The Lessons to Be Learned from the Last Three Decades of    American Legal Experience with Expert Testimony

       Edward J. Imwinkelried, Edward L. Barrett, Jr. Professor of Law, University of California, Davis, United States of America.

“Those who do not study history are doomed to repeat its errors.”

                                                                                   –George Santayana

       Worldwide, legal systems are increasingly relying on expert testimony in order to resolve factual issues.¹  However, it is probably fair to say that American courts make more extensive use of such testimony than other legal systems.  In one study of the incidence of expert testimony in American courts, the Rand Corporation found that experts testified in 86% of the trials² and that on average, there were 3.3 experts per trial.³  It is therefore hardly surprising the American jurisprudence on the admissibility of expert testimony includes thousands of judicial opinions.

       In the past three decades, that jurisprudence has been characterized by controversy.  The most publicized controversy has been over the threshold question of the standard for determining the admissibility of scientific evidence.  In 1923, the Court of Appeals for the District of Columbia rendered its decision in Frye v. United States.⁴  Frye involved a novel scientific technique, the systolic blood pressure test.  The theory was that when a person engages in a conscious attempt to deceive, the person’s systolic blood pressure changes.  Thus, by carefully monitoring a subject’s systolic blood pressure, an expert could determine whether the person was being untruthful.  In Frye, a defense expert had administered the test to the accused.  The expert was prepared to opine that the accused was being truthful when he died committing the offense. The trial judge excluded the testimony, and the appellate court affirmed.  In its opinion, the appellate court announced that before an expert could base an opinion on a scientific theory or technique, the proponent of the evidence had to demonstrate that the theory or technique has gained “general acceptance in the particular field[s] in which it belongs.”⁵   The defense evidence was inadmissible in Frye because the defense attorney had not demonstrated that the systolic blood pressure theory was generally accepted by psychologists and physiologists.  By the early 1970s, that test had become the overwhelming majority view in American jurisdictions.⁶

       However, in 1975, a new statutory scheme, the Federal Rules of Evidence, took effect in federal courts in the United States.⁷   Forty-one states have adopted evidence codes patterned more or less directly after the Federal Rules.⁸   Article VII of the Federal Rules governs the admissibility of opinion testimony, both lay and expert.⁹  The text of Article VII did not expressly incorporate the traditional general acceptance test for the admission of purportedly scientific testimony.  Rather, Article VII included Rule 702, which an enacted read:

If scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise.¹⁰

Thus, when the new Rules took effect, the question immediately arose as to whether the general acceptance test was still good law.  The lower federal courts struggled with that issue until 1993.

         In 1993 in Daubert v. Merrell Dow Pharmaceuticals, Inc.,¹¹ the United States Supreme Court confronted the question of whether the general test acceptance test had survived the adoption of the Federal Rules.  The Court answered that question in the negative.  The Court ruled that the test had been impliedly overruled when Congress adopted the Federal Rules.  However, in the same opinion Justice Blackmun made it clear that the overthrow of the Frye test did not mean that expert testimony is admissible as a matter of course under Rule 702.  Rather, under Rule 702 the trial judge has an important “gatekeeping” responsibility to screen the reliability of proffered scientific evidence.   Justice Blackmun derived a new empirical validity test from the text of Rule 702.  He focused on the expression, “scientific . . . knowledge,” in Rule 702.  He adopted an essentially methdological definition of the expression.¹²  He described the scientific method in classic Baconian terms: the process of formulating an hypothesis and then engaging in systematic, rigorous observation or experimentation to validate or falsify the hypothesis.¹³   If the proponent of the evidence can show that a scientific proposition has been validated by sound scientific methodology, the proposition qualifies as reliable “scientific . . . knowledge” and is presumptively¹⁴ admissible under Rule 702.  If the proponent made that showing, the testimony is admissible even though it relates to a novel or controversial theory.¹⁵

       The Supreme Court’s decision in Daubert was the culmination of a long legal battle that began when the Federal Rules went into effect.  As Justice Blackmun commented in his opinion,

The merits of the Frye test have been much debated, and scholarship on its proper scope and application is legion.  Indeed, the debates over Frye are such a well-established part of the academic landscape that a distinct term–“Frye-ologist”–has been advanced to describe those who take part.¹⁶

