There comes a moment in each expert's testimony where the actual opinions are delivered, and they're always delivered with something like this build-up: "I can say to a reasonable scientific certainty that...." In physician malpractice cases, substitute the word "medical" for "scientific." That is the legal standard, but what does that phrase really mean? And, particularly when an expert on the other side is using the same phrase to frame an opposite conclusion, how should a jury understand and apply it? The law doesn't seem to give much guidance on this point, since the standard is required but not defined. So it is left to the experts themselves to come to their own understanding of what "reasonable certainty" means. And it should come as no surprise that there is some inconsistency in the ways different experts will understand that phrase.

That is the conclusion of a recent study investigating what experts mean by "reasonable certainty." A group of researchers from Penn State College of Medicine (Dias et al., 2015) surveyed 294 medical specialists who testify about head trauma in cases of suspected child abuse, and found a relatively high diversity of meanings that the experts apply. The researchers found that while there were some commonalities in the way the experts would operationalize that standard, there was a significant amount of variability as well. For the greatest number, "reasonable certainty" means a 90 percent chance or greater that the claim is true. But for other experts, reasonable certainty could be found at the level of 50 percent or even lower. The lead author, neurosurgery and pediatrics professor Dr. Mark S. Dias, commented in ScienceDaily that "The juries think that everybody's testifying to the same degree of certainty, and that may not be true." He continues, "Knowing that one expert defines their degree of certainty as 98 percent and the other defines it as 50 percent would help the jury." In this post, I'll take a look at the study and share some ideas on how it can be applied to your preparation of your own expert as well as your cross of the opposing expert.

The Research Findings:

Three main conclusions standout from the study (Dias et al., 2015) on experts' understanding of "reasonable certainty" in practice.

A Lack of Comfort With "Reasonable Certainty"

Just 37 percent of survey respondents were comfortable with their own definition of reasonable medical certainty. That is understandable because this notion of there being a specific threshold for certainty is a legal concept, not a medical or a scientific one. Science and medicine tend to be recursive practices, relying on falsifiability: An explanation stands until it is overturned by new research. It isn't simply treated as true once it reaches a specific threshold of certainty.

One Shared Reference Point on What It Means

About half of the survey respondents felt that "reasonable certainty" boils down to a probability greater or equal to 90 percent. Of course, there is nothing magic about 90, compared to 80 or 95, except that it appeals to the common sense yardstick of "nine out of ten."

But a Lot of Variety in Other Individual Meanings

If about half say 90 percent or more, that means that about half say it means something else. Surprisingly, almost a third said that "reasonable certainty" means just "at least 50 percent." Two percent used an even lower threshold. In addition, the researchers asked whether the "reasonably certain" diagnosis must also be the "most likely" possibility. The vast majority, 95 percent, said that it did, but 10 experts in the sample disagreed. More than a quarter of respondents also shifted their definitions depending on the situation - e.g., a different standard for criminal versus civil or family court.

The Recommendations:

Given the variability of definitions, the question of what "reasonable certainty" means is one that should factor into your preparation for expert testimony.

Discuss the Problem With Your Expert

Ask your experts what their own standard is for "reasonable certainty," and how they would answer the survey questions. Expect that the legal standard is unlikely to match the medical or scientific standard they use on a day-to-day basis. Doctors, for example, operate based on probabilities rather than reasonable certainties. But the law will still require those magic words. In addition to being potentially unfamiliar, the percentage threshold might also be a little arbitrary. After all, in most cases the conclusion isn't something that could be run through a simulator 100 times in order to see if it turns out correctly 90 of those times. At the same time, jurors will want a rule of thumb, and 90 percent is a meaningful number that at least seems to enjoy a narrow preference among experts. In addition to reporting the number, however, experts should also address the broader question likely to be on jurors' minds: How do I know I can trust this conclusion?

Expose the Problem With Their Expert

Given that more than a third of the surveyed experts expressed a discomfort with the concept of "reasonable certainty," it is certainly worth probing the issue with the experts on the other side. You might find an expert who really hasn't thought about it, who waffles, who is reluctant to put a number to their own concept of reasonable certainty, or who is inconsistent in defining the concept. The lack of certainty could undercut that expert's credibility and reveal to the jury that there is no clear standard of what "reasonable certainty" means for this expert. As long as your expert has thought it through and has a better answer, it may be a useful approach to try with the opposing expert.

We see it all the time in mock trials and real trials: Jurors distrust the advocates and the parties, and crave a neutral voice. The expert witness doesn't quite fit that mold because jurors understand that the expert is hired and paid by one side or the other. At the same time, the expert's methods should provide that air of neutrality, and part of that method is the standard of certainty that they're applying. So it is worth it to ask.