In Reasonable Scientific Certainty, document examination expert witness Dennis Ryan writes:
A recent report by the Criminal Justice Section of the American Bar Association (ABA) to their House of Delegates addresses the common term “Reasonable Scientific Certainty.” This term or a variant of the term has been seen frequently in many expert reports and has been called a requirement by many who use our expert reports. The ABA report recommends that this phrase be avoided because it has no scientific meaning.
The ABA report calls this term ambiguous and it appears to be used to measure the degree of confidence of the expert in his or her conclusions. This ambiguity can cause the expert’s conclusions to be misinterpreted by anyone that reads the expert report.
Different courts that have required this term in expert reports have been quite surprised to hear in testimony that there was a wide gap in the interpretation of the term Reasonable Scientific Certainty; i.e. possible versus probably or identification.
The ABA report speaks about Federal Rule 401 and states “there is no requirement in the Federal Rules of Evidence that the expert’s opinion be expressed in terms of “probabilities.” In a case United States vs. Cyphers, the defense attempted to exclude expert testimony because the expert did not express their opinion to a reasonable degree of scientific certainty. The Seventh Circuit, in that case, ruled that there was no such requirement.
There are many variations that are used for the term reasonable scientific certainty. We have heard “reasonable degree of forensic certainty,” and “reasonable degree of document certainty” and in the ballistics field “reasonable degree of ballistics certainty.” These variations have likely emanated from the term “reasonable medical certainty” which some interpret as more likely than not. The ABA report expresses some concern that a jury would translate “scientific certainty” as “more likely than not.”
Dennis Ryan is a Board Certified* Forensic Document Examiner. For 16 years, he was the Forensic Document Examiner and Laboratory Supervisor for the Nassau County Police Department in New York.