Even the Court’s decision in Daubert did not end the debate.  Since the Court’s decision was based on statutory interpretation rather than constitutional principles, the decision did not bind the states.   Even the states with evidence codes modeled after the Federal Rules were free to interpret their statutes differently.  As of late 2006, although most states have decided to follow Daubert, 15 states remain committed to Frye.¹⁷  While those 15 states constitute a minority of the jurisdictions, those states include California, Florida, Illinois, New York, Pennsylvania, and Washington – some of the largest and most litigious jurisdictions.   In short, although Frye is now technically a “minority” view, even today most state trials in the United States are conducted under Frye. 

       Although the debate over the relative merits of Frye and Daubert continues,¹⁸ something else has become clear in the past three decades.  Again, Frye involved a new theory about the use of equipment to monitor systolic blood pressure test.  As we shall see, in most Frye jurisdictions the courts sharply limited the scope of the Frye test.  They realized that the test was restrictive; and they decided to confine the test to fact situations similar to Frye, that is, to fact situations involving novel, purportedly scientific theories involving instrumentation.¹⁹   If the scope of the test were limited in that fashion, it followed that certain types of expert testimony was largely exempt from scrutiny: traditional scientific theories, soft science, and non-scientific expertise.  Once those limitations on the scope of Frye became clear, those difficult questions naturally arose: Should those exemptions be ended under Daubert; and if so, how?

       The first part of this short article concentrates on the Frye test.  That part describes the test itself, its rationale, and, importantly, the limitations on the reach of the test.  The second part contrasts Frye with Daubert.  That part demonstrates that in most Daubert jurisdictions, the courts have chosen to end the former exemptions for traditional science, soft science, and non-scientific expertise.  The third part of the article explains why the courts elected to do so and how they have undertaken to scrutinize the reliability of these three, previously exempt forms of expert testimony.  Although the Frye debate has attracted most of the attention, the more important and difficult question has proven to be the reach or scope of any admissibility standard.  On its face, Frye dealt with only purportedly scientific testimony, in fact an instrumental scientific technique.  In many Daubert jurisdictions, the courts have come to realize that the admissibility of expert testimony based on hard scientific techniques is only a small part of the picture.  That is only one species of expert testimony.  There are not only grave doubts about the reliability of other species of expert testimony such as soft science and non-scientific expertise; there is also a growing appreciation that it can even more daunting to devise standards for evaluating the reliability of these other varieties of expert testimony.  In short, the lesson to be learned from the American legal experience with expert testimony in the past three decades is that it is a serious mistake to concentrate narrowly on hard scientific testimony. 

I.          THE FRYE GENERAL ACCEPTANCE TEST

       As previously stated, at one time the Frye test was the clear majority view among American courts.  It is understandable why the test was so popular.  Rather than requiring the trial judge to pass directly on the scientific merit of the expert’s theory, the test required only that the judge assess the popularity of the theory within the pertinent specialty fields.  The argument ran that both American judges and American juries lack the scientific education and training to critically evaluate scientific testimony.²⁰  In effect, the test delegates the admissibility decision to “those most qualified to assess the general validity of a scientific method,”²¹ the speciality field itself.  The majority of the members of that field ought to have “the determinative voice.”²²  Regardless of the judge’s views, the evidence should be admissible if the majority of those specialists subscribe to the theory; but, again regardless of the judge’s views, the evidence ought to be excluded if the theory does not enjoy widespread support in the pertinent scientific specialty fields. 

       The proponents of the Frye standard acknowledged “its essentially conservative nature”²³ and conceded that it could “interpose a substantial obstacle”²⁴ to the admission of logically relevant scientific testimony.  However, they believe that the courts should proceed with caution in receiving scientific evidence.  In their mind, caution is warranted because lay jurors may attach undue weight to such evidence.²⁵  A “misleading aura of certainty often envelops a new scientific process”;²⁶ and “scientific proof may . . . assume a posture of mystic infallibility in the eyes of a [lay] jury.”²⁷   If the lay jury was likely to ascribe inordinate, unjustified weight to scientific evidence, the safest course of action was to admit only testimony based on “tried and true” scientific theories. 

