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DUI LIBRARY | Daubert & Rule 702 Challenges

Daubert Challenges

The Essential Guide


Expert Witness | Rule 702

A trial court’s decision whether or not to permit expert testimony is critical to the outcome of any criminal case. In this definitive guide, you’ll learn:

  • The essential tactics for creating a successful Daubert (Rule 702) challenge.
  • The relationship between Rule 702 and the holding of Daubert in plain English
  • Step-by-step directions to persuade courts to uphold their gate keeping duty

If this is what your looking for then…


The Wolf, The Sheepskin and the Shepherd

Once upon a time…

There was a Wolf, which had a difficult time catching sheep. He was a poor hunter and did not have much of an appetite for the many plants found in the forest.

As luck would have it, there was a flock of sheep nearby. The flock was large enough to feed him for years to come. However, its Shepherd kept watch over the heard. The Wolf knew he was no match for the Shepherd. So, he stayed away.

One day, while the Shepherd was tending to the flock, the Wolf saw an empty sheepskin on a table next to the Shepherd’s home. He thought to himself “if I put it on, I can get by the Shepherd.” He then slipped into the empty sheepskin, and walked right past the Shepherd. After a pause, he strolled with the flock into the pasture. Dinner was served.

Cloaked by the stolen sheepskin, the Wolf no longer needed to worry about his palette. It was not until many months later that the Shepherd discovered the Wolf had duped him. By this time the Shepherd had lost most of his flock.

The Shepherd never thought to look for a wolf in sheep’s clothing. He was predisposed to trust: a sheep – was a sheep; and a wolf – was a wolf. This mistake cost him almost all of his sheep.

What are the lessons we can learn from this story?

Experts can be Wolves.

Judges must be vigilant Shepherds.

And jurors…can be sheep. Easily eaten by the Wolf.

With Great Privilege Great Scrutiny Must Come

A jury of your peers is, and will continue to be, the bedrock of our justice system. However, even the most solid bedrock is plagued by cracks. The trial judge is charged with guarding against things slipping through those cracks. This includes the dangers of unreliable, but persuasive, expert testimony. 

Amazing Fantasy, # 15 (Marvel)
Spiderman's Debut in Amazing Fantasy #15,
Stan Lee & S. Ditko, (Marvel)

The trial court’s decision to permit expert testimony, or not, is usually critical to the outcome of a case. As one court acknowledged, the decision to admit or excluded an expert “is often the difference between winning and losing a case.” Barabin v. AstenJohnson, Inc., 2012 WL 5669685 (9th Cir. Nov. 16, 2012).

The Purpose of Expert Testimony

The primary purpose of expert testimony is to help a jury with issues beyond the knowledge of the ordinary citizen. Daubert, at 591. (relevance is helping the jury to understand the evidence, testimony, exhibits). This is commonly the situation when scientific evidence is involved. 

For example, it would be unrealistic to expect the average citizen to know the standard of care for a doctor diagnosing an aortic dissection, or the acceptable limits of a toxin in a water supply. Accordingly, experts are permitted provide specialized knowledge that is foreign to the average person.

Special Privileges

To facilitate providing this specialized knowledge, expert witnesses are given special privileges in the courtroom. Unlike the mere lay witness, an expert may give:

(1) Opinions without firsthand knowledge;
(2) An opinion based upon hearsay; and
(3) In civil cases, they can even testify concerning the ultimate issue.

These special privileges often give the expert special influence over the jury. Therefore, before we grant an expert these privileges, it is necessary to ensure that they worthy of them. Just as the Shepherd must guard against the Wolf entering his pasture, Courts must guard against a credentialed expert who will, even unwittingly, misuse their influence.

In the courtroom, this determination is often made using Rule 702 of evidence and a line of cases starting with Daubert v. Merrell Dow Pharms. Inc., 509 U.S. 579 (1993). In Daubert jurisdictions, the trial judge must vet expert testimony before it can be presented to a jury. The so-called “gatekeeper” judge may only allow expert testimony that is the product of a reliable methodology.

The Expert Story We Tell Ourselves

Cognitive science tells us how our mind can distort reality. When you hear that someone has a title of an “expert,” you instantly tell yourself a story. Everyone has mental frames in their mind about every subject. A person with the title of “expert” will invoke several of these frames resulting in telling yourself story about what it means to be an “expert.”

Second Opinions | DUI Lawyer

Your factual and emotional experiences shape your story. Your “expert story” provides a subjective way of thinking about a person with the title expert. Scientists tell us this cognitive process happens on an unconscious level.

