Testimony Using the Term “Reasonable Scientific Certainty”

NATIONAL COMMISSION ON

FORENSIC SCIENCE

Testimony Using the Term “Reasonable Scientific

Certainty”

Subcommittee

Reporting and Testimony

Type of Work Product

Views Document

Statement of the Issue

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.” The legal

community should recognize that medical professionals and other scientists do not routinely use

“to a reasonable scientific certainty” when expressing conclusions outside of the courts. Such

terms have no scientific meaning and may mislead factfinders [jurors or judges] when deciding

whether guilt has been proved beyond a reasonable doubt. Forensic science service providers

should not endorse or promote the use of this terminology. The Commission recognizes the right

of each jurisdiction to determine admissibility standards but expresses this view as part of its

mandate to “develop proposed guidance concerning the intersection of forensic science and the

courtroom.”

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.” Outside of

the courts, this phrasing is not routinely used in scientific disciplines, a point acknowledged in

the Daubert decision (“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.”).

Daubert v. Merrell Dow Pharms., 509 U.S. 579, 590 (1993). Moreover, the terminology, 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. In the courtroom setting, the phrase’s use of “certainty” risks misleading or

confusing the factfinder.

I – Putting the Issue in Context

The NAS Report Strengthening Forensic Science–A Path Forward, explained

that the existing legal regime—including the rules governing the admissibility of

forensic evidence, the applicable standards governing appellate review of trial

court decisions, the limitations of the adversary process, and judges and lawyers

who often lack the scientific expertise necessary to comprehend and evaluate

forensic evidence—is inadequate to the task of curing the documented ills of the

forensic science disciplines. This matters a great deal, because “forensic science is

but the handmaiden of the legal system.”

NAS Report, 85 (citation omitted). Although not addressing the specific issue of using terms

such as “reasonable degree of scientific certainty,” the Report concludes its discussion of

“Forensic Science Evidence in Litigation” with the following observation pertinent to this topic:

“the adversarial process relating to the admission and exclusion of scientific evidence is not

suited to the task of finding ‘scientific truth.’” Id., 110.

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

The requirement that an expert testify to a “degree of certainty” emerged in the context of

medical testimony, when witnesses in civil cases were asked about the potential consequences of

an injury or illness. In predicting future events, courts wanted to avoid speculation, and required

that the testifying doctor express such an opinion with some degree of certitude. By contrast,

forensic discipline evidence is utilized to explain the past and needs a different vocabulary to

show its relevance and probativeness.

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).

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.

III – Emerging Criticism of the Terminology

Both academic and policy writing have addressed the lack of a requirement for, and the problems

arising from use of, the term “reasonable degree of [] certainty.” For example, The New

Wigmore: A Treatise on Evidence, explains in its 2015 supplement that there is no requirement

that such terminology be used and that the standard for admissibility only requires that the

expert’s opinion be a reasonable one, deduced from the evidence. Kaye, Bernstein, and

Mnookin, The New Wigmore: Expert Evidence, §1.5.2(c), 2d Edition, 2011 New York: Aspen

Pub. Co., 2015 Cumulative Supplement (in press). As another scholar concluded,

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.

Paul Gianelli, Scientific Evidence “Reasonable Scientific Certainty”: A Phrase in Search of a

Meaning, Crim. Just., Spring 2010, at 40, 41.

Criticism has also emerged in judicial decisions. In more than one instance in recent years, a

court has precluded all mention of “reasonable degree of certainty” in a forensic evidence case.

“The Court therefore concluded that to allow Detective Valenti, or any other ballistics examiner,

to testify that he had matched a bullet or casing to a particular gun ‘to a reasonable degree of

ballistic certainty’ would seriously mislead the jury as to the nature of the expertise involved.”

United States v. Glynn, 578 F. Supp. 2d 567, 574 (S.D.N.Y. 2008). In Glynn, the expert was

limited to testifying “in terms of ‘more likely than not,’ but nothing more.”