       The crucial question, though, proved to be the scope of the test.  Again, Frye itself dealt with a technique that was: (a) purportedly scientific; (b) novel; and (c)  based on hardware or instrumentation.  Should Frye be extended to non-scientific expertise, traditional scientific techniques, or soft science?  Although there was not unanimity on these questions, in most Frye states the courts chose to draw the line at novel, hard scientific techniques.

                                   Traditional Scientific Theories

       Frye came to be conceived as an admissibility test “focused exclusively on ‘novel’ scientific techniques . . . .”²⁸  Given the rationale for the test, that limitation made sense.  The intent of the general acceptance was to delegate the admissibility decision to the pertinent scientific community.  If the technique in question had become a “traditional” methodology within that community, the technique had presumably gained general acceptance.  In other words, the traditional status of the technique was adequate proof that it had garnered the necessary general acceptance.

                                   Soft Scientific Theories

       In many Frye jurisdictions, the understanding was that the test did not apply to soft science such as medicine or psychiatry.  California is a case in point.  The California courts have been some of the staunchest supporters of Frye.²⁹  Yet, in People v. McDonald,³⁰ the same California Supreme Court which has enthusiastically embraced Frye refused to apply the test to expert psychological testimony about the supposed unreliability of eyewitness testimony.  In the course of its opinion, the court noted that in the past, it had never extended the test to medical or psychological testimony.  The court explained that it had adopted Frye mainly because the court feared that lay jurors frequently attach exaggerated weight to scientific testimony.  However, the court added that that fear is ordinarily acute only when the testimony is “produced by a machine.”³¹  Scientific instruments are the epitome of the Black Box that can mystify ordinary jurors.  According to one commentator, a majority of Frye states have fashioned a similar exemption for “soft” scientific testimony.³²

                            Non-Scientific Expertise

       On reflection, it is clear that scientific evidence does not exhaust the category of expert testimony.  The text of Federal Rule of Evidence 702 refers in the alternative to “scientific, technical, or other specialized knowledge.”³³   In civil legal malpractice actions, experienced lawyers may testify to the standard of care for certain types of practice such as estate planning.  No one would characterize the lawyers who testify as “scientists.”  In drug prosecutions, it is commonplace to have veteran undercover police officers testify to the meaning of such expressions as “lid” in the drug trade.  Again, no one would claim that the officers who testify qualify as “scientists.”  Yet, both the lawyer and the police officer are allowed to testify as experts. 

       As in the case of traditional scientific techniques and soft science, the question arose as to whether the courts should apply the Frye standard to regulate the admissibility of non-scientific expert testimony.  As in the case of the two other types of expert testimony, the prevailing answer in Frye jurisdictions has been No.  The courts reason that as in the case of soft science, non-scientific expert testimony presents a minimal risk that the jurors will overvalue the testimony.   When a lawyer witness is describing the standard of care for a large, diverse group of attorneys such as estate planners, the testimony will not “assume a posture of mystic infallibility in the eyes of” a juror with any degree of sophistication.³⁴  Likewise, when a police officer witness is testifying about the behavior in the shady, gray world of drug dealing, there is little risk that the jurors will think that the testimony is as precise and reliable as the testimony of a chemist who has carefully measured the quantity of drug in a glassine envelope.  The trial judge may inquire whether the witness is an expert as well as whether the subject is so esoteric that lay jurors would benefit from the insight of a qualified expert.  However, as in the case of soft science, most Frye courts do not believe that non-scientific expertise presents the risk of overvaluation that is the rationale for Frye. 

       The upshot is that although its advocates champion Frye as a conservative, rigorous test, the test has a very limited scope.  Many–in fact most–types of expert testimony fall beyond the reach of the general acceptance test.   If the witness’s expertise is traditional, soft, or non-scientific, there will be little or no scrutiny of the reliability of the witness’s underlying reasoning.

II.         THE DAUBERT EMPIRICAL VALIDITY TEST

       The Supreme Court rendered its decision in Daubert in 1993.  Shortly after the decision, the lower courts began grappling with the question of the scope of the new test.  Since 1993, the courts have had occasion to address the question of whether the new test is subject to the same exemptions as the Frye standard. 

