Nearly 20 years ago, the U.S. Supreme Court issued a landmark decision that set the standards for admission of expert testimony in federal tribunals. In Daubert v. Merrell Dow Pharmaceuticals, Inc.,1 the Court concluded that the so-called Frye (or general acceptance) standard for novel scientific evidence had been superseded by the enactment of the Federal Rules of Evidence (Rule 702 in particular). The Frye approach, which required a scientific technique "to have gained general acceptance" in the relevant scientific community,2 was deemed inconsistent with the "liberal thrust of the Federal Rules."3
Thus, in the wake of Daubert, scientific and technical testimony in federal court must now satisfy a two-prong test that focuses on relevance and reliability.4 Federal trial judges are to undertake a "preliminary assessment of whether the reasoning or methodology underlying the testimony is scientifically valid and whether that reasoning or methodology properly can be applied to the facts in issue."5 Under Daubert, federal trial courts function as "gatekeepers" to screen proffered expert testimony and evidence to determine that it "is not only relevant, but reliable."6
The Court proposed a nonexclusive list of factors to consider in evaluating the expert's "theory or technique" that includes but does not require general acceptance in the scientific community.7 Because the standard is flexible, the Court naturally declined to supply a rigid framework for the trial courts but stressed that the examination should focus on the expert's principles and methodologies, not the conclusions drawn from them.8
It is now firmly established that Daubert governs federal practice. California state courts, however, are not bound to follow the federal rules. Indeed, the California Supreme Court was quick to react to Daubert. In People v. Leahy,9 Chief Justice Malcolm Lucas, writing for the majority, rejected the Daubert standard in favor of California's version of the general acceptance standard for new or novel scientific evidence, which is known as the Kelly rule.10 Since California courts had been applying the Kelly rule for decades, this meant business as usual for California courts, at least when new or novel science is concerned.
Is California behind the times? Careful review of the recent authorities suggests it is not. Over the past two decades, California courts, while resisting invitations to replace California's standards with the rule of Daubert,11 have developed an efficient statutory approach that entrusts trial courts and juries with the ability to assess and consider scientific evidence. California's Evidence Code supplies a framework for the admissibility of scientific and opinion testimony that satisfies the same flexible goals served by the Federal Rules of Evidence. While the federal standard readily adapts to rapidly advancing technology and medicine, in California--the birthplace of much of that technology--state tribunals have been adept at meeting scientific challenges as well.
Most expert challenges in California trial courts target the expert's opinions, their foundational underpinnings, and the expert's reasoning processes. An understanding of the California trial court rules applicable to medical and scientific opinions can avoid costly missteps that can lead to the exclusion of a well-qualified but unprepared expert.
Evidence Code Section 801
The admissibility of expert opinion testimony in California is governed by Evidence Code Section 801, which provides a two-pronged test. First, an expert may express an opinion that is "[r]elated to a subject that is sufficiently beyond common experience that the opinion of an expert would assist the trier of fact."12 Next, the opinion must be "[b]ased on matter (including his special knowledge, skill, experience, training and education) perceived by or personally known to [the witness]...whether or not admissible, that is of a type that reasonably may be relied upon by an expert in forming an opinion upon the subject to which his testimony relates."13
Despite this simple directive, courts are frequently invited to wade into technically daunting fields to determine not only whether an expert's opinions are properly grounded but also whether the expert has correctly interpreted and applied the information at issue. The invitation to adopt this extrastatutory approach should be resisted, as it demands that the court second-guess the judgment of a trained and qualified expert who will, after all, be subject to "as penetrating a cross-examination as the ingenuity and intellect of opposing counsel can devise."14 A series of decisions over the last decade has helped to clarify the landscape for experts testifying in the particularly challenging area of medical cause and effect, and those decisions illuminate the path for technical opinion testimony as a whole.
