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The Daubert Tutorials
by Stephen Mahle


An Introduction to the Daubert Tutorials

The Daubert Tutorials are a series of a dozen “lessons” that begin with the original Daubert opinion and work systematically through the essential Daubert progeny.

They discuss the law that governs expert testimony in federal courts, and the science that informs that law.  Eschewing technical lingo, the scientific notions are developed in the language of lawyers, and while the discussions will not always be elementary, the science discussed here will always be about novel strategies for prevailing in complex litigation.

The record shows that the original Daubert opinion was extensively informed by science and its methods. There were dozens of amicus briefs, including scientists’ briefs, and much of the scientist’s language found its way into the opinion.  Because science informs the law of expert testimony pervasively, sophisticated articulations of the law of expert testimony require that the discussion be based in the elements of the science that Daubert cites.  One senior law and science professor summarized this proposition tersely, saying, “If you are going to litigate scientific issues, its best to know some science.”

With some minor exceptions, there won’t be any math or much statistics either, because most of the scientific analysis upon which the Daubert line of cases relies can be developed in intuitive terms with only the most minimal use of mathematics and formal statistics.  The origins of these notions of science are in the philosophy literature and that is mostly how we discuss them here.

Daubert’s Roots in Science: The Origins of Testing and Error Rates

Daubert discussions often start with the four factors, treated somehow they were handed down in the manner of the Ten Commandments, written de novo, of whole cloth.  As a result, they miss one of Daubert’s critical mechanisms for controlling expert testimony: Daubert was not written de novo at all; it was written, in large part, by incorporating the elements of scientific inquiry into the law.  This suggests a range of novel strategies for litigating expert testimony, many of which center on testing and error rate analysis, and all of which have roots in what the Supreme Court and the generally-accepted peer-reviewed scientific literature call “the scientific method.”

Daubert explains the criteria that gatekeeper trial courts should use to evaluate “purportedly scientific evidence” by parsing Rule 702, focusing on the meanings of the words “scientific” and “knowledge.”  The Court said that “[t]he subject of an expert’s testimony must be ‘scientific . . . knowledge,’” because it is “the requirement that an expert’s testimony pertain to ‘scientific knowledge’” that “establishes a standard of evidentiary reliability.” In addition, the Court required that “to qualify as ‘scientific knowledge,’ an inference or assertion must be derived by the scientific method.”  Daubert v. Merrill Dow Pharmaceutical, Inc., 509 U.S. 579, 590 (1993).

The Court’s statement that “to qualify as scientific knowledge, an inference or assertion must be derived by the scientific method” presages much of the rest of Daubert, and fully parsing that phrase provides an opportunity for lawyers to prevail in matters that would be otherwise difficult. So now, the Scientific Method, as relied upon by Daubert.

Daubert’s Foundations: The Scientific Method

The phrase “the scientific method” is a term of art within the scientific community and has a specific meaning within that community. Daubert’s discussion of the scientific method quotes from seminal works on scientific inquiry enough to make it clear that the Court is using the phrase as that term of art. Indeed, much of the language relied upon by the Court in its discussion of the scientific method is strikingly similar to the language used in amicus briefs filed by or on behalf of scientists from industry and academia. Daubert says that:

Ordinarily, a key question to be answered in determining whether a theory or technique is scientific knowledge that will assist the trier of fact will be whether it can be (and has been) tested.  “Scientific methodology today is based on generating hypotheses and testing them to see if they can be falsified; indeed, this methodology is what distinguishes science from other fields of human inquiry.”  509 U.S. at 593 (emphasis added).

This famous-in-some-circles quotation (which is the citation for the first of Daubert’s four factors) usually ends after the first sentence.  But the real meat of how to control expert testimony with Daubert’s first factor comes from the emphasized first half of the second sentence.  The testing of hypotheses that the Court’s emphasized language requires is called “hypothesis testing” in the scientific community, and as the Court’s quotations indicate, hypothesis testing is the essence of the scientific method. It is noteworthy that the Daubert Court required that experts follow this “scientific method” from the first moment of their discussion of the four factors that commentators and lower courts have fixed upon, because testimony that proceeds in accordance with the scientific method will always tend to satisfy the Court’s testing and error rate criteria. Perhaps more importantly, by carefully defining what testing is, we can define what testing is not. And by defining what error rates are, we can define what they are not. Knowing what does not constitute testing and error rate analysis is at least as important for winning cases is as knowing what does.

For example, in one recent Daubert issue in a products liability case, plaintiff’s counsel claimed that his expert had tested a modification to the product by forming a prototype of a locking mechanism and installing it. But installing a locking mechanism (or a guard, or a warning) is not a Daubert test of the expert’s methods, and although the fabrication-as-test argument has carried the day for plaintiffs in reported cases, it does not win when countered by arguments that explain what testing means in Daubert. Hypothesis testing is the core of the scientific method, is carefully defined, and will be the topic of a subsequent issue in this series.  Daubert testing is likewise carefully defined: My mom tests the spaghetti noodles to see if they are done, but that is not a Daubert test.  Neither is installing a locking mechanism on a lawn chair to hold it open a test of an alternate design.  We will discuss testing more in coming issues.

The role of the scientific method is central to the Court’s analysis and the Winter, 2006 issue will develop its fundamentals in lawyer-friendly language.  For present purposes it suffices to say that the scientific method cited by Daubert requires that the analyst pose a hypothesis about some phenomenon and test that hypothesis at some specific rate of error.


In fact, all of the Daubert factors depend on the scientific method, half directly and half indirectly.  The four Daubert criteria for evaluating the admissibility of expert testimony are: whether the methods upon which the testimony is based have been tested;  the known or potential rate of error associated with that testing; whether the method has been subjected to peer review; and whether the method is generally accepted in the relevant scientific community. The first two of the Court’s four criteria amount to asking whether the techniques upon which the testimony is based are grounded in the scientific method. And virtually no scientific expert testimony will satisfy the last two factors unless it satisfies the first two, because the third criteria requires the first two and the fourth requires the first three. See  FAIGMAN, KAYE, SAKS & SANDERS, MODERN SCIENTIFIC EVIDENCE: THE LAW AND SCIENCE Of EXPERT TESTIMONY §1:15 (West Group 2005) (observing that “courts will find application of Daubert difficult if they treat testability as an optional factor. The other three factors all presuppose testability; in science, a non-testable hypothesis cannot have an error rate and is exceedingly unlikely to be published in a peer-reviewed journal and achieve general acceptance”).

Just like the scientific method, testing and error rate analysis are terms of art in the scientific literature that Daubert cites in its articulation of the evidentiary requirements for science to be admissible in federal court. They have specialized meanings that are discussed in the literature that the Court cites.  Over the coming quarters we will cite much of that literature as this column discusses how the scientific method, testing and error rate analysis can control expert testimony in a range of practice areas.

We previewed one such example above in the products area and a second example was the subject of last issues’ lead article, Controlling Regression-Based Litigation with Daubert and Statistical Analysis. Coming issues will show how the scientific basis of testing, error rate analysis and the scientific method provide important controls in most defense practice areas, including but not limited to: patent, IP, medical device and malpractice, securities, antitrust, contract, and any matter that involves statistical estimation of damages.

Stephen Mahle is a scientifically trained lawyer who concentrates his practice in litigating Daubert and expert testimony issues for insurance companies and their outside counsel. He has a doctorate in economics, has been a finance professor at several major universities, is webmaster of daubertexpert.com, and lectures and publishes regularly on Daubert and expert testimony issues.  He can be reached at smahle@daubertexpert.com, or (561) 451 - 8400 .


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