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Why Do Attorneys Ask Expert Witnesses for an Opinion “to a Reasonable Degree of Certainty”?

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written by NITA guest bloggers Iva Čechráková and Michael J. Dale

“Expert, do you have an opinion that you can state with a reasonable degree of certainty?” These are most often the exact words attorneys employ when examining expert witnesses. This also is the form of question the majority of trial advocacy text authors recommend as the appropriate or traditional question when examining an expert witness, but not all of them provide a reason why. Oddly, most authors do not explain why.

The complexity of modern society and the resulting expansion in scientific and other sources of technical knowledge increase the likelihood that expert witnesses will testify when disputes are litigated. When an issue is beyond the knowledge of a lay witness, an expert must be called to testify. The expert’s area of knowledge, governed by the Federal Rules of Evidence, may be based upon education, specialized training, and/or experience[1]. Thus, given this background, the expert can help the trier of fact understand the evidence and determine a fact in issue. Under Federal Rule of Evidence 702, when a qualified expert testifies, the testimony may be in the form of opinion or otherwise if the testimony will be helpful.

Expert witnesses occupy a special place at trial and, thus, advocates must bring all of their skills to the task of properly questioning them. Simply asking the expert whether she has an opinion might arguably be weak advocacy and less persuasive than the traditional method. Many people have opinions. But, many of their opinions may not carry much weight. One author states that “expert opinions are the primary―perhaps the only―reason for calling the expert to testify [. . .] and, thus, their disclosure should be the most dramatic and most heightened portion of the direct[2].” Therefore, the advocate should attract the attention of the jury and “choose words that will unequivocally signal that an important answer is forthcoming[3].”

Therefore, the attorney might use stronger, more compelling phrases to persuade jurors and the judge whenever possible. The commonly used language “to a reasonable degree of scientific certainty” most likely represents such language based on the frequency of its use. What is unclear about this phrase is its meaning and the real legal or evidentiary reason why the advocates phrase the question in this specific way. This article attempts to answer this question.

Some authors describe it as a traditional formula[4]. But what is the source of the current formulation? The early common law required that the expert could provide her opinion only if she was able to say the magical words of the traditional formula[5]. Only such an opinion was admissible. The Federal Rules of Evidence do not adopt that view, and many jurisdictions no longer require it. Many jurisdictions now permit experts to testify to possibilities[6]. However, one might conclude that the traditional formula represents the desired stronger and more persuasive statement because the only requirement once relevance is established is that the expert, like other witnesses, may not speculate or guess[7].

Other authors conclude that this phrase adds nothing of legal significance and is not required by the rules of evidence or case law[8]. Thus, unless the judge requires the traditional formula, some suggest that it is best to avoid it[9]. However, these conclusions do not answer the question of whether there is a jurisprudential and evidentiary reason for its use.

The phrase originated in Chicago prior to 1930 as a unique local usage, but the precise origin cannot be determined[10]. It appears that the phrase was created by the efforts of Illinois attorneys to accommodate two inconsistent rules of evidence that were adopted almost at the same time by the Illinois Supreme Court: the Reasonable Certainty Rule and the Ultimate Issue Rule[11]. The Reasonable Certainty Rule prohibited experts from expressing speculative opinions about damages, and it first acted as a rule of substantive proof to establish future damages. Also, it was interpreted to apply to the admissibility of expert medical opinion when testifying about future illnesses or conditions[12]. On the other hand, the Ultimate Issue Rule prohibited experts from usurping the province of the jury by expressing definitive opinions on the ultimate fact[13]. For these reasons, experts were required to express an opinion with a “reasonable certainty,” but were prohibited from expressing an opinion with a degree of certainty that exceeded the hypothetical[14]. The analysis of Illinois cases is consistent with the hypothesis that the phrase originated through the efforts of attorneys to offset the speculative quality of opinions[15]. The attorneys in Chicago began using the phrase frequently, and Irving Goldstein incorporated it in his 1935 manual, Trial Technique[16]. The phrase quickly found its way into nearly every American jurisdiction without regard to its peculiar local origin[17].

Some jurisdictions incorporated the phrase into legal doctrine and attributed legal significance to the phrase, viewing it as required for purposes of admissibility or as a standard of proof[18]. It “resulted from the judiciary’s uncritical acceptance of attorney’s usage without conscious consideration of its meaning[19].”