For most people, their “expert story” includes the themes of competence and trustworthiness. This is especially true when the expert is part of, or testifying on behalf of, law enforcement in cases with criminal allegations. This is no doubt caused by the preexisting frames4 we all have for someone charged with a crime.

Mystic Infallibility

Even before scientists discovered this process was occurring, courts recognized the inherent dangers of a jury being smitten by a witness with the title of expert.

Adding to the difficulty any lay person has in evaluating a cross-examination of an expert is the fact that technical opinions have an inherent aura about them which, if not properly checked by the judge, can ‘assume a posture of mystic infallibility in the eyes of a jury of laymen…’ United States v. Addison, 498 F.2d 741, 744 (D.C.Cir.1974).

Accordingly, while expert testimony is necessary, so is the need to safeguard it against its misuse. Courts should be bear in mind that there are many situations when it is human nature to trust things that are untrue. A famous non-scientist once observed there are times when we can not help but believe things we should not – “When my love swears that she is made of truth I do believe her, though I know she lies.”

  • “Because ‘science’ is often accepted in our society as synonymous with truth, there was a substantial risk of overweighting by the jury.” State ex rel. Romley v. Fields, 201 Ariz. 321, 325, ¶ 11, 35 P.3d 82, 86 (App. 2001); Morris K. Udall, et al., Arizona Practice—Law of Evidence § 102, at 212 (3d ed. 1991).
  • “…scientific evidence is ‘a source of particular judicial caution.’” State v. Superior Court, 149 Ariz. 269, 276, 718 P.2d 171, 178 (1986).
  • “[s]cientific … evidence has great potential for misleading the jury. The low probative worth can often be concealed in the jargon of some expert….”. See 22 C. Wright & K. Graham, Federal Practice and Procedure § 5217, at 295 (1978).
  • “Furthermore, this is not an area in which the jury can easily penetrate the aura of infallibility, nor one in which the principles are easily demonstrable in the courtroom.” See State v. Bible, 175 Ariz. 549, 580, 858 P.2d 1152, 1183 (1993) (citing 1 McCormick on Evidence § 205, at 897–900).
  • “Scientific evidence impresses lay jurors. They tend to assume it is more accurate and objective than lay testimony. A juror who thinks of scientific evidence visualizes instruments capable of amazingly precise measurement, of findings arrived at by dispassionate scientific tests. In short, in the mind of the typical lay juror, a scientific witness has a special aura of credibility.” Imwinkelried, Evidence Law and Tactics for the Proponents of Scientific Evidence, in Scientific and Expert Evidence 33, 37 (Imwinkelried ed. 1981).

A Magnifying Glass Isn't Helpful From 300 Feet

There are advocates for having all expert testimony dealt with by cross- examination. They would remove the judge as a gatekeeper. This position relies of the erroneous premise that cross-examination is always capable of revealing the Wolf in the expert’s clothes.

Daubert Challanges

In an ideal world, cross-examination can be the “greatest engine of truth.” However, some things are just too heavy for that engine to move. Just as a magnifying glass is a helpful tool when held directly over the fine print of a contract, so is cross-examination of the witness who has changed his story during a trial from a prior statement.

The Fine Print

A magnifying glass is no longer effective if held three hundred feet from the fine print. That situation requires a different tool. The same can be said of the expert witness subjected to a typical one-hour cross-examination during an impaired driving case. Cross-examination is often an inadequate tool to debunk unreliable expert testimony coated in the trappings of science.

Forensic science can accomplish great things for American jurisprudence. However, as history has shown us, it can also do terrible things. The recent report to the United States Congress by the National Academy of Science (NAS) revealed an epidemic of deficiencies in state crime laboratories. The NAS is perhaps the most well respect scientific organization in the world with over 200 Nobel prize winners as members.

Fine Print (Rule 702)

Serious Deficiencies

The 2009 NAS Report found the State crime laboratories system rife with “serious deficiencies,” lacking practitioner and laboratory independence, standards and oversight. Institutional bias often colored their “expert” testimony. The NAS report shows that there truly are Wolves in our crime labs.

Cross-examination can expose this kind of bias, but it’s unlikely. When considering the cognitive expert story we tell ourselves, it is a difficult endeavor. Combine this reality with the complexity of the multiple underlying sciences involved, it rare that a practitioner will have the skill, resources and opportunity to succeed. In sum, it is unrealistic to expect cross-examination be the engine of truth under these circumstances. Once the Wolf has been let past the gate, it is too late for the flock.

