Ask a defense lawyer to move against your expert and the word you will hear is "Daubert." Ask the judge, and you may get a blank look, because California never adopted Daubert v. Merrell Dow Pharmaceuticals (1993) 509 U.S. 579. The federal standard, built on Rule 702, does not run in state court. What runs instead is a patchwork the California Supreme Court has assembled over four decades: the Kelly rule for new scientific techniques, and Sargon for the reliability of an expert's reasoning. The two do different work, and conflating them is how good motions get denied.
For a plaintiff PI practice, the stakes are concrete. Your accident reconstructionist, your treating physician on future care, your economist on lost earning capacity — each is a target, and each is a target you will aim at when it is the defense's turn. Knowing which case governs which challenge, and what a trial judge is actually allowed to do, is the difference between a motion in limine that shapes the trial and one that reads as a jury argument dressed up as an evidentiary objection.
California Is Not a Daubert State, and the Difference Is Structural
The confusion is understandable. Federal courts and California courts both put the judge in a gatekeeping role, both police speculative opinion, and both use the vocabulary of reliability. But the source of authority differs, and so does the scope. In federal court, Rule 702 and the Daubert/Kumho Tire line ask the judge to assess the methodology behind any expert opinion — testability, error rate, peer review, general acceptance — as a flexible set of factors.
California's gatekeeping comes from the Evidence Code, principally sections 801 and 802, as read by Sargon Enterprises, Inc. v. University of Southern California (2012) 55 Cal.4th 747. The Sargon court was explicit that it was not importing Daubert's factor list. Section 801(b) limits expert opinion to matter "of a type that reasonably may be relied upon" by an expert in the field, and section 802 lets the court inquire into the reasons for and the basis of the opinion. That is the whole toolkit. There is no error-rate prong, no published-methodology checklist. A motion that cites Daubert factors as if they were binding is citing the wrong law, and a sharp opponent will say so.
Sargon and the Reliability of the Reasoning
Sargon is the case you will use most, because most PI expert fights are not about a novel technique — they are about whether the opinion follows from the data. The plaintiff there was a small dental-implant company projecting lost profits in the tens of millions after a botched clinical study, and the trial court excluded the damages expert. The Supreme Court affirmed, and in doing so drew the line that matters: the trial court may exclude expert opinion that is speculative, or that rests on assumptions with no evidentiary support, or where there is "too great an analytical gap between the data and the opinion offered."
The limit on that power is just as important. The court does not decide whether the expert's conclusion is correct. It does not weigh the opinion against a competing expert. It asks only whether the reasoning is supported by the material the expert relied on — a threshold question of admissibility, not a verdict on credibility. When you draft a Sargon motion, you win by showing the gap: the economist who assumes a promotion the plaintiff never received, the life-care planner who prices a surgery no treating physician has recommended. You lose when the motion is really an argument that your expert is more persuasive, which is a question for the jury.
This cuts both ways in damages-heavy litigation. When you are the one projecting a reduced work-life or a shortened lifespan, the assumptions had better be tethered to record evidence and to a methodology the expert can defend on the stand. The same theory that drove the reduced-lifespan damages fight in that Illinois mesothelioma verdict lives or dies in California on whether the expert can close the analytical gap between the diagnosis and the number. Build the foundation in the report, not in the deposition after the defense has already moved.
Kelly: The Rule Formerly Known as Kelly-Frye
The older doctrine is People v. Kelly (1976) 17 Cal.3d 24, which for years lawyers called "Kelly-Frye" after the federal Frye general-acceptance test. That hyphenate is now wrong. Once Daubert superseded Frye in federal court, California kept the general-acceptance standard on its own state-law footing and, in People v. Leahy (1994) 8 Cal.4th 587, told practitioners to drop the "Frye" half of the name. It is the Kelly rule.
Kelly is narrow, and that narrowness is the point. It applies only to a new scientific technique — a method that carries a "misleading aura of certainty" because it comes wrapped in machinery or algorithms a jury may over-trust. Where it applies, the proponent must show the technique is generally accepted in the relevant scientific community, that the witness is qualified to give the opinion, and that correct procedures were followed. Where it does not apply, invoking it is a distraction.
Most PI expert testimony is not Kelly material. An orthopedist reading an MRI, a reconstructionist applying momentum equations, a biomechanic opining on injury causation — these are established methods, and courts routinely hold that Kelly does not reach them. Save the Kelly challenge for genuinely new instrumentation or emerging analytics the other side is presenting as settled science. Point a Kelly motion at a familiar method and you will draw a citation to Leahy and a denial.
Sanchez and the Case-Specific Hearsay Trap
The quietest expert-killer in the last decade is People v. Sanchez (2016) 63 Cal.4th 665. It is a criminal case, but its rule reaches civil trials and it reshapes how much an expert can say. An expert may still rely on background material and general knowledge in the field. What the expert may not do is relate, as true, case-specific out-of-court statements that are not independently proven or admitted under a hearsay exception. The expert cannot become a conduit for facts nobody else established.
For PI work the applications are everywhere. A retained physician who recites the contents of records from providers who never testify, an economist who repeats the plaintiff's own account of a job offer, a defense expert who narrates a surveillance investigator's notes — each risks a Sanchez objection. The remedy is to get the underlying facts in through a competent witness or a records custodian, then let the expert opine on them. When it is the defense expert doing the reciting, a well-placed Sanchez objection can strip the opinion of the very facts that make it sound authoritative. This is the same discipline that governs whether a product-defect expert can tie a design theory to admissible proof rather than to assumptions the jury never hears tested.
How the Motions Actually Run
Doctrine is only half of it; the procedure decides outcomes. Expert challenges in California surface through a motion in limine, usually with a request for an Evidence Code section 402 hearing — the foundational hearing held outside the jury's presence where the judge can take the expert on voir dire before ruling. Ask for the 402 hearing expressly. A paper-only motion invites the judge to defer the whole question to trial, and a deferred ruling is a ruling you have effectively lost, because the expert is now testifying while you object.
Timing is everything. The record you need for a Sargon gap or a Sanchez problem is built in the expert deposition, so depose to the assumptions: pin down every fact the opinion rests on and ask what supports each one. Then the in limine motion writes itself from the transcript. Preserve the objection on the record at trial even after a favorable in limine ruling, because in limine rulings are often provisional. And weigh the strategic cost of moving at all — a motion that merely previews your cross can hand the other side a script for repair. Sometimes the better play is to let a shaky expert testify and dismantle the foundation in front of the jury, a calculation that belongs in the same column as your 998 and cost-shifting strategy when you are pricing the risk of trial.
Fitting the Tool to the Target
The through-line is that California gives you three distinct instruments, and each fails when pointed at the wrong target. Kelly is for new science and almost nothing else. Sargon is for the analytical gap between data and conclusion, not for who is more convincing. Sanchez is for the facts an expert smuggles in that no one proved. Match the motion to the defect, ask for the 402 hearing, and build the record in deposition — and the expert fight stops being a coin flip and starts being something you can plan around before the first witness is sworn.