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A trial court's preliminary determination whether an expert opinion is founded on sound logic is not a decision on its persuasiveness. The court must not weigh an opinion's probative value or substitute its own opinion for the expert's opinion. Rather, the court must simply determine whether the matter relied on can provide a reasonable basis for the opinion or whether that opinion is based on a leap of logic or conjecture. The court does not resolve scientific controversies. Rather, it conducts a circumscribed inquiry to determine whether, as a matter of logic, the studies and other information cited by experts adequately support the conclusion that the expert's general theory or technique is valid. The goal of trial court gatekeeping is simply to exclude clearly invalid and unreliable expert opinion. In short, the gatekeeper's role is to make certain that an expert, whether basing testimony upon professional studies or personal experience, employs in the courtroom the same level of intellectual rigor that characterizes the practice of an expert in the relevant field.
Plaintiffs, a patient and his wife, sued three related pharmaceutical companies, alleging that the patient had developed bladder cancer from ingesting a prescription drug manufactured by the companies for the treatment of type 2 diabetes mellitus. A jury found the companies liable on causes of action for strict liability failure to warn, negligent failure to warn, and loss of consortium and awarded damages to plaintiffs. After the verdict, the trial court struck the causation testimony of plaintiffs’ expert witness, a urologic oncologist, concluding that the testimony was speculative and lacking in foundation. The trial court then granted the companies’ motion for judgment notwithstanding the verdict and their motion for new trial, and subsequently entered judgment in favor of the companies. The present appeal followed.
The court held that the trial court erred in striking the testimony of plaintiffs’ expert because, by requiring that the expert rule out all other possible causes for the patient’s bladder cancer, even where there was no substantial evidence that other such causes might be relevant, the trial court exceeded the proper boundaries of its gatekeeping function under Evid. Code, §§ 801, 802, in determining the admissibility of the complex scientific testimony. Because the evidence supported giving a jury instruction on multiple causation, the trial court erred in granting the companies' new trial motion on the ground that the instruction should not have been given and resulted in prejudice to the companies. Accordingly, the judgments were reversed and the matter was remanded with directions.