What To Do About The Wolf?

Obviously all experts are not Wolves. There are reliable experts both in and out of crime labs. However, we need a system to separate the wolves from the sheep, the experts from the charlatans, science from non-science. Fortunately, courts have already given us a starting point – the Daubert lineage of cases.

While Daubert is one the most important decisions in history, it is also one of the most misunderstood. While Daubert has given us the guiding principles, there has yet to be a widespread application of them to the science of impaired driving allegations. Nowhere does science and law intersect in our courtrooms than here. This disconnect has created a chasm separating the two unfilled by the legal precedent. Let’s start filling this gap.

Arizona's Rule 702 of Evidence

In 2012, Arizona amended its version of Rule 702 of the Rule of Evidence (amended Sept. 8, 2011, effective Jan. 1, 2012).  The change effectively changed the standard for the admissibility of scientific evidence from what is known as a Frye standand to a Daubert standard.

A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if:

(a) the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue;

(b) the testimony is based on sufficient facts or data;

(c) the testimony is the product of reliable principles and methods; and

(d) the expert has reliably applied the principles and methods to the facts of the case.

Ariz. R. Evid. 702.

Key Point:

No single factor is dispositive of the reliability of the expert’s testimony.  Not all factors will apply to all experts or to every case.  

Engstrom v. McCarthy, 243 Ariz. 469, 411 P.3d 653 (App. Div.1 2018).

Who is Qualified

The Rule starts with a requirement that a witness must be “qualified.” Presumably anyone making a blood alcohol measurement in a crime lab will be qualified with the technique they used—most likely gas chromatography. However, as previously stated, if the primary area of science is metrology, then a relevant inquiry should be their expertise in this area. 

qualified as an expert by knowledge, skill (Rule 702)

The issue becomes: what is this witnesses’ level of knowledge, skill, experience, training, or education in the science of measurement? Thus, if they cannot answer basic questions about the requirements of the ISO 17025 standard (or at least the underlying principles contained therein), they should not be permitted to provide opinions under Rule 702.

Gatekeeping Duties

The Daubert standard requires courts to be gatekeepers of scientific evidence. They are required to screen expert evidence to ensure they are the product of scientific “knowledge” and supported by “appropriate validation.” These requirements must be satisfied in order to deem a blood alcohol measurement relevant and reliable.

Rule 702 Judge as Gatekeeper

Sufficient Facts and Data

The next requirement of Rule 702 is that the witness’s testimony is based on sufficient facts or data.

Sufficient Facts and Data (Rule 702)

Recall that a complete measurement includes the uncertainty associated with the reported results. Moreover, identifying, understanding, and resolving prior malfunctions is a prerequisite to relying upon the data produced by a gas chromatograph. Therefore, data from other testing runs is relevant and necessary for a reliability analysis.

Beyond data relating to instrumentation issues, data relating to the traceability of the calibration standards and how the laboratory’s uncertainty budget was calculated are also essential to ensuring reliability.

Reliably Apply Principles and Methods

Rule 702  also requires the testimony is the product of reliable principles and methods. ISO 17025 reflects the generally accepted requirement for producing a reliable measurement. The minimum requirements set forth in the standard are fairly straightforward. However, similar to legal precedent, issues primary arise as to their application. Rule 702(d) requires that an expert must reliably apply methods to the facts. Legal precedent also provides a basis for assessing the reliable application of gas chromatography as applied to a given set of facts.

United States v. Lee, 25 3d. 997 (1994) involved the science and use of gas chromatography, specifically for testing amounts of cocaine. After trial, but prior to the appeal, Daubert was decided and the appellate Court in Lee vacated the trial court’s decision and remanded for reconsideration under a Daubert analysis, stating, “Thus, on its own terms, Daubert applies not only to testimony about scientific concepts but also to testimony about the actual applications of those concepts. (Emphasis added)”

Subsequent to Lee, in 2000, Federal Rule 702 was amended in response to the Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993), and Kumho Tire Co. v. Carmichael, 119 S.Ct. 1167 (1999), holdings. The proponent must satisfy the court that the proffered testimony is both relevant and reliable.  The Supreme Court has interpreted Rule 702 as requiring that the district court act as a “gatekeeper,” ensuring that “any and all scientific testimony or evidence admitted is not only relevant, but reliable.”


The government, as proponent of blood alcohol content (BAC) test results in prosecutions for DUI, had burden of proof under Rule 702.  Their burden is a preponderance of the evidence.

