "Which answer should you believe, if you're a juror?"


Viewpoints differ on the appropriateness of using LR in court. Some of these differences stem from the view that jurors primarily need a tool to help them to determine reasonable doubt, not particular degrees of certainty. To Christophe Champod, a professor of forensic science at the University of Lausanne, Switzerland, an argument over LR's statistical purity overlooks what is most important to a jury.

"We're a bit presumptuous as expert witnesses that our testimony matters that much," Champod said. "LR could perhaps be more statistically pure in the grand scheme, but it's not the most significant factor. Transparency is. What matters is telling the jury what the basis of our testimony is, where our data comes from, and why we judge it the way we do."


"Our success in forensic science depends on our ability to measure well. The anticipated use of LR in the courtroom treats it like it's a universally observable quantity, no matter who measures it," Lund said. "But it's not a standardized measurement. By its own definition, there is no true LR that can be shared, and the differences between any two individual LRs may be substantial."



"Just because we have a tool, we should not assume it's good enough," Lund said. "We should continue looking for the most effective way to communicate the weight of evidence to a nonexpert audience."

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More information: Steven P. Lund et al, Likelihood Ratio as Weight of Forensic Evidence: A Closer Look, Journal of Research of the National Institute of Standards and Technology (2017). DOI: 10.6028/jres.122.027
Citation: NIST urges caution in use of courtroom evidence presentation method (2017, October 12) retrieved 20 May 2019 from https://phys.org/news/2017-10-nist-urges-caution-courtroom-evidence.html

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