The threshold inquiry under Section 801(b) is whether the testimony is based on matter "that is of a type that reasonably may be relied upon by an expert" and not whether the opinion is correct or even "reliable." In one case, Roberti v. Andy's Termite & Pest Control, Inc.,15 this concept was clarified. The plaintiff suffered from autism spectrum disorder, and his expert witnesses opined that exposure to pesticides during his mother's pregnancy had resulted in his condition. The defendants attacked the causation opinions, which relied on medical examination of the plaintiff and on research papers and studies in the peer-reviewed literature. The scientific literature consisted primarily of animal studies.16 The defendants' first argument for exclusion was that the opinions failed to achieve "reliability" within the meaning of the Kelly rule, and that the theory of causation did not enjoy general acceptance in the relevant medical community.17 The court rejected those arguments because the Kelly test applies only to new or novel "scientific techniques, devices, procedures or methods,"18 not medical opinion testimony. Expert medical opinions, even controversial ones, are exempt from the Kelly test when they are based on accepted diagnostic methods.19 Disagreement with the expert's conclusions does not render them a "new scientific technique."20 The court also rejected the defendants' argument that the trial court should have undertaken a foundational analysis of the science, akin to the extensive preliminary admissibility test described in Daubert. The court departed from the reasoning of Daubert, holding that expert opinion testimony in California is not subject to a threshold screening for reliability.21 Since the inquiry in California state court is different from that in federal court, the court's function is different as well. In the federal system, the court acts as a gatekeeper to ensure that the "reasoning or methodology underlying the testimony is scientifically valid,"22 while state courts are instructed to trust the trier of fact to reach appropriate conclusions about the opinions rendered by experts. As the Roberti court observed, "When a witness gives his personal opinion on the stand--even if he qualifies as an expert--the jurors may temper their acceptance of his testimony with a healthy skepticism born of their knowledge that all human beings are fallible."23 Because the expert had relied on studies and protocols of a type that reasonably may be relied upon by medical experts, the requirements of Section 801(b) were satisfied, and the testimony and opinions were admissible.
Roberti built on a theme expressed two years earlier in People v. Bui,24 which upheld an expert's reliance on scientific literature, statistical data, and an epidemiological study to support the opinion that the use of methamphetamine in greater-than-therapeutic doses results in impaired driving. Because this was a method of research generally accepted in the scientific community, and because the matters relied upon satisfied Section 801(b), the testimony did not implicate the Kelly test. Even if the opinions were controversial, they were admissible.25
While the Section 801(b) inquiry does not impose a gatekeeper function on the court to evaluate the quality of an expert's reasoning, the expert should, nonetheless, have some reasoning to link cause and effect, because an opinion untethered to its foundation does not assist the trier of fact. An example of this is Jennings v. Palomar Pomerado Health Systems, Inc.26 During surgery, a retractor had been left in the plaintiff's abdominal cavity. In the retrieval surgery, an abscess was found, and the plaintiff contracted a subcutaneous infection that kept him from returning to work. The defendants admitted negligence in leaving the retractor behind but denied that the retractor was the source of infection. The plaintiff's expert on infectious disease explained that the retractor must have been the source of the infection and that the bacteria could have migrated outward through layers of tissue, finally seeding the subcutaneous tissue where the infection flourished. As the expert witness testified, this fact pattern "just sort of makes sense." The court disagreed and excluded the expert's explanation,27 concluding that the expert's "because I say so" opinion had not supplied a reasoned explanation of the etiology connecting the starting point in the peritoneal cavity to the subcutaneous infection point. Therefore, said the court, the expert opinion did not assist the trier of fact on medical causation.28
The Lockheed Litigation Cases
Many practicing lawyers believe that the Lockheed Litigation Cases,29 decided the following year, went several steps too far by inserting into Section 801(b) an uncodified extra condition: that the matter on which the expert relies not only be of the type on which other experts rely but also reasonably support the expert's opinion.30 Lockheed was an appeal from summary judgment rendered after the exclusion of causation opinions. The plaintiffs had been occupationally exposed to five industrial chemicals. The causation opinion relied on a meta-analysis of cancer in painters who had been exposed to over 130 different compounds, including three of those to which the plaintiffs had been exposed. The court concluded that the study supplied no reasonable basis for the expert's opinions because it did not identify which of the many compounds was responsible for the greater incidence of cancer. Thus, according to the court of appeal, the trial court did not abuse its discretion in excluding the opinions under Section 801(b).31 The Lockheed court arguably took a step beyond the limitation drawn by Roberti by examining not only whether the expert's scientific data was "of a type that reasonably may be relied upon" but also whether the expert's reliance on this type of data to reach his conclusion was, in fact, reasonable. Notably, the Lockheed court reached its result without mention of Roberti, which had been decided the preceding year.