State v. Bernstein, 234 Ariz. 89, 317 P.3d 630, as amended, review granted, vacated in part 237 Ariz. 226, 349 P.3d 200 (App. Div.1 2014).

The Relationship of the Daubert Criteria

A Court’s inquiry in evaluating the admissibility of proffered expert testimony is flexible and fact-specific. A  court should use, adapt, or reject Daubert factors as needed based on facts of the particular case. See Federal Rules Evidence, Rule 702; and In re Prempro Products Liability Litigation, 765 F. Supp. 2d 1113 (W.D. Ark. 2011).

As one Arizona court stated , the Daubert test for determining admissibility of expert testimony offersa list of non-exclusive factors.  Theses include:

  • empirical testing,
  • peer review,
  • error rate,
  • the existence of standards and controls, and
  • the degree to which the theory and technique is generally accepted by a relevant scientific community.

See State v. Romero, 236 Ariz. 451, 341 P.3d 493, (App. Div.2 2014)review continued, review granted in part, vacated in part 239 Ariz. 6, 365 P.3d 358, on remand 240 Ariz. 503, 381 P.3d 297, review denied. 

General Acceptance

In most matters, there is a range of generally accepted opinions.  While not all scientists agree on all things, there are often established boundaries of what is reliable science and what falls outside it.   Things that fall outside of the spectrum of where reasonable minds may differ are often referred to as pseudoscience or junk science.

In the scientific community, opinions that fall within a spectrum of reliable opinions are traditionally subjected to some form of peer review. That is, other members of the scientific community have vetted the assertion. 

The general acceptance criterion requires a trial court to ask: does the opinion being offered fall outside of the spectrum?

The Relevant Scientific Community

The relevant scientific community will vary from case to case and may involve more than one group for different aspects of the same technology. The relevant scientific community must be individuals “other than those who have professional and personal interest in the outcome of the evaluation” Blake, 149 Ariz. at 277, 718 P.2d at 179.


“Finally, ‘general acceptance’ can yet have a bearing on the inquiry. A ‘reliability assessment does not require, although it does permit, explicit identification of a relevant scientific community and an express determination of a particular degree of acceptance within that community.’ United States v. Downing, 753 F.2d, at 1238. See also3 Weinstein & Berger ¶ 702[03], pp. 702–41 to 702–42. Widespread acceptance can be an important factor in ruling particular evidence admissible, and ‘a known technique which has been able to attract only minimal support within the community,’ Downing, 753 F.2d, at 1238, may properly be viewed with skepticism.

Daubert v. Merrell Dow Pharms.,Inc., 509 U.S. 579, 594 (1993) (citations omitted) (quoting United States v. Downing, 753 F.2d 1224, 1238 (3d. Cir. 1985)).


“Just when a scientific principle or discovery crosses the line between the experimental and demonstrable stages is difficult to define. Somewhere in this twilight zone the evidential force of the principle must be recognized, and while courts will go a long way in admitting expert testimony deduced from a well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs.”

Frye v. United States, 292 F. 1013, 1014 (D.C. Cir. 1923).


Under Rule 702, expert evidence must be the product of a reliable methodology. The court is not charged with determining if the evidence is correct. In the case of a measurement, it is still the jury’s role to make the determination if evidence is what the State claims. However, as the gatekeeper of evidence, a court should preclude any measurement failing to meet the criteria of Rule 702 and the Daubert standard.

The Daubert standard requires courts to be gatekeepers of scientific evidence. They are required to screen expert evidence to ensure they are the product of scientific “knowledge” and supported by “appropriate validation.” These requirements must be satisfied in order to deem a blood alcohol measurement relevant and reliable.

The Daubert Trilogy

Kumo Tire Co. v. Carmichael

After examining the transcript in “some detail,” 923 F.Supp., at 1518–1519, n. 4, and after considering respondents’ defense of Carlson’s methodology, the District Court determined that Carlson’s testimony was not reliable. It fell outside the range where experts might reasonably differ, and where the jury must decide among the conflicting views of different experts, even though the evidence is “shaky.” Daubert, 509 U.S., at 596, 113 S.Ct. 2786. In our view, the doubts that triggered the District Court’s initial inquiry here were reasonable, as was the court’s ultimate conclusion.

Kumho Tire Co., Ltd. v. Carmichael, 526 U.S. 137, 153, 119 S. Ct. 1167, 1177, 143 L. Ed. 2d 238 (1999)

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