Even though this phrase has achieved a certain status, it appears that the phrase lacks a definite and ascertainable meaning. The phrase apparently has different meanings in different jurisdictions and in different contexts, generating substantial confusion among experts, legal professionals, and courts. Some experts suggest that the phrase is a legal term of art meaning “more probable than not”—or a 51 percent probability[20]. Others assume that it means something more than a probability—almost a near absolute certainty[21]. Legal professionals use this phrase, but they have not achieved any consensus as to the meaning of the phrase. Some courts interpret it as a “substantive comment about the likelihood that a proposition is true[22].” A majority of courts equate it with a preponderance of the evidence standard[23]. Courts in some jurisdictions interpret it as more demanding, perhaps even approaching the beyond a reasonable doubt standard[24]. One hint of a solution to the evidentiary standard inconsistency could be the recent decision in United States v. Snider[25], where a federal district court in Oregon noted that the court did not ask the experts whether they could express their opinions to a “reasonable degree of medical certainty,” as the question is usually posed in civil cases as a reference to the preponderance standard because in criminal cases this requirement can create confusion about the burden of proof[26].

This confusion results from the incoherence of the phrase itself. The noun “certainty” implies it is the absolute, while the adjective “reasonable” essentially negates the absolute implication of “certainty[27].” While texts on trial advocacy technique reflect the extensive use of the phrase, neither the texts nor the early reported decisions suggest that the phrase has any legal significance. In none of the opinions did the court indicate that this particular phrase was essential or even helpful[28].

In the 1969 case of Boose v. Digate, an Illinois court declared that an expression of reasonable degree of certainty does not relate to substantive certainty about the likelihood of the future condition, but rather to “the general consensus of recognized medical thought and opinion concerning the probabilities” The phrase simply represented an appropriate locution for introducing substantial evidence. The only reasonable explanation for the spread of this phrase are trial advocacy texts and, most specifically, the Goldstein text, Trial Technique[31]. Prior to its publication, the phrase did not appear in a single opinion outside of Illinois[32]. However, the author did not explain that this phrase represented local usage in Illinois courts. Thus, it is understandable that attorneys throughout the United States, without knowledge of this local custom, would assume that the phrase was essential and required.

Interestingly, based on the National Commission on Forensic Science’s recommendations regarding the Testimony Using the Term “Reasonable Scientific Certainty,” it is certain that the phrase was not initiated by physicians[33]. In its recommendations, the Commission stated that “medical professionals and other scientists do not routinely express opinions or conclusions ‘to a reasonable scientific certainty’ outside of the courts[34].” “Such terms have no scientific meaning and may mislead factfinders when deciding whether guilt has been proved beyond a reasonable doubt[35].” Significantly, the Commission commented that there is no common definition as to what threshold establishes certainty[36].

While use of the phrase has become custom and practice, the modern view recognizes that the term is not required. In People v. Ramirez[37], the Colorado Supreme Court found the phrase outdated and inappropriate for determining the admissibility of expert testimony under the current standard codified in Colorado Rules of Evidence[38]. Thus, the court overruled the cases to the extent they approved this standard[39]. A recent decision in West Virginia, Edwards v. McElliotts Trucking, LLC[40], clarifies that the use of the phrase “by experts in court contributes to false sense of confidence in expert opinion and sows confusion about the ‘objectivity’ of the expert’s process[41].” The court also held that “because the phrase has no relation to scientific inquiry and no stable meaning in the law, resort to it only serves to obscure the reliability of the expert’s opinion[42].” Furthermore, the United States Department of Justice has recommended that its attorneys refrain from its use in court[43].

Nevertheless, courts continue to require some level of certitude by experts. Thus, experts can express opinions that a fact was “more likely than not” or “probably” true, but they cannot base their opinions on a mere “possibility” or “speculation” that a fact “might” or “could” be true[44]. Still, courts in several jurisdictions refer to “reasonable medical certainty or probability” without clarifying whether the terms were meant to be synonymous or to represent distinct alternative criteria[45]. On the other hand, courts in most recent cases have held that the experts are not required to phrase their opinion by using the traditional formula and may convey their testimony in a language, which sufficiently shows and persuades the judge that she reached her own best judgment to a reasonable certainty even if not using these exact words[46].

In conclusion, there is not a consistent definition of what these words mean, and, thus, it does not follow that experts should be required to recognize and express their opinions in that way. There is no clear answer provided by case law or secondary sources that demonstrate whether this phrase equates to “more probable than not standard” or “beyond a reasonable doubt standard.” In addition, the United States District Court for the Southern District of West Virginia stated in the footnote to Edwards v. McElliotts Trucking LLC[47] that “the origin story of the phrase reveals its arbitrary birth, unthinking adoption, and most importantly, its vacuity of meaning. Accordingly, it has no place in the admissibility analysis[48].”