Bushling v. Fremont Medical Center32 was decided shortly after Jennings. The plaintiff in Bushling had undergone gallbladder surgery and soon after experienced pain in his shoulder. The plaintiff's expert anesthesiologist and surgeon were prepared to testify that a shoulder injury probably resulted from the plaintiff being dropped or improperly positioned during surgery, although his treating physician saw no evidence of such trauma, and there was no record of repositioning during the surgery. According to the majority opinion, the experts' opinions were properly excluded because they had done no more than to assume cause from the fact of injury.33 The experts' conclusion that the injury had "more probably than not" been caused by the defendants' negligence was speculation that was not likely to assist the trier of fact.34 On the other hand, one justice hearing the case observed, "We do not have to set aside our common sense so as to forget that the damage to plaintiff's shoulder first manifested itself on the morning following his abdominal surgery when plaintiff was in the hospital. Thus...it may be the case that 'these things just happen' but the thing that happened to plaintiff just happened to happen when plaintiff was in the hospital for abdominal surgery. This is, of course, a wholly remarkable coincidence."35 From this point of view, the experts had "established a case of negligence premised on the doctrine of res ipsa loquitur."36
In Geffcken v. D'Andrea,37 the plaintiffs argued that they had been sickened by household mold and the mycotoxins that mold produces. The trial court had excluded the plaintiffs' expert testimony on causation in domino fashion, and those rulings were affirmed on appeal. The court noted that test samples had been mishandled, and they identified only spores, not mycotoxins. Since the sampling did not answer the question of whether the plaintiffs had been exposed to mycotoxins, the evidence was excluded under Evidence Code Section 352.38 An immunological antibody assay and serology test fared no better, as neither test had achieved general acceptance in the relevant community, failing the Kelly rule applicable to new scientific methodologies.39 Since the medical expert on causation did not have sufficient foundational materials in the excluded sampling and serology, his opinions were excluded under Section 801(b) because they were not based on matter "that is of a type that reasonably may be relied upon by an expert in forming an opinion."40
The unifying theme in these cases is that an expert may appropriately base opinions on the types of matters that would be relied upon by other experts, but the opinion must assist the trier of fact. The methods must usefully (but not necessarily correctly) assimilate the data upon which the expert relies. As is also true in federal tribunals, there are no fixed guidelines or checklists. California courts embark on the evaluation of scientific testimony without the benefit of the factors discussed in Daubert.41 Thus, it is the lawyer's job to ensure that the expert has based the opinions on matter of the type that reasonably may be relied on (and the Daubert factors may be helpful but not essential). The court's role is to satisfy itself that the study or matter is appropriate to the inquiry. California trial judges are not expected to serve as amateur scientists in conducting this examination.42
Expert opinions on causation are admissible in California if based on the right matter, and the inquiry is necessarily flexible because of the vast array of scientific endeavors that may come to bear on a particular question. Experts typically assimilate an array of materials to arrive at reasoned conclusions about causation. In Roberti, for example, the court approved "generally accepted diagnostic methods and tests, including statistical studies that are not definitive,"43 as well as peer-reviewed literature (including animal studies) as appropriate predicates for the opinions. Most successful opinions linking cause and effect--and virtually all opinions doing so in the medical setting--rely upon a differential diagnosis or differential etiology, which brings data and science together in a logical sequence.
