“Reasonable Degree Of Certainty”: Understanding What The Phrase Entails


When an expert testifies in open court and states their opinion on the record, it is oftentimes a climactic moment. As seen in courtrooms throughout the country (and as portrayed in innumerable courtroom television and film dramas), the expert usually phrases their opinion in the form of reaching a “reasonable degree of certainty.” Sometimes the standard is stated specifically to the expert’s own practice area, i.e., a reasonable degree of medical or scientific certainty. But experts across all practice areas, not just those of the sciences, use this term. Despite the ubiquity of this often invoked phrase, there is no concrete definition as to what “reasonable degree of certainty” constitutes. The standard can be as fluid and varied as the fields of expertise and opinions it is used to describe. But in light of its general acceptance in the courtroom, and the likely expectations on behalf of the jury to hear an opinion crafted this way, it is important to understand the extent of what a “reasonable degree of certainty” can mean.

Reasonable Degree of Certainty: A History on the Term

The term, “reasonable degree of certainty” was first recorded in a 1935 case, Herbst v. Levy, 279 Ill. App. 353, 358 (Ill. App. Ct. 1935), during which a witness was asked “whether he could determine with reasonable scientific certainty” the cause of a boat capsizing. Interestingly, the term was not devised by the Court but rather, by the lawyer. The phrase was considered a stylistic approach to the examination and was not adopted as an actual standard of admissibility until 1969 in Twin City Plaza, Inc. v. Central Surety & Ins. Corp., 409 F.2d 1195, 1203 (8th Cir. 1969) when the Court held that: “If the witness, based upon his background skill, possesses extraordinary training to aid laymen in determining facts and if he bases his answer upon what he believes to be reasonable scientific or engineering certainty, generally the evidence should be admitted, subject, of course, to the cross-examination of the adversary.”

In federal courts (and state courts that adopt the federal rules), the term has never been defined. The Federals Rules of Evidence governing the admissibility of expert testimony (Rules 702 through 705) are silent as to the definition. Likewise, the seminal case Daubert v. Merrell Dow Pharmaceuticals, Inc. 509 U.S. 579 (1993), which enumerates the factors a court should consider when admitting expert testimony, along with its progeny, also fails to address what exactly “reasonable degree of certainty” means. For example, in United States v. Moman, 413 F.3d 372, 381 (3d Cir. 2005), the Circuit stated that it “has been unable to find any authority to support the position that questions regarding the expert’s ‘degree of scientific certainty’ categorically renders expert testimony inadmissible.”

Although the definition of “reasonable degree of certainty” is implied under both Daubert and Frye standards of admissibility, there is no guidance as to how the terminology should be used when stating an opinion. Unlike the more legally defined standards of “beyond a reasonable doubt,” “by a preponderance of the evidence,” or “by clear convincing evidence,” courts have shied away from giving “reasonable degree of certainty” a defined parameter or numerical probability.

Reasonable Degree of Certainty in the Medical Context

The ambiguity of the standard has led to some inconsistency in its application among different types of experts. Among medical experts, the term “reasonable degree of medical certainty” has garnered different opinions. In a 2015 study conducted at Penn State College of Medicine, 294 physicians who provide expert testimony in child abuse cases were questioned about the term. Although 95% of these experts testified in court, only 37% were comfortable with defining the term “reasonable degree of medical certainty.” In addition, only 45% of them had received any training in the definition of the standard. The study showed a 95% consistency among the experts in opinion, but found a high degree of variability when defining the term. Most of those surveyed defined “reasonable certainty” as a 90% or greater chance that the claim is true.

The study noted that “there is nothing magical about 90, compared to 80 or 95, except that it appeals to the common sense yardstick of ‘nine out of ten.’” Other medical experts, however, found that reasonable certainty could be reached at a chance of 50% or lower. The majority of those surveyed stated the standard could be defined as “most likely,” though ten experts in the sample disagreed with this contention. About a quarter of the medical experts changed their definition depending on the venue, finding different standards should be used in criminal, civil, and family courts. The researchers disagreed with this interpretation.

Dr. Mark S. Dias, professor of neurosurgery and pediatrics explained: “If you’re testifying to a reasonable degree of medical certainty, it doesn’t matter what court you’re in or what the charges are…The court requires you to use the same standard, although it won’t define it for you.” Dias recognized the likelihood for confusion when experts are testifying to different thresholds of certainty yet using the same “reasonable degree of medical certainty” language. “The juries think that everybody’s testifying to the same degree of certainty, and that may not be true,” Dias reasoned. “Jurors are listening to the expert witnesses and they’re being told two different things by two different experts…The jury then has to decide which of these experts is more believable.

Knowing that one expert defines their degree of certainty as 98% and the other defines it as 50% would help the jury.” The study concluded that medical experts should more definitively state their threshold of reasonable medical certainty when they testify before a jury. The researchers are now conducting a study to determine how judges and attorneys can more accurately and uniformly apply a reasonable degree of medical certainty standard when trying child abuse cases.

The Future of the Standard

Over the years, experts have voiced their concern over the standard. As stated in the authoritative treatise, “The New Wigmore: A Treaty on Evidence,” the standard of expert admissibility only requires that the opinion be reasonable and deduced from evidence. Another scholar on the subject found that:

The term “reasonable medical certainty” has no scientific meaning. Its legal meaning is at best ambiguous, at worst misleading. It is not required by the Federal Rules of Evidence, nor any other evidence code. More importantly, the term (“scientific certainty”) is problematic for a different reason–misleading the jury, and should be excluded under Federal Rule 403 for that reason alone.

See Paul Gianelli, Scientific Evidence “Reasonable Scientific Certainty”: A Phrase in Search of a Meaning, Crim. Just., Spring 2010, at 40, 41.

Another issue is that there is no common definition across scientific fields or disciplines as to what the term means. Thus, a reasonable degree of medical certainty will not be the same as a reasonable degree of certainty in other areas, especially those of the less “hard science” variety. Some experts, such as those within the National Commission on Forensic Science, suggest completely abandoning the term for more appropriate language that represents the likelihood of the conclusion, opposed to the expert’s personal confidence in his opinion.

While we are nowhere near describing “reasonable degree of certainty” to the extent that other legal standards, such as “beyond a reasonable doubt” have been defined, it is important that the usage and meaning behind this term be fully explored before presenting it to a jury.

About The Author

Anjelica Cappellino, J.D. is an accomplished defense attorney and legal writer who has represented numerous federal criminal defendants in the Southern and Eastern Districts of New York.

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