                                   Traditional Scientific Theories

       In footnote 11 of his opinion, Justice Blackmun highlighted one of the differences between the traditional Frye standard and the test enunciated in Daubert construing Federal Rule 702:

Although the Frye decision itself focused exclusively on “novel” scientific techniques, we do not read the requirements of Rule 702 to apply specially or exclusively to unconventional evidence.³⁵

The evident implication was that the new test applied across the board to all scientific theories, both novel and conventional.  The language implied that under Daubert, the proponent cannot “grandfather” a scientific technique into admissibility, based solely on the fact that the technique has been in use for a long period of time.

       The courts quickly seized upon that implication.  The first case to do so was United States v. Starzecpyzel,³⁶ a 1995 decision by a federal District Court in New York.  In Starzecpyzel, the prosecution wanted to present testimony by questioned document examiners, experts who analyze documents in order to determine whether the writings have been forged or altered.  In the United States, the courts had accepted questioned document testimony for decades.³⁷  It certainly qualified as a conventional or traditional technique.  Yet, the Starzecpyzel court tested questioned document expertise under Daubert and found it wanting.  The court concluded that there had been inadequate empirical validation of many of the underlying premises of the discipline.  Consequently, the court refused to admit the testimony as “scientific” evidence.  Instead, the court indicated that it would admit the testimony only as non-scientific expertise.  The District Court judge, Judge McKenna, stated that he intended to give the jury a cautionary instruction, warning them not to attach undue weight to the testimony:

Forensic document examiners, as a group, may develop skills not possessed by members of the general public, skills that may give rise to opinions useful to you [jurors] in your deliberations.  The Court has studied the nature of the skill claimed by forensic document examiners, and finds it to be closer to a practical skill, such as piloting a vessel, than to a scientific skill, such as that which might be developed by a chemist or a physicist.  That is, although forensic document examiners may work in “laboratories,” and may rely on textbooks with titles like “The Scientific Examination of Documents,” forensic document examiners are not scientists–they are more like artisans, that is, skilled craftsmen.³⁸

Questioned document examiners found the Starzecpyzel decision shocking because, for decades, they had referred to themselves as forensic “scientists.”  The attack on questioned document examination has continued and been pressed in numerous cases.³⁹

       Starzecpyzel was only the first in what is now a long line of cases challenging traditional scientific techniques.⁴⁰  For instance, there have been attacks on such diverse conventional techniques as microscopic hair comparison,⁴¹ bitemark comparisons,⁴² and comparative bullet lead analysis.⁴³  Even fingeprint analysis has been questioned.⁴⁴  In the words of federal District Court judge Nancy Gertner, Daubert and its progeny “plainly invite a reexamination even of ‘generally accepted’ venerable . . . fields.”⁴⁵

                                   Non-Scientific Expertise

       Just as Frye jurisdictions routinely admitted testimony based on traditional scientific techniques, they admitted testimony resting on non-scientific expertise as a matter of course.  Post-Daubert, the question posed was whether non-scientific expertise enjoys a similar exemption under Federal Rule of Evidence 702.  In federal practice, the answer is clear: There is no such exemption.

       To begin with, the Supreme Court itself refused to recognize such an exemption.  After the Court rendered its decision in Daubert and lower courts began rigorously applying Daubert, many proponents of expert testimony attempted to circumvent Daubert by (1) arguing that Daubert applied only to scientific expertise and (2) characterizing their expert testimony as non-scientific.  A footnote in Daubert lent some credibility to that argument.  In footnote 10, Justice Blackmun remarked: “Rule 702 also applies to ‘technical, or other specialized knowledge.’  Our discussion is limited to the scientific context because that is the nature of the expertise offered here.”⁴⁶  Proponents of expert evidence sometimes urged that the footnote was intended as a signal that the Daubert requirement for a showing of reliability is limited to purportedly scientific testimony.  If so, even under Rule 702 non-scientific expert testimony is admissible with little or no scrutiny as it was under Frye.

