The following article originally appeared on Law360 (Part 1 and Part 2).
In the 2013 case Comcast Corp. v. Behrend, the U.S. Supreme Court explained that a party seeking class certification must satisfy the requirements of Federal Rule of Civil Procedure 23 through evidentiary proof.
What few practitioners may recall, however, is the question for which the court granted certiorari: “Whether a district court may certify a class action without resolving whether the plaintiff has introduced admissible evidence, including expert testimony, to show that the case is susceptible to awarding damages on a class-wide basis.”
While the court’s opinion stressed the need for evidentiary proof, it did not directly address whether the proffered evidence must be admissible.
As a result, the question of whether plaintiffs moving to certify a class must use admissible evidence remains unresolved. In this article, we survey the circuit approaches to this question.
This survey reveals a patchwork of differing decisions. The circuits are clearly split as to whether evidence must be admissible to be considered at class certification. Yet what is most notable is that the circuits take different approaches to whether fact or expert evidence must be admissible to be considered at class certification.
When it comes to fact evidence, the current plurality position — expressly adopted by the Sixth, Seventh and Ninth Circuits and followed by district courts in several other circuits — is that a district court may consider inadmissible fact evidence at class certification.
When it comes to expert evidence, the plurality position — adopted by the Third, Fifth, Ninth, Eleventh and D.C. Circuits — is that a district court must conduct a full Daubert analysis and as a result consider only admissible expert evidence.
Thus, the circuits are split not only on whether evidence must be admissible at all, but even on the question of whether fact and expert evidence should be treated equivalently in this regard.
Our survey covers the decisions of each circuit court of appeals regarding the admissibility of both fact and expert evidence. Where a court of appeals has not directly addressed the issue, we examine relevant holdings suggestive of that court’s approach. Where a court of appeals has not had occasion to opine on evidence at class certification at all, we consider how district courts within the circuit approach fact and expert evidence at class certification.
Following this survey, we offer several observations regarding the current state of the law.
At bottom, the different treatment of fact and expert evidence cannot be reconciled. Moreover, decisions holding that evidence — whether fact or expert — need not be admissible to be considered at class certification appear to ignore the plain language of the Federal Rules of Evidence and the Supreme Court’s guidance regarding evidentiary proof.
Yet while the circuits appear riven over how to treat evidence at class certification, the Supreme Court has conspicuously refused to take up the question.
As a result, these differing standards are likely to remain in place until the circuits coalesce around consensus positions or the Supreme Court decides to take up the issue once more.
Fact Evidence at Class Certification
The Plurality Position: Inadmissible Fact Evidence May Be Considered at Class Certification
The Sixth, Eighth and Ninth Circuits have all held that fact evidence need not be admissible at trial to be considered at class certification. The U.S. Court of Appeals for the Sixth Circuit‘s 2021 opinion in Lyngaas v. Ag is the most recent appellate decision endorsing this position.
In Lyngaas, the district court certified a class of recipients of unsolicited faxes in purported violation of the Telephone Consumer Protection Act. On appeal, defendants argued that “Lyngaas failed to put forth admissible evidence showing which individuals actually received the fax advertisements,” instead relying on unauthenticated summary-report logs that purported to show fax recipients.
The Sixth Circuit rejected this argument, declaring it was joining the Eighth and Ninth Circuits by holding that the evidentiary proof needed to satisfy Rule 23 “need not amount to admissible evidence, at least with respect to nonexpert evidence.”
The court explained it found the reasoning of its sister circuits persuasive in that because class certification “must occur at an early practicable time” and is inherently tentative, requiring presentation of only admissible evidence would “transform a preliminary stage into an evidentiary shooting match.”
The U.S. Court of Appeals for the D.C. Circuit, in an unpublished per curium opinion in In re: Rand Corp. from 2002, arguably prior to the Supreme Court’s modern class action jurisprudence, also adopted this position.
The court noted, “the propriety of a district court’s refusal to scrutinize for admissibility and probative value evidence proffered to demonstrate that the requirements of [Rule 23] are satisfied is well-settled.”
At least one district court, the U.S. District Court for the District of Columbia in its 2006 opinion in Disability Rights Council of Greater Washington v. Washington Metropolitan Area Transit Authority, has cited this opinion holding “evidence need not be admissible at trial in order to be submitted for purposes of class certification.”
