An expert witness is a person authorized by a judge to share professional opinions in a court of law about subjects in which the expert was found to be sufficiently knowledgeable and competent.  

During criminal or civil litigation, the permission granted to expert witnesses to give their opinions about topics of interest (DNA, fingerprints, accounting, ballistics, engineering, toxicology, and so forth) gives them considerable power and influence in the courtroom as compared to other kinds of witnesses.

Expert witnesses serve as consultants - so to speak - to the courts.  In this capacity, they help make sense of information that is simply too specialized for laypersons to understand without proper assistance, especially when the testimony under consideration is scientific in nature.

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The Federal Rules of Evidence, passed into law in 1975, set forth guidelines for determining if a person may be qualified to testify as an expert witness.  Nearly twenty years later, in 1993, the United States Supreme Court handed down its landmark ruling in Daubert v. Merrell Dow Pharmaceuticals, which reminded judges of their "gatekeeping" role in applying the Federal Rules of Evidence while weighing the potential reliability of expert testimony.

When scientists serve as expert witnesses, however, testimony can be highly technical and very compelling.  For instance, a forensic DNA scientist that identifies a defendant's blood at a crime scene - or a drug chemist that identifies a white powder as being cocaine - is in a position to exert impressive influence over the court.  It makes complete sense, then, that judges take their gatekeeping role very seriously, although it is among the most challenging of all the responsibilities they hold in American jurisprudence.

Unfortunately, the gatekeeping role of judges is complicated by another challenge for which neither the Federal Rules of Evidence nor Daubert provide much help:  How much latitude should scientists be afforded in communicating the likelihood that the opinions they are offering are actually true and accurate?  It is one thing to give a scientific opinion, but expressing a particular degree of certainty is an entirely different matter.

A traditional approach for both trial attorneys and expert witnesses in addressing matters of scientific reliability has been to employ a simple phrase that has remarkable legal and judicial precedent:  

To a Reasonable Degree of Scientific Certainty.  

The phrase "to a reasonable degree of scientific certainty" is a bold expression of assuredness.  It implies that no unreasonable degrees of certainty should be assumed by a trier of fact, namely the judge and jury.  It is more or less a legal way of saying, "don't worry, I got this."

As convenient as the phrasing is and has been for expert witnesses, it appears that things may be changing in the world of expert witnessing when it comes to expressions of certainty.

In January 2016, The National Commission on Forensic Science, an advisory body managed jointly by the U.S. Departments of Justice and Commerce, issued a formal recommendation on testimony that refers to expressions of certainty:

"These terms have no scientific meaning and may mislead factfinders about the level of objectivity involved in the analysis, its scientific reliability and limitations, and the ability of the analysis to reach an individualized conclusion."

In the wake of the commission's recommendation, many expert witnesses are now scratching their heads in search of alternative ways to express the extent of their professional certainties about the accuracy of their opinions.  What was once an efficient and comfortable way to communicate one's certainty is now looked upon with suspicion - nothing more than creative wordsmithing intended to manipulate people into believing things that are not justified.

So what exactly are expert witnesses to do?

First, they can take comfort from knowing that Reasonable Degree of Scientific Certainty was never a scientific term nor ever intended to be.  It is a legal phrase used by legal professionals to evaluate expert witness testimony.  Exactly when expert witnesses claimed ownership of it is hard to know.  But it likely arose, as it still does, from trial litigators who specifically ask expert witnesses if their testimony before the court is proffered to a reasonable degree of scientific certainty.  A simple "yes" to this question becomes a sort of scientific endorsement of a legal determination, something that a reputable scientist need not attempt to do. 

Second, scientists serving as expert witnesses can focus on simply communicating their opinions without feeling obligated to "decorate" them with pithy statements about degrees of scientific certainty or other types of flowery language like putting a cherry on a hot fudge sundae.  Improving the cosmetic value of one's testimony helps no one if it doesn't help establish the reliability of expert determinations in the eyes of the court.

Any scientist asked in court if his or her testimony meets a reasonable degree of scientific certainty might simply acknowledge that such language is of a legal nature, not a scientific one.  A skilled witness can then pivot the testimony to address what matters most in science - confidence.  

Yes, reputable scientists called as expert witnesses have confidence in their opinions.  If they didn't, they would not take the risk of sharing them under oath.  But to fully assess how confident a scientist is and why that confidence is justified, requires a conversation to be had.  If attorneys and judges are too impatient or lazy to have that conversation, then so be it.  That is not the concern of the expert witness. 

The National Commission on Forensic Science has this one right.  Degrees of certainty in science are complex matters that need to be discussed and evaluated in depth, not baked into bite-sized morsels of legal jargon that taste bitter to a scientific palate.