Although differential diagnosis is most commonly undertaken to arrive at opinions about human disease and its causes, the technique is also "a standard scientific technique of identifying the cause of a medical problem by eliminating the likely causes until the most probable one is isolated."44 This process is nothing more than a thoughtful, stepwise45 exercise of deductive reasoning to arrive at probable causes, which has been accepted in California courts for a long time. As the court held in People v. Jackson, "An expert medical witness may give his opinion as to the means used to inflict a particular injury based on his deduction....A medical diagnosis based on probability...is admissible; the lack of scientific certainty does not deprive the medical opinion of its evidentiary value."46
As part of the differential diagnostic process, the expert assembles and evaluates a list of potential causes (or hypotheses) that may explain the outcome.47 The expert assimilates the medical and scientific literature and data, epidemiological data, toxicological data, case reports,48 and other data appropriate to the question at hand. These are all the sorts of materials upon which experts reasonably rely in the Section 801(b) metric. To this assembled knowledge base, the expert applies his or her clinical judgment and experience, using probabilistic reasoning.49 The expert then begins the process of elimination of those potential causes to arrive at the most likely cause of the findings in a particular case.50
Because the inquiry is flexible and varies from case to case, there is no fixed checklist that defines what an expert may reasonably rely upon. It is clear, however, that the scientific data need not be conclusive and may and should be drawn from a number of sources. Experts typically rely on a broad range of materials that, taken alone, need not stand for the expert's conclusion. The U.S. Supreme Court recently explained:
[L]ack of statistically significant data does not mean that medical experts have no reliable basis for inferring a causal link between a drug and adverse events....[M]edical experts rely on other evidence to establish an inference of causation....It suffices to note that... "medical professionals and researchers do not limit the data they consider to the results of randomized clinical trials or to statistically significant evidence."51
Human data are sometimes scarce, as it is unethical and illegal to experiment on humans to accomplish much of the research that populates the peer-reviewed literature. Animal studies routinely supply information that supports the expert's opinions about biological plausibility.52 Data from animal research, cellular research, and human studies are all appropriately considered, weighed, and assimilated by the expert, applying his or her professional judgment.
An expert need not have all the answers in order to assist the trier of fact. As one U.S. District Court explained, "Causation can be proved even when we don't know precisely how the damage occurred, if there is sufficiently compelling proof that the
agent must have caused the damage somehow."53 The differential diagnosis permits the expert to establish a cause-and-effect relationship even in the absence of conclusive causal evidence.
When an expert undertakes an analysis of a spectrum of information of the type reasonably relied upon by an expert, he or she has satisfied the requirement of Section 801(b). If that opinion will assist the trier of fact, it satisfies the test posed by Section 801 and should be admissible. Any infirmities in the opinion subject its author to searing cross-examination.
One approach that has recently come under judicial scrutiny in California courts is the practice of challenging expert reasoning by nonstatutory motions, such as a motion in limine, that are nominally addressed to the admissibility of evidence. Although this practice is not uncommon, a nonstatutory motion should not address the sufficiency of evidence to establish liability, as the sufficiency test is associated with a summary judgment motion.54 A challenge to an expert's foundational data should not circumvent such procedural safeguards as those found in Code of Civil Procedure Section 437c.