The concern over this terminology has been recognized in the forensics community as well. The

2012 report Latent Print Examination and Human Factors: Improving the Practice Through a

Systems Approach: The Report of the Expert Working Group on Human Factors in Latent Print

Analysis emphasizes that:

Outside the courtroom, however, scientists do not communicate their findings in this

fashion. An astronomer who reports the discovery of an exoplanet does not characterize

the finding as satisfying some “reasonable degree of scientific certainty.” A chemist who

deduces the identity of a compound from its nuclear magnetic resonance spectrum has no

table of degrees of scientific certainty with which to label the deduction. Scientists might

refer to personal degrees of confidence in a finding or to the degree of controversy

surrounding it, but there is no generally accepted or working definition of a “reasonable

degree of certainty” in scientific discourse.

LATENT PRINT EXAMINATION, 119.

IV – The Problems Arising from this Terminology

The phrase “reasonable degree of scientific certainty,” which combines two words of concern—

“scientific” and “certainty”—has no scientific meaning. One scholar summed it up this way:

The reasonable-degree-of-scientific-certainty language almost certainly was drafted by

the lawyers. Scientists have no use for this phrase (outside the courtroom). Indeed, “a

reasonable degree of scientific certainty” is not a defined concept in scientific disciplines

or even in law. . . . It is legal mumbo jumbo derived from archaic cases in which lawyers

discovered that if a medical doctor did not utter the incantation “to a reasonable degree of

medical certainty,” his testimony might be excluded because doctors were not supposed

to talk about mere probabilities. Modern cases usually recognize that suitably explained

information about less-than-certain possibilities can be helpful in various circumstances,

but experts want to (or are induced to) incant not only “medical certainty” but also

“clinical certainty,” “psychological certainty,” “psychiatric certainty,” “engineering

certainty,” “architectural certainty,” “ballistic certainty,” “professional certainty,” and

even “forensic certainty” and “legal certainty.”

David H. Kaye, The Double Helix and the Law of Evidence 82 (2010).

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.

o A juxtaposition to the term “beyond a reasonable doubt” is appropriate here.

Although not precisely quantifiable, it can be measured by comparison (it is

greater than both the “preponderance” standard of 50.1% and the much higher

“clear and convincing” standard) and has a definition as being convinced beyond

having a doubt of the size that would cause a reasonable person to pause or refrain

from acting in a matter of high importance.

 Use of the term “scientific” implies that the discipline is indeed a science, which is not

true of all disciplines.

 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 susceptibility of the term to varying definitions is illustrated in Burke v. Town of Walpole,

405 F.3d 66 (1st Cir. 2005), a bite mark identification case. The U.S. Court of Appeals for the

First Circuit had to interpret the term as used in an arrest warrant:

[W]e must assume that the magistrate who issued the arrest warrant assigned no more

than the commonly accepted meaning among lawyers and judges to the term “reasonable

degree of scientific certainty”—“a standard requiring a showing that the injury was more

likely than not caused by a particular stimulus, based on the general consensus of

recognized [scientific] thought.” Black’s Law Dictionary 1294 (8th ed. 2004) (defining

“reasonable medical probability,” or “reasonable medical certainty,” as used in tort

actions). That standard, of course, is fully consistent with the probable cause standard.

Id. at 91. The case involved a magistrate, not a jury, and it seems doubtful that a jury would

understand that the term “reasonable scientific certainty” meant only “more probable than not”

—that is, any probability greater than 50%. It is more likely that the jury would understand the

term to mean 95% certain or perhaps “beyond a reasonable doubt.”

V. 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. The expert’s subjective certainty may be an appropriate concern in a

particular case and is a factor distinct from the certainty or likelihood of the particular

conclusion.

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 (e.g., the underlying information, studies, observations)

and the limitations relating to the results of the examination.

Developing such a framework and language may best be undertaken through the OSAC structure

or this Commission. As a first step, however, the term “reasonable degree of scientific [or

discipline] certainty” has no place in the judicial process.