       However, in 1999 in Kumho Tire Co. v. Carmichael,⁴⁷ the Supreme Court revisited the issue and repudiated any suggestion that the reliability requirement applies only to explicitly scientific expert testimony.  Justice Breyer authored the Court’s opinion.  He invoked both policy analysis and statutory construction arguments to support his conclusion that the reliability requirement applies to all types of expert testimony. 

       As a matter of policy, Justice Breyer asserted that it would be impractical to require trial judges to administer differing evidentiary requirements based on a distinction between “scientific” and “non-scientific” testimony.  The Justice stated:

[I]t would prove difficult, if not impossible, for judges to administer evidentiary rules under which a gatekeeping obligation depended upon a distinction between “scientific” knowledge and “technical” or “other specialized” knowledge.  There is no clear line that divides the one from the others.  Disciplines such as engineering rest upon scientific knowledge. Pure scientific theory itself may depend for its development upon observation and properly engineered machinery.  And conceptual efforts to distinguish the two are unlikely to produce clear lines capable of application in particular cases.  We do not believe that [evidence law should] create[] a schematism that segregates expertise by type while mapping certain kinds of questions to certain kinds of experts.  Life and the legal cases that it generates are too complex to warrant so definitive a match.⁴⁸

       Justice Breyer added that the statutory language of Rule 702 did not justify limiting the reliability requirement to scientific testimony.  He conceded that the statute referred in the altenative to “scientific, technical, or other specialized” testimony.⁴⁹  However, he stressed that all of those adjectives modify the noun, “knowledge.”⁵⁰   In Justice Breyer’s view, that noun was the key to interpreting Rule 702.  That word is the source of the requirement for a showing of reliability.⁵¹  By choosing the word, “knowledge,” Congress indicated that any expert testimony must rest on more than the witness’s subjective speculation.⁵²   In short, the proponent cannot escape from Daubert’s reliability requirement simply by labeling his or her testimony “non-scientific expertise.”

       I,n 2000, Federal Rule of Evidence 702 was amended.  The Advisory Committee Note accompanying the amendment states:

Consistently with Kumho, the Rule as amended provides that all types of expert testimony present questions of admissibility for the trial court in deciding whether the evidence is reliable and helpful. [T]he amendment does not distinguish between scientific and other forms of expert testimony.  The trial court’s gatekeeping function applies to testimony by any expert.  While the relevant factors for determining reliability will vary from expertise to expertise, the amendment rejects the premise that an expert’s testimony should be treated more permissively because it is outside the realm of science.  An opinion from an expert who is not a scientist should receive the same degree of scrutiny for reliability as an opinion from an expert who purports to be a scientist.  “[I]t seems exactly backwards that experts who purport to rely on general engineering principles and practical experience might escape screening by stating that their conclusions were not reached by any particular [scientific] method or technique.”⁵³

                                   Soft Scientific Theories

       The final issue was the impact of Daubert on the existence of the exemption for soft scientific testimony that had developed under Frye.   For three reasons, a consensus has developed that there is no longer such an exemption.

       The first reason was the breadth of the definition of science in Daubert.  As previously stated, Justice Blackmun adopted the methodological definition of formulating hypothesis and then conducting systematic observation or experimentation to validate or falsify the hypothesis.  Although it is easier to control experimental variables in the hard sciences, many soft science hypotheses can also be subjected to empirical investigation.⁵⁴  Given the applicability of empirical investigative techniques to soft science, the lower courts quickly concluded that the requirement for a showing of reliability extends to soft science theories and methodologies.⁵⁵

       The second reason, of course, was the sweeping nature of the Court’s holding in Kumho.  There Justice Breyer explicitly rejected the suggestion that any type of expert testimony ought to be exempt from the reliability requirement. 

       Thirdly and finally, if any doubt at all persisted after Kumho, the Advisory Committee Note for the 2000 amendment to Rule 702 eliminated the uncertainty.   

       Part I of this article pointed out that in most Frye jurisdictions, exemptions emerged for traditional scientific theories, soft science, and non-scientific expertise.  In short order, between 1993, the date of the original Daubert decision, and 2000, the date of the most recent amendment to Rule 702, the federal courts and drafters effectively repealed each of those exemptions.