The Black Box
As the Roberti55 court and others noted, an expert's opinion is not subject to the Kelly rule, which applies only to new or novel devices and processes--the figurative black box. In that narrow class of evidence to which Kelly applies, three factors guide a court's analysis: 1) the reliability of the method, 2) a properly qualified witness supplying the evidence, and 3) a showing that proper scientific procedures were used.56
By contrast, in a federal tribunal, new or novel science is evaluated in a preliminary admissibility assessment under Daubert. The federal courts thus bear a heavy burden. The nonexclusive list of factors considered by a federal trial court permits trial judges to admit cutting-edge science even when it has not earned community acceptance. The Daubert standard also imposes a responsibility on federal courts to immerse themselves in the relevant science to make assessments of admissibility, which is an often uncomfortably daunting task.57
In state courts, it is only the rare new or novel technique or device that is scrutinized under Kelly, not opinion. There is a difference between an expert's opinion testimony and the product of a machine or process.58 The reason for judicial caution for novel devices and processes is the misleading aura of infallibility potentially created by a machine. If the expert's opinion itself is what is being examined, that "black box" risk is eliminated. As one court noted, "This was not a case in which a magical device was unveiled to astound a gullible jury. It was...simply the testimony of an expert who had examined a subject and whose diagnosis might have been helpful in determining the mental state of the subject."59
The Kelly rule tests the validity of new scientific methodology, but not the degree of professionalism brought to its application. Errors in testing affect the weight of the evidence, not its admissibility.60 Kelly's reach is thus quite narrow, as it applies only to that limited class of expert testimony that is based on a novel technique,61 and it does not condition admissibility on correctness of the conclusions produced by that technique.
The California Supreme Court recently supplied additional guidance in People v. Cowan.62 In that case, an expert had performed a ballistics analysis using a process that involved lasers and castings. The expert photographed his work and was prepared to explain to the jury what the images depicted. The court explained that this testimony did not implicate Kelly concerns because neither ballistic comparison nor tool mark identification is so foreign to everyday experience as to be difficult for laypersons to understand. When the expert's work, captured in photographs, simply "isolates physical evidence whose existence, appearance, nature, and meaning are obvious to the senses of a layperson, the reliability of the process in producing that result is equally apparent and need not be debated under" the Kelly rule.63
Inferences and Probabilities
Medical and scientific opinions in California are expressed in terms of probabilities, not certainties. Opinions may be the product of deduction and inference and need not be expressed in terms of certainty. In People v. Mendibles, the court explained an expert is qualified "'to give an opinion of the cause of a particular injury on the basis of the expert's deduction from the appearance of the injury itself.'" The court also observed, "Such a diagnosis need not be based on certainty but may be based on probability; the lack of absolute scientific certainty does not deprive the opinion of evidentiary value."64
This is nothing new to scientists, as they are trained and experienced in inferential reasoning. The Federal Judicial Center's Reference Manual on Scientific Evidence defines inferential reasoning as the "reasoning process by which a physician assimilates the various findings on a given patient and forms hypotheses that lead to testing and further hypotheses until a coherent diagnosis is reached."65
These inferences and deductions are grounded in observed and known data and thus are not speculative.66 Inferential reasoning allows an expert to draw inferences and conclusions after synthesizing data, just as jurors may properly draw inferences from evidence.67 Inferential reasoning and deduction are part of the scientific expert's skill set, and scientists and physicians are permitted and expected to draw scientific inferences from the data.
California litigants and trial courts rely on expert testimony. Some researchers have estimated that nearly 90 percent of civil trials conducted in superior court involve expert testimony,68 and many of those experts testify on medical issues. Any trial is a costly proposition, and the stakes are particularly high when the investment includes medical and scientific experts. That investment can be protected by careful attention to the rules governing their admissibility.
California Evidence Code Section 801 supplies litigants and trial courts with ample and flexible guidance to assess the admissibility of scientific testimony. If the opinion will assist the trier of fact, then the driving clause of Section 801(b) instructs that the opinion is admissible if based on matter that is of a type that reasonably may be relied upon by an expert. This two-step approach to evaluating the admissibility of expert opinions ensures that expert testimony derives from scientifically appropriate matter and that it will assist the trier of fact to evaluate whether a particular expert's reliance on a particular type of study was reasonable. n