The Minority Position: In the Fifth Circuit, Fact Evidence Must Be Admissible at Class Certification
Only one court of appeals, the U.S. Court of Appeals for the Fifth Circuit, has expressly held plaintiffs must satisfy their burden of proof at class certification through admissible evidence in its 2005 decision.
In the 2005 case of Unger v. Amedisys Inc., the court held “a careful certification inquiry is required and findings must be made based on adequate admissible evidence to justify class certification.”
Unger involved a securities fraud class action. Since the class was being pursued under a fraud on the market theory, the plaintiffs were required to show that the small-cap stocks in question traded on an efficient market in order to establish predominance under Rule 23(b)(3).
In support, the plaintiffs relied on an internet printout purporting to show multiple market makers for the stock in question along with affidavits by the plaintiffs’ witnesses that were admitted without the opportunity for cross-examination.
The district court accepted this proffer and certified the class. On appeal, however, the court of appeals vacated, explaining “[a]t the certification stage, reliance on unverifiable evidence is hardly better than relying on bare allegations.”
The Remaining Circuits
The First, Second and Third Circuits: Opinions Suggesting Fact Evidence Must Be Admissible
The First, Second and Third Circuits have issued opinions strongly suggesting fact evidence must be admissible to be considered at class certification. The U.S. Court of Appeals for the First Circuit‘s 2018 opinion in In re: Asacol Antitrust Litigation explained that “[t]he fact that plaintiffs seek class certification provides no occasion for jettisoning the rules of evidence and procedure, the Seventh Amendment, or the dictate of the Rules Enabling Act.”
Buyers of an ulcerative colitis drug brought a putative class action against drugmakers alleging they violated state antitrust laws by coordinating to prevent production of a generic version of the drug. The plaintiffs proposed that uninjured class members could be separated from injured ones through collection of affidavits by a claims administrator. Each affidavit would come from a drug buyer who would affirm that he or she would have switched to the generic brand if it had been available.
The district court found this proposal sufficient and granted certification. The First Circuit reversed, holding it could not “sanction the use of inadmissible hearsay to prove injury to each class member at or after trial.” Thus, while the court’s holding was that the plaintiffs could not use inadmissible evidence to prove injury to class members, the court’s reasoning suggests that plaintiffs may not use inadmissible evidence to prove the elements of Rule 23 are met.
The U.S. Court of Appeals for the Second Circuit in In re: Initial Public Offering Securities Litigation, while speaking in less definitive terms, held in 2006 that “a district judge is to assess all of the relevant evidence admitted at the class certification stage.” The class certification decision can be made “only if the judge resolves factual disputes relevant to each Rule 23 requirement and finds that whatever underlying facts are relevant to a particular Rule 23 requirement have been established.”
The requirement that a judge consider “relevant evidence admitted,” resolve factual disputes, and make factual findings strongly suggests that only admissible evidence may be considered at class certification. Indeed, that is the conclusion reached by district courts within the circuit.
The U.S. Court of Appeals for the Third Circuit, in In re: Hydrogen Peroxide Antitrust Litigation, held in 2008 that “in deciding whether to certify a class under Fed. R. Civ. P. 23, the district court must make whatever factual and legal inquiries are necessary and must consider all relevant evidence and arguments presented by the parties.”
Although the issue of admissibility was not before the court, the use of the term “relevant evidence” suggests that only admissible evidence may be considered at class certification.
The Fourth, Seventh, Tenth and Eleventh Circuits: No Clear Appellate-Level Guidance, District Courts Taking Varying Approaches
The remaining courts of appeal have not opined, either directly or tangentially, on the admissibility of fact evidence at class certification. A review of relevant decisions reveals courts reaching opposing conclusions, even within the same circuit.
In the Fourth Circuit, district courts have taken a variety of approaches. At least one court has stricken inadmissible evidence at class certification. Others have accepted and considered such evidence.
In the Seventh Circuit, district courts express a willingness to consider inadmissible evidence at certification.
In the Tenth Circuit, Judge James Browning of the U.S. District Court for the District of New Mexico squarely addressed the issue and held he would “consider only admissible evidence” at class certification in Anderson Living Trust v. WPX Energy Production LLC in 2015.
