Scientific Evidence

Reasonable Certainty: A Term It Is Certainly Reasonable to Repudiate

By Jules M. Epstein

Virtually every expert witness examination concludes with the question “and do you hold those beliefs/opinions/conclusions to a reasonable degree of [discipline] certainty.” The phrase, like the term “insane,” is a legal construct; but in science and elsewhere the words have no meaning, as science may include a measure of uncertainty, scientific knowledge evolves, and there is no agreed metric within or across disciplines for what degree of certainty—32 percent, 45 percent, 97 percent—is “reasonable.” The standard is subjective to the declarant.

There is more. Some courts use the term as a measure of the probability of a predicted cause or outcome; others use it as a statement of how confident the expert is in the conclusion.

That the phrase is devoid of substance and is instead ritualistic is confirmed by the variety of contexts in which courts have allowed it:

  • “a licensed architect, who opined, to a reasonable degree of architectural certainty. . . .” (Lorenzo v. Millennium Mgmt., 2015-Ohio-2614, ¶ 45 (Ct. App. June 30, 2015).)
  • “Q: And is it your opinion . . . to a reasonable degree of . . . automotive certainty . . .” (Gray v. Ford Motor Co., No. 05-CV-2236, 2011 U.S. Dist. LEXIS 118307, at *30 (C.D. Ill. Oct. 13, 2011)
  • Mr. Tieger also avers that, “within a reasonable degree of Master Plumbing certainty at the time of the scalding incident . . .” (Clindinin v. N.Y.C. Hous. Auth., 2013 NY Slip Op 31718(U), ¶ 12 (Sup. Ct. July 23, 2013)

In a recent iteration, a court noted, without comment, that there was testimony of a “diagnosis within a reasonable degree of social worker probability . . .” (Morris v. T.D. Bank, Docket No. A-2268-16T1, 2018 N.J. Super. LEXIS 58 (N.J. Super. Ct. App. Div., Apr. 10, 2018)).

Ritual has its place, even in the law, but terminology that suggests a scientific metric of validity when none exists deserves no protection. To this end, in 2016, the then-extant National Commission on Forensic Science (NCFS) approved a “views document” calling for the term to no longer be used. [This writer was a member of the Commission and a principal author of the document.]

To understand why this was a recommendation of the Commission, one must appreciate where and how the term emerged and the problems caused by its use. “Reasonable certainty” originated as a requisite for opinion testimony predicting the future, such as the need for medical care in years to come or the likely harm a plaintiff would suffer later in life. “[T]he alleged permanent disability, in order to be a ground for damages, must be one that is reasonably certain to result from the injury complained of.” (Lake Shore & M. S. R. Co. v. Conway, 48 N.E. 483, 484 (Ill. 1897).) The purpose of the language was to avoid speculation or conjecture when deciding monetary damages. (Jeff L. Lewin, The Genesis and Evolution of Legal Uncertainty About “Reasonable Medical Certainty, 57 Md. L. Rev. 380 (1988).)

What began as a standard of assurance regarding predictions—are you guessing the person will never walk again or are you reasonably certain based on other cases you have seen—morphed into one applied retrospectively to address causation or, in the case of forensics, origin.

And the problems the words cause? They breed the likelihood of confusion in at least two regards: an overinflation of the probativeness and reliability of the testimony and the potential conflation of that standard with the burden of proof, particularly in criminal cases.

With this history and these problems in mind, and because the term is not required in most jurisdictions, the now-decommissioned NCFS approved a “views document” at its March 2016 meeting calling for the term to no longer be used. [This writer was a member of the Commission and was one of the principal authors of the document.]

It is worth considering portions of the document:

It is the view of the National Commission on Forensic Science (NCFS) that legal professionals should not require that forensic discipline testimony be admitted conditioned upon the expert witness testifying that a conclusion is held to a “reasonable scientific certainty,” a “reasonable degree of scientific certainty,” or a “reasonable degree of [discipline] certainty.” . . .

. . . Such statements have no scientific meaning and may mislead factfinders when deciding whether guilt has been proved beyond a reasonable doubt. Outside of the courts, this phrasing is not routinely used in scientific disciplines. In the courtroom setting, the phrase’s reference to “certainty” risks misleading or confusing the factfinder.

In the courts, this phrase is almost always a matter of custom, but in some jurisdictions, it results from an appellate court ruling or trial judges’ or lawyers’ belief that it is a necessary precondition for admissibility. Not until 1969 was the terminology linked to the admissibility determination:

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.

(Twin City Plaza, Inc. v. Central Surety & Ins. Corp., 409 F.2d 1195, 1203 (8th Cir. 1969).)

