Unreasonable Certainty: A Call To Abandon “Reasonable Degree of Scientific Testimony” Terminology

The prevailing practice in many jurisdictions, usually compelled by custom rather than law, is to ask a testifying expert whether the opinion proffered or the conclusion drawn is held “to a reasonable degree of scientific certainty.” Yet scientists do not proclaim certainty in their domains; instead they acknowledge and embrace scientific knowledge as an area of change and evolution. And any attempt to define the term fails – how certain is “reasonable” certainty, and how is that measured except as a subjective appraisal rather than a uniform measure within a discipline?

The origins of the term also confirm its inutility and inappropriateness in most if not all circumstances. The history of this legal term – and its beginnings with medical experts – was traced in Lewin, The Genesis and Evolution of Legal Uncertainty About “Reasonable Medical Certainty,”, 57 Md. L. Rev. 380 (1988).

The emergence of this terminology came from early precedent in regard to expert testimony predicting future consequences 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. Lewin, at 410. Without judicial or scientific/medical analysis, the phrase then began to be used in testimony that was retrospective to address causation. Lewin, 421-424.

The persistence of this term breeds the likelihood of confusion in at least two regards – an over-inflation 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 these problems and others in mind, and because the term is not required in most jurisdictions, the National Commission of Forensic Science approved a “views document” at its March, 2016 meeting calling for the term to no longer be used. [This writer is a member of the Commission and was one of the principal authors of the document.]

The rest of this column is that document [excerpted]:

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.”…

Forensics experts are often required to testify that the opinions or facts stated are offered “to a reasonable scientific certainty” or to 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. Thus, in prescribing a different standard for admissibility in Daubert v. Merrell Dow Pharmaceuticals., 509 U.S. 579, 590 (1993), the Supreme Court acknowledged that “it would be unreasonable to conclude that the subject of scientific testimony must be ‘known’ to a certainty; arguably, there are no certainties in science.” In the courtroom setting, the phrase’s reference to “certainty” risks misleading or confusing the factfinder.

Moreover, the phrase in its varying forms, is not formally defined in standard medical or scientific reference materials. 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.

I – The Historic Background to Use of the “Reasonable Degree of Certainty” Terminology

As best as can be ascertained, the “reasonable degree of certainty” formulation was first applied to scientific evidence in 1935, when a witness was “asked whether he could determine with reasonable scientific certainty the cause of the capsizing of the boat.” Herbst v. Levy, 279 Ill. App. 353, 358 (Ill. App. Ct. 1935).

This was not the mandate of the court but a stylistic approach adopted by a lawyer. 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).

This statement was made without legal or scientific analysis as to what the term meant or why its use was being mandated.

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,’”).

The same is true in federal courts–neither the Daubert trilogy of cases [Daubert v. Merrell Dow Pharmaceuticals, Joiner v. General Electric, or Carmichael v. Kumho Tire] nor Federal Rules of Evidence 702-705 require such language. As well, both the Daubert and Frye tests, when properly implemented, serve to screen out speculative testimony and thus further demonstrate the lack of need for the “reasonable degree of certainty” language.

II – The Problems Arising from this Terminology

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?

III – Toward a Meaningful Alternative (or Alternatives):

The Commission recognizes that recommending the abandonment of a long-used phrase is a first step and an incomplete one.

Additional work is needed in both the scientific and legal communities to identify appropriate language that may be used by experts to express conclusions and opinions to the trier of fact based on observations of evidence and data derived from evidence. 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.