In a lengthy footnote to his opinion, Browning noted, “The Tenth Circuit does not appear to have addressed this question, but most courts have concluded that the Federal Rules of Evidence do not apply to class certification proceedings.” Nevertheless, following a detailed analysis of relevant precedent and the Federal Rules of Evidence, Judge Browning concluded the Federal Rules apply.
In the Eleventh Circuit, district courts, such as the U.S. District Court for the Southern District of Alabama in its 2004 opinion Fisher v. Ciba Specialty Chemicals Corp., conclude they “may consider evidence that may not ultimately be admissible at trial.”
Expert Evidence at Class Certification
When it comes to expert evidence, the plurality position — expressly adopted by the U.S. Court of Appeals for the Third Circuit, the U.S. Court of Appeals for the Fifth Circuit, the U.S. Court of Appeals for the Seventh Circuit, the U.S. Court of Appeals for the Ninth Circuit and the U.S. Court of Appeals for the Eleventh Circuit — is that the district court must conduct a full Daubert analysis and rule on the admissibility of expert testimony when it is challenged at class certification.
While the U.S. Court of Appeals for the First Circuit, the U.S. Court of Appeals for the Second Circuit, the U.S. Court of Appeals for the Fourth Circuit and the U.S. Court of Appeals for the D.C. Circuit have not ruled definitively, courts within those circuits similarly conduct a full Daubert analysis.
The U.S. Court of Appeals for the Eighth Circuit, however, has held that a court should conduct only a limited Daubert analysis at class certification. This approach likewise appears to be favored within the U.S. Court of Appeals for the Tenth Circuit.
The Plurality Position: Expert Evidence Must Meet Be Admissible Under Daubert
The Third, Fifth, Seventh, Ninth and Eleventh Circuits have held that expert testimony must be admissible under the standard set forth in Daubert to be considered at class certification. The Seventh Circuit explained in 2012 the need for this “full Daubert” analysis in Messner v. Northshore University HealthSystem, as follows:
“[W]henever an expert’s report or testimony is critical to a class certification decision, a district court must rule conclusively on a challenge to the expert’s qualifications or opinions before ruling on class certification, without regard to whether the district court ultimately grants or denies that motion. … The ruling is just as important to the plaintiffs as it is to the defendants. Northshore’s proposed rule would also create an unworkable logical conundrum, requiring a court to determine first whether to certify a class before considering the admissibility of the evidence it relied upon in making that determination. … The district court should have ruled definitively on plaintiffs’ Daubert motion and objections before ruling on their motion for class certification.”
The First, Second, Fourth, Sixth and D.C. Circuits: Moving Toward the Daubert Admissibility Standard
Although the First, Second, Fourth and D.C. Circuits have not definitively ruled on these issues, district courts within those circuits similarly apply a full Daubert approach.
The U.S. Court of Appeals for the Sixth Circuit has expressly noted the difference in circuit approaches while recognizing that it has “yet to settle this matter” in its 2020 Hicks v. State Farm Fire & Casualty Co. opinion.
The Minority Position: The Eighth Circuit’s “Limited Daubert” Approach, Also Followed Within the Tenth Circuit
The Eighth Circuit applies a more limited view of Daubert’s application to class certification. This approach focuses the inquiry on the requirements of Rule 23 at class certification, evaluating whether the testimony offered may eventually develop to be later admissible at trial, and not requiring that the evidence pass Daubert.
In explaining its reasoning in 2011, in In re: Zurn Pex Plumbing Products Liability Litigation, the Eighth Circuit stated:
“[B]ecause class certification decisions are generally made before the close of merits discovery, the district court’s analysis is necessarily prospective and subject to change, and there is necessary uncertainty about the evidence … [and] because a decision is far from a conclusive judgment at trial, it is not accompanied by the traditional rules and procedure applicable to trials.”
The Tenth Circuit has not yet directly addressed this question, though its district courts tend to follow this limited, more focused approach.
What to Make of the Current State of the Law
Treating Fact and Expert Evidence Differently
Our review of the law shows a clear distinction between the way appeals courts treat fact and expert evidence at class certification. Thus, the first question that arises is whether there is any meaningful basis for treating these types of evidence differently.
