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NATIONAL COMMISSION ON

FORENSIC SCIENCE

Testimony using the term “Reasonable Scientific Certainty”

Type of Work Product

Views Document prepared by the Reporting and Testimony Subcommittee

Statement of the Issue

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. 

Moreover, the terminology, in its varying forms, is not formally defined in standard 

medical or scientific reference materials. With respect to its use in the courts, this phrase 

is almost always interjected as a matter of custom, but in some jurisdictions 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 risks misleading or confusing the 

factfinder. 

It is the view of the National Commission on Forensic Science (NCFS) that the scientific 

community should not promote or promulgate the use of this terminology. Additionally, 

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 since there is no foundational scientific basis for its use. Therefore, 

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,” as such terms have no scientific meaning and may mislead 

factfinders [jurors or judges] when deciding whether guilt has been proved beyond a 

reasonable doubt. The Commission recognizes the right of each court 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.”

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. 
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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, 

one 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 of an expert testifying that his/her conclusion is held to a “degree of 

certainty” emerged in the context of medical testimony, when witnesses in civil cases 

were asked about the potential future consequences of an injury or illness. In predicting 

future events, courts wanted to avoid speculation, and as a result required that the 

testifying doctor make a prediction with some degree of certitude.

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.

Use of the term became custom and practice, but in fact only one state – Ohio – has an 

apparent mandate requiring use of this term as a precondition to admissibility. The 

modern view recognizes that the term is not required. The most current review of state 

court caselaw, 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, Joyner, 

or Kumho Tire] nor Federal Rules of Evidence 702-705 require such language. See, e.g., 

United States v. Mornan, 413 F.3d 372, 381 (3d Cir. 2005) (“this Court has been unable 
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to find any authority to support the position that questions regarding the expert's ‘degree 

of scientific certainty’ categorically renders expert testimony inadmissible”). 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.” The 

authoritative Evidence treatise, 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, §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. In 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 this is emphasized:

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 
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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 this phrase, whether “scientific certainty” or “[discipline] 

certainty.” These include the following:

 There is no common definition across science or within disciplines as to what 

threshold establishes certainty. Therefore, whether couched as “scientific 

certainty” or “[discipline] certainty,” the term is idiosyncratic to the witness.

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

While 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 

in some instances may be putting the cart before the horse. As the Supreme 

Judicial Court of Massachusetts explained, “The phrase ‘reasonable degree of 

scientific certainty’ should also be avoided because it suggests that forensic 

ballistics is a science, where it is clearly as much an art as a science.” 

Commonwealth v. Pytou Heang, 458 Mass. 827, 849 (Mass. 2011).
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 Coupled with the term “reasonable,” a juror might equate it with certainty at the 

level of beyond a “reasonable” doubt.

 The term invites confusion when presented with probabilistic evidence. 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 ambiguity of the term is illustrated in Burke v. Town of Walpole, 405 F.3d 66 (1st 

Cir. 2005), a bite mark identification case. 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” — i.e., 51%. It is more likely that the jury would understand the term 

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