III.       THE REAL LESSONS FROM THE AMERICAN EXPERIENCE: WHY AND HOW THE EXEMPTIONS WERE ENDED

       The Introduction noted that the debate over Frye in the past three decades attracted much of the scholarly and popular attention.  However, as we have seen, during the same period of time it has become clear to the American legal system that the reach or scope of an admissibility test is as important, if not more important, than the nature of the test.   Why has the American legal system proceeded to repeal the exemptions that existed under Frye, and how have American courts attempted to do so?

                                          Why

       In part, the answer to this question turns on Congress’ decision to enact the Federal Rules in 1975 and the subsequent decision of most states to adopt evidence codes patterned after the Federal Rules.  In the American legal hierarchy, statutes trump common law rules.  Frye was a common law decision, and the limitations on the scope of the Frye test developed in case law.  However, as a statutory scheme the Federal Rules can supersede common law doctrines.  Hence, if Rule 702 abolishes the distinction between the various types of expert testimony and mandates a showing of reliability for any proffered expert testimony, a proponent of soft science or non-scientific expertise can no longer invoke the exemption that previously existed at common law.         However, it would be naive to claim that the language of Rule 702 was so crystal clear that it dictated the courts’ conclusions.  Neither the text of Rule 702 nor the Advisory Committee Note accompanying the original statute expressly stated that there was a reliability requirement or that the requirement applied to all types of expert testimony.⁵⁶   Candor requires recognizing that policy considerations influenced the courts’ decisions to end the common law exemptions. 

       One such policy consideration was the eventual realization that scientific evidence represents only a relatively small percentage of the expert testimony proffered in American trials. In the early 1990s the Rand Corporation released its study of the use of expert testimony in American courts.   The Rand researchers found:

Half of the experts in our data were medical doctors, and an additional 9% were other medical professionals–clinical psychologists, rehabilitation specialists, dentists, etc.  Engineers, scientists and related experts made up the next largest category, nearly 20% of the total.  The only other sizeable categories were experts on various aspects of business and finance (11%), and experts in reconstruction and investigation (8%).⁵⁷

In this database, the clear majority of experts were not full-fledged “scientists.” 

       A second consideration was that it became increasingly clear that there were good reasons to be concerned about the reliability of expert testimony other than scientific evidence.  There have long been doubts about the trustworthiness of psychiatric and psychological testimony.⁵⁸  As previously stated, the Starzecpyzel court characterized questioned document testimony as non-scientific expertise.⁵⁹   The proficiency studies of questioned document examiners reveal that there is a substantial margin of error even in analyses conducted by experienced examiners.⁶⁰

       For this complex of reasons, a judicial consensus developed that under Daubert, the courts should no longer recognize the exemptions that existed under Frye.  Rather, the courts resolved to test the reliability of all types of proffered expert testimony.

                                          How

       Of course, it is one thing to decide to do something.  It can be much more difficult to implement that decision.  As Judge McKenna wrote in Starzecpyzel,

In the long term, we may discover that Daubert addressed the easier part of the problem of expert testimony; it was a relatively straightforward matter for the Daubert Court to deduce objective reliability standards from the nature of the process of experimental Newtonian science.  However, although ultimately all types of expert knowledge are inferences from underlying experience, the epistemology of nonscientific expert knowledge is quite different from that of scientific propositions . . . . [T]he development of objective validation standards to nonscientific opinions is likely to prove to be a more difficult task than the formulation of such tests for scientific testimony.⁶¹

The American courts are now engaged in the enterprise of attempting to formulate such tests.  In part, the task is proving to be a difficult one precisely because, under Frye, the courts rarely undertook to assess the reliability of expert testimony other than scientific evidence.  Thus, post-Daubert the courts do not have a pre-existing body of precedent to build on.

       Nevertheless, the courts have made strides in the right direction.  The pattern that is emerging that the courts are employing a three-step analytic framework.