The circuit courts have not considered this broader question. Admissibility under Daubert consists primarily of analysis of an expert’s qualifications and testimony, and usually does not require making context-specific factual determinations.
On the other hand, the admissibility of fact evidence may depend on the specific context in which it is developed and offered, which may not be known until the evidence is presented at trial. Additionally, one might argue that fact and expert testimony have different degrees of importance at certification.
Class certification often involves a war of the experts, with each side proffering expert witnesses who opine that a class either can or cannot be certified. The party whose expert carries the day is most likely to achieve their desired result on a motion to certify.
Accordingly, although courts have not addressed the question of whether these two forms of evidence should be treated differently, it might be argued that a court can, and should, determine whether expert testimony is admissible at class certification, while foregoing ruling on fact evidence until trial.
While this position has some appeal, it may be difficult to justify analytically. Fact and expert evidence are simply two different forms of evidence; applying an admissibility standard to one and not the other appears illogical, and has no foundation in either the Rules of Evidence or any jurisprudential doctrine.
In short, evidence is evidence. Moreover, treating fact and expert evidence differently may potentially lead to evidentiary arbitrage.
In the Ninth Circuit, fact evidence need not be admissible, but expert evidence must meet the Daubert standard for admissibility. As a result, parties are incentivized to find creative ways to submit “expert-like” evidence as though it were fact evidence. Indeed, this appears to have already happened in at least one case, as detailed below.
Requiring That All Evidence Be Admissible at Class Certification
If there is no logical or legal reason to treat fact and expert evidence differently, the next question is whether a court should consider any form of evidence without first determining that it is admissible.
The circuit courts that have held that a trial court may consider evidence without ruling on its admissibility in connection with class certification tend to emphasize that certification is a preliminary determination that is subject to revision or later vacatur.
Indeed, courts of appeals have used this justification both in permitting the consideration of inadmissible fact evidence and in endorsing the limited Daubert approach. On this theory, the district court should not be required to resolve evidentiary disputes at class certification because the record is still developing, with potential additional discovery to follow the certification decision.
We see at least two problems with this approach. First, while the courts of appeals that have endorsed consideration of inadmissible evidence describe class certification as a preliminary decision, the practical reality is that the court’s certification decision usually sounds the death knell of the case.
If a class is certified, the potential exposure to the defendant is so large that the great majority will settle rather than continue to litigate. If certification is denied, plaintiffs and their counsel see little incentive to continue to litigate their individual claims.
Moreover, few courts revisit and alter their certification decisions once made. Thus, while class certification may formally constitute a preliminary ruling, it is, in effect, a final one.
Second, the argument that the Rules of Evidence do not apply at class certification is contradicted by the rules themselves. Rule 1101 provides that the rules “apply to proceedings” before the federal courts in both civil and criminal cases, with certain specific delimited exceptions set forth in Rule 1101(d) (principally concerning criminal proceedings).
Class certification is not among the delineated exceptions. By their own terms, then, the Rules of Evidence apply to class certification.
Nor does Rule 23 provide any basis for suggesting the Rules of Evidence should be relaxed. Rule 23 is a procedural tool for aggregating individual claims; it does not alter the evidentiary rules applicable to those claims.
The Supreme Court’s Curious Silence
Although the Supreme Court asked the Comcast parties to brief the question of the admissibility of evidence at class certification, it ultimately did not address the issue directly in its opinion.
The court’s statement that plaintiffs must demonstrate they satisfy Rule 23 through evidentiary proof could be read as implicitly requiring that evidence be admissible: It stands to reason that a party cannot prove something through evidence unless that evidence is admissible.
Nevertheless, it is clear that the courts of appeals have not uniformly adopted this reading. As a result, the question remains as to whether the Supreme Court will rule definitively on the admissibility of evidence — either fact or expert — at class certification.
Given the circuit split identified above, this issue would appear ripe for resolution by the Supreme Court. Yet the court’s silence thus far is not for lack of opportunity. In February 2018, the court rejected a petition for a writ of certiorari in Taylor Farms Pacific Inc. v. Pena, where the petitioner squarely asked the court to address this issue.
