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Uncertainty Ahead - A Shift in How Federal Scientific Experts Can Testify

NCJ Number
Date Published
February 2018
8 pages
Publication Series
This article discusses how state and local forensic scientists and attorneys will alter their testimonial practice under a new directive that instructs U.S. Justice Department forensic scientists working in federal laboratories and U.S. Attorneys to stop using the phrase "reasonable degree of scientific certainty" when testifying.
The phrase at issue has been used in courts throughout the nation for decades to signal that the expert's testimony is based on sound scientific or technical analysis rather than conjecture. The rejection of this phrase stems from a concern that juries not be influenced by words that imply certainty when that level of assurance cannot be substantiated, regardless of the discipline. In addition, the phrase has never received a cross-disciplinary definition and continues to carry differing interpretations. It will be a difficult task to identify a single phrase that all forensic disciplines can use to distinguish between speculation and the relative degree of confidence that a scientist has in his/her findings and opinions. One likely approach will be that each forensic discipline will develop and define its own terminology, based on a formal standards-development process. The unique aspects of each discipline and the needs of those who will use this information will be taken into account. The available research has shown that confident testimony by an experienced forensic expert has a positive influence on juries. Conversely, the use of highly technical methods, validation studies, cross-examination, exculpatory evidence, concessions of error, explanations of the limitations of a methodology, and jury instructions had little overall impact on juror decision-making in the research setting. Informed by studies of juror comprehension, scientists must continue to develop and define terminology that accurately conveys both their opinions and the limitations of their methodology.

Date Published: February 1, 2018