       In the initial step, the court identifies the specific theory or technique that the expert proposes relying on.    The best reading of the Supreme Court precedents is that the proponent need not demonstrate the “globa[l]” validity of his or her discipline.⁶²  In Daubert, Justice Blackmun insisted that the proponent establish that the expert’s theory enables the expert to perform the specific “task at hand.”⁶³   In the Court’s 1997 decision, General Electric Co. v. Joiner,⁶⁴ the late Chief Justice Reqhnquist stated that the issue was not “whether animal studies can ever be a proper foundation for an expert’s opinion.”⁶⁵  Rather, he defined the issue posed as “whether these experts’ [particular] opinions were sufficiently supported by the animal studies on which they purported to rely.”⁶⁶  Finally, the Court’s 1999 decision in Kumho fits the same mold.  In Kumho, Justice Breyer declared:

[C]ontrary to [plaintiffs’] suggestion, the specific issue before the [trial] court was not the reasonableness in general of a tire expert’s use of a visual and tactile inspection to determine whether overdeflection had caused the tire’s tread to separate from its steel-belted carcass.  Rather, it was the reasonableness of using such an approach, along with Carlson’s particular method of analyzing the data thereby produced, to draw a conclusion regarding the particular matter to which the expert testimony was . . . relevant.⁶⁷

The Justice stressed that the inquiry must be “specific, not general.”⁶⁸

       Identifying the specific theory or technique makes the most sense.  At any given time in virtually any expert field, there will be a mix of various types of current propositions⁶⁹–some of which will later be discredited and others which will be validated.  The fact may be that many of the propositions in the expert’s field may be valid.  However, it is fallacious to leap from that fact to the conclusion that the particular proposition the expert is relying on is valid.  Conversely, many of the propositions current in the expert’s specialty may eventually be exposed as invalid.  Yet, that does not dictate the conclusion that the specific theory the expert is utilizing is also invalid.  The trilogy of Daubert, Joiner, and Kumho indicate that in passing on the admissibility of the expert’s testimony, the judge should begin by focusing on the specific theory or technique underlying the expert’s opinion; and that is the soundest approach.

       The next step in the analytic framework is to characterize the type of claim that the expert is making about the theory or technique.   There are various types of potential claims.⁷⁰  In some instances, the expert is making a limited descriptive claim.⁷¹   By way of example, suppose that the expert witness is a police officer prepared to testifying that in drug transactions, the word “lid” means a certain quantity of contraband drugs.  The witness may be relying on his or her experience as an undercover narcotics officer.  The officer is merely summarizing the results of his of her experience as an undercover.  The officer is claiming only that a linguistic convention exists among drug dealers.

       In other cases, though,  a soft scientist or non-scientific expert may be making a radically different type of claim.  The expert may be making an inferential claim.⁷²  The expert’s proponent wants the expert “to do far more than merely recite or summarize [the witness’s] experience.”⁷³  The proponent invites the expert to draw a further inference from his or her experience.⁷⁴   A psychologist might attempt to testify that because a patient displayed a certain set of symptoms, he or she had probably experienced a particular type of traumatic event such as a rape.  Or, a police officer may be ready to testify that because the dog he or she handles alerted in a particular way to the accused’s luggage, the luggage had recently contained explosives.  The expert implicitly claims that his or her theory enables them to correctly draw the desired further inference. 

       As a third and final step, having characterized the type of claim that the expert is making about the specific theory or technique, the judge inquires whether the expert has presented “appropriate validation” for that claim about the theory.  Suppose, for instance, that the expert is the police officer prepared to testify about the historical claim that the word “lid” has a certain usage among drug traffickers.  In that setting, an adequate foundation would consist of proof that the witness has participated in several transactions in which the word “lid” was used and that in all or most of those transactions, the traffickers acted as if the word had the specified meaning.⁷⁵  That is, after one of the parties used the term “lid,” that quantity of drugs changed hands. 