Therefore, if lawyers are to consider continuing to use the phrase “to a reasonable degree of certainty,” they should use it because it is more persuasive, the judge expects it, or it is local practice, but not because there is some jurisprudential basis for its use.

Iva Čechráková is a graduate in law from Charles University, Prague, Czech Republic, and a dual degree graduate of Nova Southeastern University College of Law, where she was until recently a research assistant to Professor Michael J. Dale. She can be reached at

Michael J. Dale has been a member of the faculty at Nova Southeastern University Shepard Broad College of Law since 1985. He teaches regularly for the National Institute for Trial Advocacy. He can be reached at If you’d like to know more Professor Dale, please read his “Asked and Answered” interview with The Legal Advocate here.

To read more articles by Professor Dale’s research assistants at Nova Southeastern, click here.

[1] Fed. R. Evid. 702.
[3] Id.
[4] RONALD L. CARLSON & EDWARD J. IMWINKELRIED, DYNAMICS OF TRIAL PRACTICE: PROBLEMS AND MATERIALS 342 (4th ed. 2010); see also DAVID D. MALONE & PAUL J. ZWIER, EFFECTIVE EXPERT TESTIMONY 91 (3rd ed. 2014) (the authors indicate that the counsel should make the presentation of the expert’s opinion dramatic, ask in formal language to call the juror’s attention to it and to assure the judge that the counsel followed the legal formalities because the formal language gives it weight and the terminology “reasonable degree of certainty” is required in almost identical language in many jurisdictions). [Note: However, the authors do not mention any specific rule, case law or the reason for this use.]
[5] CARLSON & IMWINKELRIED, supra note 4, at 342; see also State v. Holt, 246 N.E.2d 365 (Ohio 1969) (holding that “. . . where in answer to question whether, as result of the analysis, he had an opinion ‘based on reasonable scientific certainty as to the’ [. . .], his response was that ‘the samples are similar and are likely to be from the same source,’ such response fell short of the legal requirement of ‘reasonable certainty’ or ‘probability’, so that admission in evidence of such answer over objection constituted prejudicial error.”) [Note: State v. Holt was later negatively impacted by State v. Brenner, 533 N.E.2d 701 (Ohio 1988).]
[6] Dupree v. County of Cook, 677 N.E.2d 1303 (1997); United States v. Davis, 44 M.J. 13, 16 (C.A.A.F. 1996). However, the opinion can be vulnerable to effective cross-examination because the advocate can compel the expert to admit that the opinion is a mere possibility, not a probability or a certainty and may convince the trier of the fact that it is a speculative opinion and the other party does not met its burden of proof.
[7] CARLSON & IMWINKELRIED, supra note 4, at 342.
[8] THOMAS A. MAUET, TRIAL, STRATEGY, SKILLS, AND THE NEW POWER OF PERSUASION 410 (2nd ed. 2009); see also STEVEN LUBET & J.C. LORE, MODERN TRIAL ADVOCACY: ANALYSIS AND PRACTICE 236-37 (5th ed. 2015) (the authors are not expressly saying that the traditional formula is a proper way to ask an expert about her opinion but call for testimony in straightforward, unequivocal terms that emphasizes accuracy and certainty in order to prevent miscommunication and loss of expert’s credibility); see also STEVEN LUBET, EXPERT TESTIMONY: A GUIDE FOR EXPERT WITNESSES AND THE LAWYERS WHO EXAMINE THEM 8 (1998) (the author in the earlier edition of the text emphasizes that in most jurisdictions an expert’s opinion must be stated to a “reasonable degree of certainty” even though he admits that the term is difficult to define may be ambiguous or an oxymoron, and thus, he asks a variety of questions such as, “What is a reasonable degree of certainty?” or “How do you know when you have it?” and answers that most likely the best approach is self-referential; however, he also warns that “reasonable certainty” is merely the minimum level of confidence required for the expression of an opinion in court).
[9] MAUET, supra note 8, at 410.
[10] Jeff L. Lewin, The Genesis and Evolution of Legal Uncertainty About “Reasonable Medical Certainty,” 57 MD. L. REV. 380, 381 (1998).
