The Admissibility of Scientific Evidence: a Primer on Federal Law
| Publication year | 2013 |
| Pages | 0010 |
A Look at the Law
by Hon. William S. Duffey Jr.
Scientific evidence is common in judicial proceedings. Expert testimony is the most common vehicle through which parties can offer technical and scientific evidence because it aids the trier of fact in understanding the evidence or determining an issue in the case.[1] Most judges and juries lack the scientific training that would permit them to evaluate thoroughly claims that turn on scientific questions.[2] Admitting expert testimony facilitates the evaluation of scientific and technical claims and evidence, and helps ensure that decisions are based on scientifically sound knowledge.[3]
Expert testimony plays a key role in arriving at scientifically sound determinations. Determining what evidence and expert testimony to admit requires careful, deliberate consideration by the court to ensure the evidence admitted is reliable and trustworthy.
Not all scientific evidence is equally reliable, and admitting scientifically unsound evidence risks confusing or misleading the jury, or may ultimately allow the jury to reach an unjust conclusion.[4] To obtain the benefits of expert testimony and minimize the dangers of permitting unsound science to enter the courtroom, the Federal Rules of Evidence and the U.S. Supreme Court's decision in Daubert v. Merrell Dow Pharmaceuticals guide federal judges in ensuring that only reliable evidence is admitted at trial to assist the jury in reaching a just outcome.[5]
The Legal Framework for Evaluating the Admissibility of Expert Testimony
The admission of expert testimony in federal courts is governed by Rule 702 of the Federal Rules of Evidence, which states:
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 upon sufficient facts or data;
(c) the testimony is the product of reliable principles and methods; and
(d) the witness has applied the principles and methods reliably to the facts of the case.
Daubert sets forth the analysis courts must use to determine whether proffered evidence is sufficiently reliable to be admissible under Rule 702. After the adoption of the Federal Rules of Evidence (FRE) in 1975, but prior to Daubert being decided in 1993, courts applied the Frye test, which originated from a 1923 court of appeals case, when assessing the admissibility of expert testimony.[6]Under Frye, expert testimony only was admissible when it was based on a technique or theory that was "generally accepted" as reliable in the scientific community.[7]
In Daubert, the Supreme Court held that the Frye admissibility standard was inconsistent with the more liberal standard for the evaluation of evidence under the FRE.[8] The Supreme Court articulated a new standard in Daubert, holding that the Frye test had been superseded by the FRE.[9] In 1999, the Supreme Court decided in Kumho Tire Company v. Carmichael that Daubert applies not only to expert testimony based on scientific evidence, but also to testimony based on technical or other specialized knowledge.[10]
Under Daubert, for an expert's testimony to be based on "scientific knowledge," the opinion must be "derived by the scientific method."[11] At Daubert's core is the requirement that courts focus "solely on principles and methodology, not on the conclusions that they generate."[12] Each step of the expert's analysis must be demonstrated to be reliable; if any step fails the Daubert test, then the entire testimony is inadmissible.[13]
The judge serves a "gatekeeper" function in determining the admissibility of proffered expert testimony, excluding unreliable and irrelevant testimony. The court exercises broad discretion in determining whether evidence, including expert testimony, is admissible. Courts of appeal defer substantially to a trial judge's admissibility decision.[14] The critical gatekeeper inquiry is into the scientific validity of the expert's underlying reasoning or methodology.[15] The court determines not only whether the principles and methods used by the expert are reliable, but also whether those principles and methods have been properly applied to the facts of the case.[16] The inquiry under Daubert thus is process-oriented.[17] Judges focus on the methodology the expert used to reach his conclusions, rather than on the conclusions themselves.[18]
Applying the Daubert Standard
When evaluating whether to admit expert testimony, a court considers whether: (1) the expert is qualified to testify regarding the matters he intends to address; (2) the expert's methodology is sufficiently reliable; and (3) the expert's testimony assists the trier of fact to understand the evidence or to determine a fact in issue.[19] Daubert sets forth a non-exclusive checklist to use in evaluating the reliability of scientific expert testimony. The factors include:
(1) whether the expert's technique or theory can be or has been tested—that is, whether the expert's theory can be challenged in some objective sense, or whether it is instead simply a subjective, conclusory approach that cannot reasonably be assessed for reliability;
(2) whether the technique or theory has been subject to peer review and publication;
(3) the known or potential rate of error of the technique or theory when applied;
(4) the existence and maintenance of standards and controls; and
(5) whether the technique or theory has been generally accepted in the scientific community.[20]
The inquiry is flexible, and the criteria the judge applies may vary based on the facts of the case and the nature of the proffered testimony.[21] The court is not required to consider each of these factors, and the court should consider any additional factors that may advance its Rule 702 analysis.[22]
The second prong of Rule 702-that the expert testimony "assist the trier of fact"-addresses the relevance of the evidence after the court has determined it is reliable. The extent to which the trial court may inquire into the sufficiency of evidence after making the reliability determination is viewed differently in different appellate courts.[23]Some courts exclude expert testimony as lacking relevance where it is insufficient to prove the matter for which the party seeks its introduction. In others, judges only are required to find that the evidence meets a minimal relevance threshold to be admissible.
After determining that evidence is reliable, the trial judge may consider whether the evidence is "sufficiently tied to the facts of the case so that the evidence will aid the trier of fact in resolving the dispute."[24] This inquiry is sometimes described as a "fit test." Rule 702's requirement that the evidence be helpful to the trier of fact requires a valid scientific connection-a fit-to the pertinent inquiry as a precondition to admissibility.[25]
After a judge finds that the expert testimony is reliable and that it sufficiently "fits" the evidence, the judge also may evaluate whether the evidence is nonetheless inadmissible under FRE 403. Under FRE 403, a court:
May exclude relevant evidence if its probative value is substantially outweighed by a danger of one or more of the following: unfair prejudice, confusing the issues, misleading the jury, undue delay, wasting time or needlessly presenting cumulative evidence.
Part of the judge's role as gatekeeper is to preclude the introduction of evidence that, although relevant on an issue in the case, is likely to be so prejudicial, misleading or confusing that it risks causing the jury to reach an unjust result.[26]
Considering and Applying the Daubert Criteria
Applying the Daubert criteria is not a rigid process resulting in a uniform template to evaluate whether evidence is or is not allowed to be introduced. A short survey of cases analyzing each Daubert factor illustrates this point, although certain general themes emerge from these evaluations, as illustrated below.
Factor 1: Whether the expert's technique or theory can be or has been tested-whether the expert's theory can be challenged in some objective sense, or whether it is instead simply a subjective, conclusory approach that cannot reasonably be assessed for reliability
Whether a theory or method can or has been tested is one of the most important indicators of its reliability.[27] That a theory can be empirically challenged, has been tested in controlled circumstances, and is reproducible weighs in favor of admissibility. Epidemiological studies are good...
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