The modern view recognizes that the term is not required as a condition of admitting expert evidence. A review of state court case law, undertaken by the Hawaii Supreme Court in 2014, confirmed this and concluded, for its state, that “trial courts should not require a ‘reasonable degree of scientific certainty’ before admitting expert opinions but may exclude expert testimony based on speculation or possibility.” (State v. DeLeon, 131 Haw. 463, 484 (Haw. 2014); see also State v. Johnson, 290 Neb. 862, 862 N.W.2d 757, 773 (Neb. 2015) (“An expert does not have to couch his or her opinion in the magic words of ‘reasonable certainty’”).)

Multiple problems abound with phrases such as “scientific certainty” or “[discipline] certainty.”

These include the following:

  • There is no common definition across science or within disciplines as to what threshold establishes “reasonable” certainty. Therefore, whether couched as “scientific certainty” or “[discipline] certainty,” the term is idiosyncratic to the witness.
  • Use of the term “scientific” cloaks the opinion with the rigor, acceptance and reproducibility of scientific study.
  • Coupled with the term “reasonable,” a juror might equate it with certainty at the level demanded by the “beyond a reasonable doubt” standard of proof.
  • The term invites confusion when presented with testimony expressed in probabilistic terms. How is a lay person, without either scientific or legal training, to understand an expert’s “reasonable scientific certainty” that evidence is “probably” linked to a particular source?

The Commission recognized that finding a suitable replacement is an involved process. But it proposed a general approach:

Rather than use “reasonable . . . certainty” terminology, experts should make a statement about the examination itself, including an expression of the uncertainty in the measurement or in the data.

The expert should state the bases for that opinion (the underlying information, studies, observations) and its limitations.

(Nat’l Comm’n on Forensic Sci., Testimony Using the Term “Reasonable Scientific Certainty,” (last visited Apr. 13, 2018).)

The Commission’s voice did not go unheeded. In September 2016, the US Department of Justice directed that its laboratories ensure that the phrase is not used in reports or testimony and that “Department prosecutors . . . abstain from use of these expressions . . . unless required by a judge or applicable law.” (Memorandum from the Office of the Att’y Gen. for Heads of Department Components (Sept. 6, 2016).)

The concept still confuses courts. In a January 2018 decision, the Maryland Court of Appeals accepted in part the prosecution argument that relied on the NCFS views document and held that a statement of “reasonable . . . certainty” was not necessary for all scientific conclusions. (Taylor v. State, No. 1174, 2018 Md. App. LEXIS 16, at *28 (Jan. 5, 2018).) In Taylor, the language was omitted when the expert testified to a YSTR profile being “consistent” with that of the suspect/defendant. But the same court seemed to indicate that when a DNA profile identifies a particular person, the “reasonable degree” language is needed. (Id.)

Another court confused the term, linking it to not how certain the expert is but how likely a causal connection is between an injury and behavior. In a capital case, a mitigation witness sought to testify that a brain injury “might, but might not, have impacted [the defendant’s] behavior. . . .” (State v. Schierman, No. 84614-6, 2018 Wash. LEXIS 272, at *125 (Apr. 12, 2018).) The Washington court explained that “expert opinion does not satisfy [a relevance] standard unless it is expressed to a reasonable degree of probability.” (Id.) The court nonetheless allowed the testimony because “the ‘reasonable medical certainty’ standard should not be applied to mitigating expert testimony in a capital sentencing proceeding.” (Id. at *126.)

That confusion notwithstanding, the law remains the same—in most state and federal courts there is no requirement that “reasonable degree” language be used as a predicate for admission. “[T]he magic words ‘to a reasonable degree of certainty’ are not required of all experts. . . . [I]f the expert offers a definitive opinion based on research and expertise, it is likely not excludable as speculation.” (Soto v. Aria Resort & Casino, LLC, No. 2:16-cv-00064-JAD-PAL, 2017 U.S. Dist. LEXIS 135903, at *13 (D. Nev. Aug. 23, 2017) (internal quotation and citation omitted).) Indeed, one judge recently urged that “[n]o matter the context, the phrase has no coherent meaning and should be torn out root and branch from the legal lexicon.” (Edwards v. McElliotts Trucking, LLC, No. 3:16-1879, 2017 U.S. Dist. LEXIS 133803, at *12 n.3 (S.D. W. Va. Aug. 22, 2017).)

What are needed are advocates who can explain to courts why the language misleads and is unnecessary, all as part of a culture of ensuring that scientific or quasi-scientific evidence is not presented in a nonscientific, misleading manner.

Jules M. Epstein

Jules M. Epstein is a professor of law and director of advocacy programs at Temple Beasley School of Law and served as a member of the National Commission on Forensic Science.