This section contains the nature of the case and procedural background.

P served as a groundskeeper and golf course superintendent at several Pittsburgh area golf courses. His work involved the regular application of various pesticides. P kept a detailed record of his activities including a detailed log of the specific products and the dates of their applications. In the early years, gloves were the only protective gear used, although in later years they also wore overalls, rubber boots, and masks. Eventually, they began wearing disposable protective gear. Even with these protections, a co-worker explained, exposure to the dust from the pesticide products would occur while opening the bags and mixing the chemicals. On October 5, 2008, P was suffering from fever, chills, and a cough when he arrived at an emergency room. A bone marrow biopsy resulted in a diagnosis of Acute Myelogenous Leukemia (AML). Cytogenetic testing revealed significant chromosomal aberrations. On February 2, 2009, Walsh died. His treating oncologist, James Rossetti, D.O., later opined that P's extensive exposure to pesticides raised a high degree of suspicion that said exposure played a significant role in the development of his AML. P sued Ds for wrongful death and survival action against the manufacturers of various pesticides that he had applied during his career, asserting claims of strict products liability, negligence, and breach of warranty. Based upon a lack of expert testimony identifying various pesticides as substantial contributing factors, the trial court granted summary judgment in favor of a large number of manufacturers. Three of the current Ds filed a Frye motion to exclude the testimony of the Executor's expert witnesses, Nachman Brautbar, M.D. (Dr. Brautbar) and April Zambelli-Weiner, Ph.D. (Dr. Zambelli-Weiner). Ds alleged that Drs. Brautbar and Zambelli-Weiner failed to apply methodologies generally accepted in the relevant scientific communities. or general causation, Dr. Brautbar described the generally accepted methodology he employed as follows: (1) identify all relevant studies, (2) read and critically evaluate all the relevant studies, (3) evaluate all the data based upon recognized scientific factors (the Bradford Hill viewpoints) and other factors relevant to the chemical and the disease; (4) exercise best professional judgment in reaching a conclusion on the issue of whether a particular chemical or class of chemicals can cause a particular disease; and (5) explain the factual basis and the reasoning supporting the conclusion. With respect to causality, Dr. Brautbar indicated that he utilized the 'Bradford Hill criteria' or 'viewpoints,' a set of nine factors used to determine whether a recognized association is in fact a causal link. In his 1965 article, Sir Bradford Hill indicated that his new methodology was necessary to decide whether an observed association between a potential toxin and a particular disease was in fact the outcome of an actual causal relationship. The nine criteria are (1) consistency of the observed association, (2) strength of the observed association, (3) specificity of the observed association, (4) temporal relationship of the observed association, (5) biological gradient, (6) biological plausibility, (7) coherence, (8) experimental evidence from human populations, and (9) analogy. According to Dr. Brautbar, these criteria are widely accepted in the relevant scientific community, including by scientific bodies (e.g., the World Health Organization) and in modern textbooks on epidemiology, occupational and environmental medicine, and toxicology. Dr. Brautbar stresses that there is no formula or algorithm for assessing the factors, but rather it requires judgment and scientific expertise to measure the weight and significance of each factor when conducting a particular evaluation. Dr. Brautbar provided a detailed analysis of his observations and conclusions with respect to each factor, including 'the facts and studies that I rely on [to] provide evidence supportive of causality under each of the Bradford Hill factors... .' He explained that the differing conclusions of the Appellants' experts 'is due to differences in professional judgment, rather than methodology,' indicating that 'different experts may reach differing conclusions based upon their best exercise of professional judgment, even though they may employ the same generally accepted methodology, they may rely on generally the same body of literature and may base their opinion on the same facts relative to the patient and the workplace.' Dr. Brautbar conducted his Bradford Hill factors analysis on the association between AML and pesticides. He then proceeded to conduct product-specific analyses of each product manufactured by Ds, including detailed tables attached to his report identifying every exposure that P had to each product (including by date and specific product) as well as the carcinogenic potential for the active ingredients in each product. Dr. Brautbar opined that the causal link could be demonstrated indirectly. In particular, he identified studies that demonstrated that the active ingredients in these specific pesticides are genotoxic and thus cause chromosomal abnormalities. In what he refers to as the 'fingerprint theory,' he cited studies showing that when exposure to pesticides or benzene causes AML, cytogenetic review identifies a unique set of chromosomal abnormalities (abnormalities of the fifth and seventh chromosomes), which in turn start the process to carcinogenicity. According to Dr. Brautbar, this specific pattern of chromosomal abnormalities was identified in the cytogenetic testing performed on P. Dr. Zambelli-Weiner conducted what she described as a critical review of 'the published epidemiological literature on pesticide exposure and leukemia.' She employed a variety of methodologies, 'including reviews, meth-analysis, weight-of-the-evidence analyses and application of the Bradford Hill criteria for causality.' She concluded that 'exposure to organophosphate pesticide formulations, individually or in combination, is causally related to an increased risk of leukemia in humans exposed to them.' The court ordered the parties to conduct depositions and file briefs. Upon review of these materials and after oral arguments by counsel, the trial court granted Ds' Frye motions by written opinion. A Frye hearing was never conducted. The court concluded that Dr. Brautbar's expert report did not establish either general or specific causation for pesticides not containing benzene. With respect to pesticides containing benzene, the trial court determined that Dr. Brautbar's expert report demonstrated general causation, as it cited numerous studies showing that exposure to benzene 'at some level' may cause AML. The trial court rejected Dr. Brautbar's specific causation analysis with regard to benzene products because it concluded that the reports and studies he cited did not support the contention that low-level exposures to benzene could cause AML. The court refused to credit Dr. Brautbar's alternative theory of specific causation, namely the 'fingerprint theory' that Dr. Brautbar claimed resulted in no cases of idiopathic instances of AML (no known external cause). The parties stipulated to the entry of summary judgment 'in favor of all remaining defendants and against P. P appealed. The appeals court reversed. The court affirmed that the application of the Frye standard is necessary when a trial court has 'articulable grounds to believe that an expert has not applied generally accepted scientific methodology in a conventional fashion in reaching his conclusions.' It held that the trial court, by making itself the arbiter of what research could be scientifically relied upon to support an expert's opinion, had overstepped its 'gatekeeper' function because that is 'not the proper role of the trial court in a Frye inquiry.' It reaffirmed that the Frye test was adopted to avoid having judges try to understand the complexities of modern science, as it is better to allow the scientists to do that to ensure reliability. The Court held that an expert need not rely on studies that mirror the exact facts under consideration. It is sufficient if the synthesis of various legitimate studies reasonably permits the conclusion reached by the expert. The absence of a treatise or study directly on point goes to the weight, not the admissibility, of expert opinion. An expert's opinion will satisfy Frye when it is deduced from generally acceptable scientific principles and supported by studies or literature, even where the expert could not point to one study involving precisely parallel circumstances. The court overturned the trial court. Ds appealed.

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