The Daubert Factors

The sentence that comes immediately before the list

The considerations attorneys call the Daubert factors appear at 509 U.S. 593–94, and the Court introduced them with a disclaimer that is part of the holding rather than an aside: “Many factors will bear on the inquiry, and we do not presume to set out a definitive checklist or test.” A page later it added, of the inquiry Rule 702 envisions, that it “is … a flexible one.” 509 U.S. at 594.

Both sentences are routinely dropped when the factors are reproduced, and dropping them inverts what the opinion says. The list is a set of considerations a district judge may find useful in assessing whether reasoning or methodology is scientifically valid. It is not a set of elements, and nothing in the opinion, in FRE 702 or in the Advisory Committee Notes requires a court to work through it item by item or an expert to satisfy every entry. Six years later Kumho Tire restated the point in terms that leave no room: the test of reliability is flexible, and the list of specific factors “neither necessarily nor exclusively applies to all experts or in every case.”

The accurate phrase is the non-exclusive factors identified in Daubert. The phrase the four-part Daubert test describes something the Court declined to create, and a brief that uses it hands the other side a quotation that answers it.

The considerations the Court actually named

Whether the theory or technique can be, and has been, tested

“Ordinarily, a key question to be answered in determining whether a theory or technique is scientific knowledge that will assist the trier of fact will be whether it can be (and has been) tested.” This is the falsifiability consideration, and it carries the most weight of the four in scientific disputes. A method that cannot in principle be shown to be wrong is not being assessed for reliability at all.

Whether it has been subjected to peer review and publication

“Another pertinent consideration is whether the theory or technique has been subjected to peer review and publication.” The opinion treats publication as evidence bearing on validity rather than as a proxy for it, and notes that some propositions are too particular or too new to have attracted a literature.

The known or potential rate of error, and controlling standards

“The court ordinarily should consider the known or potential rate of error … and the existence and maintenance of standards controlling the technique’s operation.” Two ideas sit in one sentence: what the method gets wrong and how often, and whether there is a protocol governing its application that the examiner in fact followed.

General acceptance

“Finally, ‘general acceptance’ can yet have a bearing on the inquiry.” The test the decision displaced was not abolished as a consideration; it was demoted from a sufficient and necessary condition to one factor among several.

Why the count varies between four and five

The opinion does not number the considerations, and it does not say how many there are. The variation in the count comes from a single sentence: error rate and the existence and maintenance of controlling standards are stated together, and different writers treat them as one item or as two. Neither reading is wrong, which is the clearest available evidence that the list was not meant to be enumerated.

The practical consequence is small but real. A brief that announces “the four Daubert factors” and then argues five, or that treats an unaddressed factor as a failure of proof, invites a response that begins with the checklist sentence and does not have to go much further. Counting them is not the exercise; matching them to the method in front of the court is.

There is a further reason the list was never going to harden into elements. The reliability discussion was not the unanimous work of the Court. Chief Justice Rehnquist, joined by Justice Stevens, agreed that the Federal Rules displaced Frye but dissented from the remainder of the opinion, objecting that its general observations about reliability were unnecessary to the decision and offered vague guidance to district judges. On a strict reading, the factor discussion is the portion of the opinion most exposed to being characterized as dictum. That argument has since been overtaken: Kumho Tire treated the considerations as governing guidance in 1999, and the 2000 amendment to FRE 702 wrote the reliability requirements they serve into the rule. But it explains why the opinion is worded as illustration rather than as instruction.

Which experts the criteria reach, and which they fit

Two different questions are often collapsed into one. Kumho Tire Co. v. Carmichael, 526 U.S. 137 (1999), settled the first: the gatekeeping obligation reaches technical and other specialized knowledge, not only science, so no expert is outside the reliability requirement. It answered the second in the opposite direction: the Daubert list “neither necessarily nor exclusively applies to all experts or in every case,” and the trial judge has “considerable leeway in deciding in a particular case how to go about determining whether particular expert testimony is reliable.”

What replaces the list where it does not fit is the standard the same opinion supplies: the obligation 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.” For an experience-based witness, that formulation does more work than peer review or error rate ever will — it asks what a competent practitioner would have done outside litigation, and whether this witness did it.

The reliability criteria that are not in the opinion at all

The Advisory Committee Note to the 2000 amendment to FRE 702 collects five further considerations that courts had found relevant before and after 1993. They are cited far less often than the Daubert list and are frequently the more workable set, particularly where the method is not experimental:

• whether the expert is “proposing to testify about matters growing naturally and directly out of research they have conducted independent of the litigation, or whether they have developed their opinions expressly for purposes of testifying”;
• whether the expert “has unjustifiably extrapolated from an accepted premise to an unfounded conclusion”;
• whether the expert “has adequately accounted for obvious alternative explanations”;
• whether the expert “is being as careful as he would be in his regular professional work outside his paid litigation consulting”;
• whether the field “is known to reach reliable results for the type of opinion the expert would give.”

Three of the five are about the conduct of this witness in this engagement rather than the abstract validity of a technique, which is why they travel so well into disciplines where nothing is peer reviewed. The Committee’s own framing is that these are additional factors courts have found relevant, not a competing list — the reliability inquiry has never had a closed set of inputs.

What the criteria do not measure

They do not measure whether the opinion is right. The Advisory Committee, quoting In re Paoli R.R. Yard PCB Litigation, 35 F.3d 717, 744 (3d Cir. 1994), put it as plainly as it can be put: “The evidentiary requirement of reliability is lower than the merits standard of correctness.” A method can be reliable and the conclusion still lose in front of the jury.

Nor do they permit the court to look only at method and never at result, which is how the 1993 sentence “[t]he focus, of course, must be solely on principles and methodology, not on the conclusions that they generate” (509 U.S. at 595) is often deployed. General Electric Co. v. Joiner, 522 U.S. 136 (1997), qualified it four years later: “conclusions and methodology are not entirely distinct from one another,” and a court “may conclude that there is simply too great an analytical gap between the data and the opinion proffered.” 522 U.S. at 146. Rule 702(d), as amended in 2023, is written on the Joiner side of that line: the opinion must reflect a reliable application of the method to the facts of the case.

How the considerations are actually used in a challenge

On the proponent’s side, the factors function as an organizing device for a showing the rule already requires: identify the method, show what makes it reliable in terms the field itself would recognize, and show that the opinion stays inside what the method supports. Where a listed factor does not apply, the more durable answer explains why the criterion is inapt for this kind of method and identifies what does bear on reliability instead, rather than leaving a silent gap.

On the opponent’s side, the factors are a source of questions rather than a scoring sheet. The 2000 Note’s five are usually the more productive line of examination, because they are answerable from the file: what was the opinion developed for, what alternatives were considered and rejected, what does the method support and where does the report go further. Bill Hartzer, who has answered those questions from the witness chair rather than asked them from counsel table, notes that a factor-by-factor recitation in a deposition is easy to answer in the abstract, while a question about what a competent practitioner in the field would have done differently outside litigation is not.

Primary sources: the opinion at 509 U.S. 579 and Rule 702 with the 2000 and 2023 Committee Notes.

Frequently Asked Questions

Published 2026-08-12