       In contrast, in the case of an inferential claim, the judge should demand a foundational showing that the prior results of the use of theory or technique demonstrate that the use of the techniques enables the expert to accurately draw the desired inference.⁷⁶   In the final analysis, why we accept the findings of modern science?  We see place faith in science because on a daily basis we see an “immense body of results” of the use of scientific theories, practical technologies that prove the “worth [of science] in the real of material technique.”⁷⁷  Those results provide the pragmatic basis for belief in science.⁷⁸ 

       Similarly, we should have faith in a scientific proposition in court–enough faith to allow it to be used as a basis for expert testimony–if the concrete results of the previous use of the theory show that its use ordinarily accurately leads to the desired inference.⁷⁹  As the Advisory Committee Note to the 2000 amendment to Federal Rule of Evidence 702 states, the judge should carefully weigh the consideration that “the field of expertise claimed by the expert is known to reach reliable results for the type of opinion the expert would give.”⁸⁰  If the expert claims that this dog’s alert supports the inference that the suitcase recently contained explosive material, the expert should detail the prior experience with the dog.  On how many occasions has the witness seen the dog alert in that fashion?  In what percentage of those cases was the luggage later searched?  In what percentage of those cases did the search yield explosive material or lead to the discovery of other evidence that the luggage had contained explosives?  If the expert claims that the subject’s display of a particular set of symptoms indicates that the subject had been subjected to a certain type of trauma, the judge should pose a parallel set of questions?  Essentially, the witness is basing his or her opinion on a comparison between the subject’s symptoms and a profile derived from a database.⁸¹  Was there proof that the reported incidents included in the database were in fact instances of the trauma?⁸²    

IV.  CONCLUSION

       In the United States, at least at times the highly publicized debate over the Frye test has tended to obscure an even more significant question about the legal treatment of expert information: the scope or reach of any test governing the admissibility of expert testimony.  In both Frye and Daubert, the courts dealt with expert testimony that the proponents themselves had categorized as “scientific.”  As controversial as the debate has been, Judge McKenna was correct when he observed in Starzecpyzel that the admissibility of purportedly scientific evidence is “the easier part of the problem of expert testimony.”⁸³   Any reasonably literate person is familiar with and has at least a rudimentary understanding of the “scientific” methodology of framing hypotheses and empirically testing the hypotheses.   A basic understanding of that methodology should enable a judge to make a reasonably confident assessment of the objective merit of proffered “scientific” testimony.⁸⁴

       However, the classic scientific model is less useful to the judge when the judge must pass on the admissibility of expert testimony that does not fit the traditional paradigm of an instrumental technique such as DNA typing or polygraph.  Once the judge ventures beyond that paradigm and is asked to gauge the admissibility of soft science and non-scientific expertise, the judicial decision becomes much more problematic.  Of course, those are precisely the problems that arise once a legal system makes the fundamental decision to extend the scope of its admissibility test to other types of expert testimony.

       To date, the United States Supreme Court has offered relatively little guidance to the lower courts.  In Daubert, Justice Blackmun declared that “[t]he inquiry envisioned by Rule 702 is . . . a flexible one.”⁸⁵  To be sure, he did mention a number of factors that trial judges should consider in determining whether the proffered testimony amounts to reliable “scientific knowledge.”⁸⁶  However, he prefaced the list with the statement that “we do not presume to set out a definitive checklist or test.”⁸⁷   In Kumho dealing with non-scientific expertise, the Court’s language was even vaguer.  After harking back to Justice Blackmun’s list in Daubert, Justice Breyer wrote:

“The factors identified in Daubert may or may not be pertinent in assessing reliability, depending on the nature of the issue, the expert’s particular expertise, and the subject of his testimony.” [W]e can neither rule out, nor rule in, for all cases and for all time the applicability of the factors mentioned in Daubert, nor can we now do so for subsets of cases categorized by category of expert or by kind of evidence.  Too much depends upon the particular circumstances of the particular case at issue.⁸⁸

In Joiner, the Court made clear that the trial judge has a measure of discretion in applying the Daubert factors to decide the admissibility of proffered scientific testimony.⁸⁹  In Kumho, Justice Breyer went farther.  He announced that in the case of non-scientific expertise, the judge also possesses discretion to choose the factors to use to evaluate the reliability of the testimony.⁹⁰

       The three-step analytic framework outlined in Part III can be synthesized from the more thoughtful judicial opinions applying the Daubert test.  However, it must be conceded that the courts are just beginning to develop this framework.  The American jurisprudence on the admissibility of expert testimony other than instrumental scientific techniques is still very much a work in progress.  It is hoped that this article will give readers in other countries such as China an insight into the American experience in the past three decades–and aid them to avoid some of the mistakes committed by the American legal system.