KF&C California Law Blog

June 12, 2023 | Business Litigation

Trial Court Properly Granted Summary Judgment Motion After Excluding the Only Causation Expert’s Opinion for Lack of Reliable Methodology

An expert’s opinion is properly excluded when the opinion does not contain a reliable methodology for weighing the evidence. While a court may not weigh an expert opinion’s probative value or persuasiveness, it must consider whether the opinion is logically sound, i.e., whether the matter relied on can provide a reasonable basis for the opinion and is not a leap of logic or conjecture.

Expert testimony is generally required when proof of an element of a claim or defense calls for testimony that is outside the jurors’ common knowledge. California Evidence Code sections 801 and 802 set forth certain standards governing the admissibility of expert opinion testimony. The California Supreme Court clarified the standards under sections 801(b) and 802 in Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747. As the Supreme Court explained, a trial court must act as “a gatekeeper” under sections 801(b) and 802 “to exclude expert opinion testimony that is (1) based on matter of a type on which an expert may not reasonably rely, (2) based on reasons unsupported by the material on which an expert relies or (3) speculative.” Id. at 771-72.

A recent case, Onglyza Product Cases (2023) 90 Cal.App.5th 776, reh’g denied (May 9, 2023), review filed (May 30, 2023), offers further insights into how courts apply the Sargon standard. In Onglyza, plaintiffs alleged injuries from saxagliptin, the main ingredient found in two medications manufactured and distributed by defendants to treat type-2 diabetes. Following expert discovery, defendants first moved to exclude plaintiffs’ causation expert’s opinions, including that saxagliptin can cause heart failure. After the trial court granted the motion to exclude, defendants then moved for summary judgment on the ground that, without expert testimony, plaintiffs could not show a triable issue of material fact as to general causation. The trial court agreed, and in the same order denied plaintiffs’ request to extend discovery deadlines to allow them to identify a new causation expert. The Court of Appeal affirmed the trial court’s decision.

As the Court noted, “whether the 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.” Onglyza, 90 Cal.App.5th 776.

The goal of trial court gatekeeping under Sargon is not to resolve scientific controversies, but to exclude “clearly invalid and unreliable” expert opinion. A “circumscribed inquiry” is conducted 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.” Onglyza, 90 Cal.App.5th 776 (citation omitted); id. (“While we agree that the trial court may not weigh an expert opinion’s probative value or persuasiveness, it must still consider whether the opinion is logically sound.”).

Because the trial court explained in its detailed exclusion order that “[the expert’s] opinion does not contain a reliable methodology for weighing the evidence but a shifting results-based methodology that fails to logically and consistently weigh all relevant evidence,” the Court of Appeal found no abuse of discretion.

With respect to defendants’ summary judgment motion, the Court of Appeal pointed out that, “[t]he law is well settled that in a personal injury action causation must be proven within a reasonable medical probability based upon competent expert testimony. This is especially true when the cause of a disease or harm is beyond the experience of laymen and can only be explained through expert testimony.” Id. (citations and quotation marks omitted.) Therefore, none of the plaintiffs’ non-expert evidence (since the motion to exclude plaintiffs’ expert was properly granted) were sufficient to create a triable issue of fact as to general causation.

Finally, plaintiffs sought to identify a new expert after their only expert was excluded. The Court of Appeal stated, “[a]lthough the court’s decision to exclude [plaintiffs’ sole expert] may have been unexpected to plaintiffs, they made the strategic decision to identify only one expert in this area, despite knowing how crucial it was to prevail on the issue of general causation.” Id. As such, the Court found that the trial court did not abuse its discretion in concluding that allowing plaintiffs to designate a new expert would prejudice defendants given the amount of time and resources needed to conduct additional expert discovery.

Practice Tips

The court’s gatekeeping obligation for an expert’s testimony requires the court to inquire into not only the type of material on which the expert relies, but also whether that material supports the expert’s reasoning. As discussed above, in Onglyza, the plaintiffs’ expert’s failure to base his opinion on a reliable methodology was fatal to the entire case. The plaintiffs not only lost on the dispositive motion after the expert was excluded, but also were denied any opportunity to designate a new expert. Clearly, for practitioners, expert designation is a very strategic move that must be carefully contemplated. Based on Onglyze, it may be wise to consider designating multiple experts when expert testimony is needed to prove the case. And in any event, make sure your experts’ opinions—and the methodologies employed in reaching them—are adequately supported and explained.

For further questions or comments on the content of this post, please email Matt Cave at [email protected] or Sara Borjigin at [email protected].

Matthew J. Cave

Founding Partner

(310) 409-0475

Shiqi (Sara) Borjigin


(310) 409-4678

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