The psychology of interpreting expert evaluative opinions

Not all identification conclusions are equal: Quantifying the strength of fingerprint decisions

In the pattern comparison disciplines, forensic practitioners evaluate two impressions with respect to the same-source and different-sources propositions. The results are communicated using a pre-determined conclusion scale, and in the friction ridge discipline Identification is typically the highest category on the scale for reporting evidence supporting the same source proposition. Although error rates have been measured in most disciplines, there are no widespread quantitative approaches and therefore most conclusions rely on subjective human evaluations. The current work uses articulation decisions provided by fingerprint examiners in error rate studies to produce a quantitative likelihood ratio measure that characterizes the strength of the support for the two propositions. We use an ordered probit model to summarize the distribution of responses of examiners who participated in error rate and validation studies. We then aggregate the data for all image pairs in a database to construct a set of likelihood ratios based on the ratio of the two strength-of-support values. We find that these values are modest relative to values typically produced by DNA analysis or implied by current fingerprint articulation language. The technique can be applied to any pattern comparison discipline for which error-rate data is available, and therefore can be used to appropriately weigh the evidence from different disciplines.

Calibrating the perceived strength of evidence of forensic testimony statements

Pattern comparison disciplines use categorical statements to express conclusions. We measured the strength of evidence for six different scales as perceived by members of the general public and fingerprint examiners. The statements came from different types of scales, and included categorical conclusions, likelihoods, strength of support statements, and random match probabilities. We used an online interface that required participants to first correctly sort the statements in a given conclusion scale, and then place each statement on a single evidence axis that ranged from most support imaginable for same source to most support imaginable for different sources. We analyzed the data using both the raw values and a Thurstone-Mosteller model based on ordinal values. We found systematic differences between examiners and members of the general public, such that examiners distinguished between Identification and Extremely Strong Support for Common Source, while members of the general public did not. Statements that included numerical values tended to be placed lower than categorical conclusions, and members of the general public tended to place the highest categorical conclusion in each scale at the very top of the evidence axis. The results suggest that laypersons can distinguish between statements meant to represent moderate vs strong evidence, but tend to place categorical conclusions above statements that involve numerical values.

Must the random man be unrelated? A lingering misconception in forensic genetics

A nearly universal practice among forensic DNA scientists includes mentioning an unrelated person as the possible alternative source of a DNA stain, when one in fact refers to an unknown person. Hence, experts typically express their conclusions with statements like: “The probability of the DNA evidence is X times higher if the suspect is the source of the trace than if another person unrelated to the suspect is the source of the trace.” Published forensic guidelines encourage such allusions to the unrelated person. However, as the authors show here, rational reasoning and population genetic principles do not require the conditioning of the evidential value on the unrelatedness between the unknown individual and the person of interest (e.g., a suspect). Surprisingly, this important semantic issue has been overlooked for decades, despite its potential to mislead the interpretation of DNA evidence by criminal justice system stakeholders.

Juror comprehension of forensic expert testimony: A literature review and gap analysis

Forensic scientists and commentators including academics and statisticians have been embroiled in a debate over the best way to present evidence in the courtroom. Various forms of evidence presentation, both quantitative and qualitative, have been championed, yet amidst the furor over the most "correct" or "accurate" way to present evidence, the perspective of the fact-finder is often lost. Without comprehension, correctness is moot. Unbeknownst to many forensic practitioners, there is a large, though incomplete, body of literature from the cognitive psychology domain that explores the question of what jurors understand when forensic scientists testify. This body of work has begun to test different proposed methods of testimony in an effort to understand which are most effective at communicating the strength of evidence that is intended by the expert. This article is a review of that literature that is intended for the forensic scientist community. Its aim is to educate that community on the findings of completed studies and to identify suggestions for further research that will inform changes in testimony delivery and ensure that any modifications can be implemented with confidence in their effectiveness.

Cognitive neuroscience in forensic science: understanding and utilizing the human element

The human element plays a critical role in forensic science. It is not limited only to issues relating to forensic decision-making, such as bias, but also relates to most aspects of forensic work (some of which even take place before a crime is ever committed or long after the verification of the forensic conclusion). In this paper, I explicate many aspects of forensic work that involve the human element and therefore show the relevance (and potential contribution) of cognitive neuroscience to forensic science. The 10 aspects covered in this paper are proactive forensic science, selection during recruitment, training, crime scene investigation, forensic decision-making, verification and conflict resolution, reporting, the role of the forensic examiner, presentation in court and judicial decisions. As the forensic community is taking on the challenges introduced by the realization that the human element is critical for forensic work, new opportunities emerge that allow for considerable improvement and enhancement of the forensic science endeavour.