The Daubert and Frye expert witness standards are the two competing tests American courts use to decide whether an expert’s testimony is reliable enough for a jury to hear. Frye, the older rule, asks a single question: has the expert’s method gained “general acceptance” in its field? Daubert asks more. It makes the trial judge a gatekeeper who weighs several factors, including testability, peer review, known error rates, professional standards, and yes, general acceptance too. Federal courts use Daubert. Most states have followed. A handful of large jurisdictions still apply Frye, and that split matters because the same expert can be admitted under one standard and excluded under the other.
What the Frye Standard Asks
Frye comes from a 1923 D.C. Circuit decision on an early blood-pressure-based lie detector. The court refused to admit the results and announced a rule that governed scientific evidence for the next seventy years: a technique is admissible only if it has gained general acceptance among experts in the relevant field.1Justia Law. Frye v. United States
Under Frye, the judge doesn’t independently evaluate the science. The judge looks outward at the scientific community and asks whether practitioners have coalesced around the method. That’s the whole test.
The strength of Frye is also its limitation. It keeps novel or experimental techniques out of court until the underlying community endorses them, which filters out junk science. But it can also block reliable new methods that haven’t yet accumulated enough published endorsements to clear the acceptance threshold. A forensic technique might produce solid results for years before it satisfies Frye.
What the Daubert Standard Asks
The Supreme Court replaced Frye in federal court with its 1993 decision in Daubert v. Merrell Dow Pharmaceuticals.2Justia U.S. Supreme Court Center. Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993) Daubert assigns the trial judge a gatekeeping role: before any expert testimony reaches the jury, the judge must find the expert’s reasoning and methodology scientifically valid and relevant to the case.
The Court identified factors the judge should consider. None of them is required and none is dispositive:
- Whether the theory or technique can be, and has been, tested
- Whether it has been subjected to peer review and publication
- The known or potential error rate
- Whether there are standards governing how the technique is performed
- Whether the method is generally accepted in the relevant field
General acceptance survives as one factor. It is no longer the only one. A method that hasn’t yet achieved consensus can come in if it holds up on the other factors, and a widely accepted method can be excluded if the expert applied it sloppily to the case at hand.
How the Two Standards Treat the Same Expert Differently
Imagine a forensic analyst using a new imaging technique that has been peer-reviewed, produces a documented error rate under 2%, and follows published protocols, but hasn’t yet been embraced by a majority of the field. Under Daubert, that testimony has a strong shot: three of the five factors clearly favor admission, and the judge weighs them together. Under Frye, the same testimony likely fails, because the general-acceptance question is the only question and the answer is not yet.
Run it the other way. A long-established technique that most practitioners still use, but that has never been rigorously tested and has no known error rate, sails through Frye on acceptance alone. Under Daubert, the judge can look past acceptance and exclude it because the underlying reliability isn’t shown. This is why identifying which standard governs is one of the first strategic decisions in any case where expert testimony will be contested.
Which Standard Applies in Your Case
Federal courts apply Daubert to all expert testimony.2Justia U.S. Supreme Court Center. Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 U.S. 579 (1993) State courts split. The majority have adopted a version of Daubert. A handful of significant jurisdictions still use Frye, including California, Illinois, New York, Pennsylvania, and Washington. A few states use hybrid approaches that borrow from both.
The practical impact is real. An expert whose testimony survives in federal court can face exclusion in a state court across the street, or the other way around. Getting the standard wrong at the outset can waste months of preparation and substantial expert fees on a witness who never testifies.
How an Expert Qualifies Under Either Standard
Before a court reaches the reliability question, the witness has to be qualified to give the opinion in the first place. Federal Rule of Evidence 702 lists five pathways to qualification: knowledge, skill, experience, training, and education.3Legal Information Institute. Federal Rule of Evidence 702 These are alternatives, not cumulative requirements. A witness can qualify through any combination of them.
Formal credentials help but aren’t mandatory. A mechanic with decades of hands-on diagnostic experience can qualify as an expert on engine failures without an engineering degree. What courts look for is whether the person’s background gives them insight an ordinary juror wouldn’t have. Advanced degrees, professional certifications, published research, and years of practical work all count. The party calling the expert walks the witness through their background at the start of testimony, and the opposing side gets to challenge whether the qualifications actually match the opinions being offered.
Rule 702 then adds four substantive conditions on the testimony itself:3Legal Information Institute. Federal Rule of Evidence 702
- The specialized knowledge will help the jury understand the evidence or resolve a factual dispute
- The testimony is based on sufficient facts or data
- The testimony is the product of reliable principles and methods
- The expert has reliably applied those methods to the facts of the case
The 2023 Amendment to Rule 702
On December 1, 2023, an amendment took effect that added an explicit burden of proof: the party offering the expert must show admissibility by a preponderance of the evidence.4Legal Information Institute. Federal Rule of Evidence 702 – Section: Committee Notes on Rules 2023 Amendment In plain terms, the court must find it more likely than not that the methods are reliable and were reliably applied. Before the amendment, some courts had drifted toward a looser standard, letting shaky testimony through for the jury to weigh. The change reinforces that the judge, not the jury, decides reliability.
Challenging or Defending an Expert
The standard mechanism for challenging an expert is a motion in limine filed before trial, typically after discovery closes, asking the judge to exclude the testimony on reliability or qualification grounds. The judge rules without the jury present.5Legal Information Institute. Motion in Limine The proceeding is often called a Daubert hearing, though format varies. Some judges hold full evidentiary hearings with live expert testimony and cross-examination. Others decide the motion on paper, reviewing depositions, affidavits, and the expert’s report.
The three most common grounds for exclusion track Rule 702 directly. Courts throw out testimony when the expert reached conclusions without sufficient supporting data, when the methodology itself isn’t reliable or can’t be replicated, or when the methodology was sound but was misapplied to the facts of the specific case.
Losing this motion can end a case. In many civil actions, the plaintiff can’t prove an essential element (causation in a product liability suit, for example) without expert testimony. Exclude the expert, and the defendant follows with a summary judgment motion arguing there’s no admissible evidence on that element. Courts grant these motions regularly, and defense attorneys pair Daubert challenges with summary judgment motions as a deliberate strategy.
A Note on Lay Witness Opinions
Neither Daubert nor Frye applies to ordinary witnesses giving everyday opinions. Rule 701 lets a non-expert offer opinions based on personal perception, so long as they’re helpful to the jury and don’t rest on the specialized knowledge Rule 702 covers.6Legal Information Institute. Federal Rule of Evidence 701 A factory worker can testify that a machine sounded unusual on a particular day. The moment the testimony shifts to explaining why the bearing failed at a metallurgical level, it crosses into expert territory and triggers whichever reliability standard the court applies.
That line matters because parties sometimes try to dress up expert opinions as lay observations to sidestep the reliability requirements. Courts watch for it, and the same witness can give both types of testimony in one case as long as the expert portions clear Rule 702.