[11] Id. at 406–30.
[12] Id.
[13] Id.
[14] Id. at 410–14.
[15] Philadelphia & Reading Coal & Iron Co. v. Industrial Comm’n, 165 N.E. 161, 162 (Ill. 1929); Armour Grain Co. v. Industrial Comm’n, 153 N.E. 699, 701 (Ill. 1926); Benton Coal Mining Co. v. Industrial Comm’n, 151 N.E. 520, 525 (Ill. 1926) (these three cases represent worker’s compensation cases); Herbst v. Levy, 279 Ill. App. 353, 358 (App. Ct. 1935) (this case evidences that this phrase was used not only in worker’s compensation claims but also in different practices―in this case, the expert witness was asked “whether he could determine with reasonable scientific certainty the cause of the capsizing of the boat”).
[16] Lewin, supra note 10, at 425–54, 456–57.
[17] In Ohio, New York, Arizona, Delaware, Florida, Michigan, Hawaii, California, and Montana, the phrase entered the vocabulary of attorneys with only mild endorsement from the courts. On the other hand, in Washington, Minnesota, Missouri, Wisconsin, Pennsylvania, New Mexico, Tennessee, Oregon, Colorado, and South Carolina, the phrase received a strong endorsement from the courts.
[18] Twin City Plaza, Inc. v. Central Surety & Ins. Corp., 409 F.2d 1195, 1203 (8th Cir. 1969) (holding that if the witness possesses extraordinary training to aid laypersons 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); Penteluk v. Stark, 244 Minn. 337, 340 (Minn. 1955) (the court incorporated the phrase in its description of the standard of proof of permanent injuries, stating that “the rule is that for a person to recover for permanent injuries it must appear to a reasonable medical certainty that there will be permanent injury”). Later Minnesota courts held that it would be also satisfied by testimony expressed in terms of probability.
[19] Lewin, supra note 10, at 396.
[20] Id. at 402; see also Douglas Danner & Elliot L. Sagall, Medicolegal Causation: A Source of Professional Misunderstanding, 3 AM. J.L. & MED. 303, 305 (1977).
[21] State v. Austin, 368 N.E.2d 59, 61, 66–67 (Ohio Ct. App. 1976).
[22] Lewin, supra note 10, at 403–04.
[23] Dallas v. Burlington N., Inc., 689 P.2d 273, 277 (Mont. 1984); Wheeler v. Central Vt. Med. Ctr., Inc., 582 A.2d 165, 170 (Vt. 1990).
[24] Cf. Hashimoto v. Marathon Pipe Line Co., 767 P.2d 158, 165–67 (Wyo. 1989).
[25] 180 F. Supp. 3d 780, 795, 798 (D. Or. 2016), appeal dismissed (Sept. 7, 2016).
[26] Id.
[27]Edwards v. McElliotts Trucking, LLC, 2017 WL 3611848, at *3, 5 (S.D.W. Va. Aug. 22, 2017) (reference to footnote 2); see also Lewin, supra note 10, at 400–01.
[28] Not until 1937 decision in Shell Petroleum Corp. v. Industrial Commission, 10 N.E.2d 352 (Ill. 1937), where the court attached significance to the failure of a witness to state his opinion with reasonable medical certainty. However, the Court did not provide any reason why it was so crucial to express the opinion in this way or explain the meaning of the phrase.
[29] 246 N.E.2d 50, 53 (Ill. App. Ct. 1969).
[30] Id.
[32] Lewin, supra note 10, at 453.
[34] Id.; see also Daubert v. Merrell Dow Pharms., 509 U.S. 579, 590 (1993) (stating 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”); see also Danner & Sagall, supra note 20, at 308.
[35] NAT’L COMM’N ON FORENSIC SCI., supra note 33.
[36] Id.
[37] 155 P.3d 371, 375–76 (Col. 2007).
[38] Id.
[39] Id.
[40] 2017 WL 3611848, at *3 (S.D.W. Va. Aug. 22, 2017).
[42] Edwards, 2017 WL 3611848, at *4.
[43] Id. at *3; see also Seth Augenstein, DOJ’s Code of Conduct: No More ‘Reasonable Scientific Certainty’ (Sept. 2016) available at
[44] Lewin, supra note 10, at 462–63.
[45] Holecek v. Janke, 171 N.W.2d 94, 101 (N.D. 1969); State v. Holt, 246 N.E.2d 365 (Ohio 1969); Lane v. State Farm Mut. Auto. Ins. Co., 308 N.W.2d 503, 512–13 (Neb. 1981).
[46] Marpaka v. Patel, 2017 WL 84655, at *2 (N.J. Super. Ct. App. Div. 2017); Aspiazu v. Orgera, 535 A.2d 338, 342 (Conn. 1987); State v. Jarrell, 608 P.2d 218, 230–31 (Utah 1980).
[47] 2017 WL 3611848, at *4, 5 (S.D.W. Va. Aug. 22, 2017).
[48] Id. (reference to footnote 2).

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