In Taylor, hourly workers brought a class action alleging wage-and-hour violations against the owner of two food-processing facilities. In support of certification, the plaintiffs submitted, through an attorney declaration, a spreadsheet created by their counsel of what they characterized as a compilation of the defendant’s timekeeping records.
Taylor Farms Pacific objected, but the district court held that “evidence in support of class certification need not be admissible at trial,” and certified certain subclasses. Taylor Farms petitioned the Supreme Court, phrasing the question presented as “whether a district court may certify a class based on information that does not meet the standards of admissibility set forth in the Federal Rules of Evidence and Civil Procedure.”
In its petition, Taylor Farms argued that the spreadsheet was expert-like analysis, but had been proffered as factual evidence, and thus evaded a Daubert review — a kind of evidentiary arbitrage made possible by the differing standards under which fact and expert evidence are considered in the Ninth Circuit.
The Supreme Court declined to grant the Taylor Farms petition. The result — for the moment — is that the circuits are approaching fact and expert evidence differently at class certification. Until the high court addresses the issue definitively, or the law evolves toward uniformity, practitioners would be well advised to stay carefully attuned to courts’ differing evidentiary standards.
 Comcast Corp. v. Behrend , 529 U.S. 27, 33 (2013).
 Lyngaas v. Ag , 992 F.3d 412 (6th Cir. 2021).
 Id. at 428.
 Id. at 428-29.
 Id.; accord In re Zurn Pex Plumbing Prod. Liab. Litig. , 644 F.3d 604, 611 (8th Cir. 2011) (“We have never required a district court to decide conclusively at the class certification stage what evidence will ultimately be admissible at trial”); Sali v. Corona Reg’l Med. Ctr. , 909 F.3d 996, 1005 (9th Cir. 2018) (“Inadmissibility alone is not a proper basis to reject evidence submitted in support of class certification.”).
 In re Rand Corp ., No. 02-8007, 2002 WL 1461810, at *1 (D.C. Cir. July 8, 2002).
 Disability Rts. Council of Greater Washington v. Washington Metro. Area Transit. Auth ., 239 F.R.D. 9, 24 (D.D.C. 2006) (citing Rand).
 Unger v. Amedisys, Inc ., 401 F.3d 316, 319 (5th Cir. 2005).
 Id. at 324.
 In re Asacol Antitrust Litigation , 907 F.3d 42, 53 (1st Cir. 2018).
 Id. at 53.
 In re IPO Sec. Litig ., 471 F.3d 24, 41 (2d Cir. 2006).
 See Lujan v. Cabana Mgmt., Inc ., 284 F.R.D. 50, 64 (E.D.N.Y. 2012) (“[T]his Court is of the opinion that the Second Circuit would require [fact evidence in support of class certification] be admissible.”).
 In re Hydrogen Peroxide Antitrust Litigation , 552 F.3d 305, 307 (3d Cir. 2008).
 See Soutter v. Equifax Info. Servs. LLC , 299 F.R.D. 126, 129–33 (E.D. Va. 2014) (striking an affidavit under Rule 602 where the declarant lacked personal knowledge).
 See Brandriff v. Dataw Island Owners’ Ass’n, No. CV 9:07-3361-CWH, 2010 WL 11534520, at *6 (D.S.C. Aug. 6, 2010) (“[I]n making a decision on class certification, it can consider evidence that would be inadmissible at trial.”); accord Baxley v. Jividen , No. CV 3:18-1436, 2020 WL 7061752, at *3 (S.D.W. Va. Dec. 2, 2020). And another court has held that, while the rules of evidence apply, they are “relaxed” at certification. See Todd v. XOOM Energy Md., LLC , No. GJH-15-154, 2020 WL 4784767 (D. Md. Aug. 18, 2020) (applying a “relaxed” approach for “determining whether plaintiffs have met the requirements of Rule 23 with sufficiently reliable evidence”).
 See, e.g., Craftwood Lumber Co. v. Essendant, Inc ., No. 16 C 4321, 2020 WL 3000255, at *5 (N.D. Ill. June 4, 2020) (“[E]vidence need not be admissible for purposes of resolving class certification issues.”); accord Coan v. Nightingale Home Healthcare, Inc ., 05–CV–0101, 2005 WL 1799454, at *1 n. 1 (S.D. Ind. June 29, 2005) (“plaintiffs need not come forward with evidence in a form admissible at trial”).
 Anderson Living Tr. v. WPX Energy Prod., LLC , 306 F.R.D. 312, 320 (D.N.M. 2015) (Browning, J.).
 Id. at 378 n.39.
 See id.
 Fisher v. Ciba Specialty Chems. Corp ., 238 F.R.D. 273, 279 (S.D. Ala. 2006); accord Pines Nursing Home (77), Inc. v. Rehabcare Grp., Inc., No. 14-cv-20039, 2014 WL 12531512, at *4 (S.D. Fla. June 20, 2014) (“While it is likely true that expert evidence must be admissible under Daubert to be considered during class certification, the same is not true of other types of evidence.”).
 In Daubert v. Merrell Dow Pharmaceuticals Inc ., the Supreme Court explained that district courts must conduct a “gatekeeping function” to ensure expert testimony is reliable and relevant. Federal Rule of Evidence 702 codifies the requirements for expert testimony to be admissible.
 Messner v. Northshore Univ. HealthSystem , 669 F.3d 802, 813 (7th Cir. 2012); accord In re Blood Reagents Antitrust Litigation (“[A] plaintiff cannot rely on challenged expert testimony, when critical to class certification, to demonstrate conformity with Rule 23 unless the plaintiff also demonstrates, and the trial court finds, that the expert testimony satisfies the standard set out in Daubert”); Prantil v. Arkema Inc ., 986 F.3d 570, 575–76 (5th Cir. 2021) (“[I]f an expert’s opinion would not be admissible at trial, it should not pave the way for certifying a proposed class”); Grodzitsky v. Am. Honda Motor Co ., 957 F.3d 979, 984 (9th Cir. 2020) (“[I]n evaluating challenged expert testimony in support of class certification, a district court should evaluate admissibility under the standard set forth in Daubert”); Sher v. Raytheon Co ., 419 F. App’x 887, 890–91 (11th Cir. 2011) (holding that the district court’s failure to conduct a Daubert analysis was an error).
 See In re Loestrin 24 FE Antitrust Litig ., 410 F. Supp. 3d 352, 385 (D.R.I. 2019) (holding that before taking up a motion for class certification, the court must address the parties’ challenges to the expert analysis under Daubert); Scott v. Chipotle Mexican Grill Inc ., 315 F.R.D. 33, 55 (S.D.N.Y. 2016) (“Despite the lack of a clear legal standard on [whether Daubert applies at the class certification stage], trial courts in this circuit often subject expert testimony to Daubert’s rigorous standards insofar as that testimony is relevant to the Rule 23 class certification analysis”); Good v. Am. Water Works Co. Inc. , 310 F.R.D. 274, 284 (S.D.W. Va. 2015) (concluding court should apply Daubert analysis); Childress Campbell v. Nat’l R.R. Pass. Corp. , 311 F. Supp. 3d 281, 295 (D.C. Cir. 2018) (“The Court is persuaded that it must conduct a full Daubert inquiry at the class-certification stage”).
 Hicks v. State Farm Fire & Cas. Co ., 965 F.3d 452, 465 (6th Cir. 2020). Courts within the Circuit apply a full Daubert analysis. See, e.g., Hosp. Auth. of Metro. Gov’t of Nashville v. Momenta Pharms. Inc ., 333 F.R.D. 390, 400 (M.D. Tenn. 2019) (applying Daubert analysis).
 In re Zurn Pex Plumbing Prods. Liab. Litig ., 644 F.3d 604, 610–11 (8th Cir. 2011).
 See, e.g., Beltran v. InterExchange Inc ., No. 14-CV-03074, 2018 WL 1509258, at *4 (D. Colo. Mar. 27, 2018) (“[A]n exhaustive and conclusive Daubert inquiry before the completion of the merits discovery cannot be reconciled with the inherently preliminary nature of pretrial evidentiary and class certification rulings”) (citing cases).
 See Zurn Pex Plumbing, 644 F.3d at 613 (“[A] decision to certify as class is far from a conclusive judgment on the merits of the case” and “there is bound to be some evidentiary uncertainty”).
 Pena v. Taylor Farms Pacific Inc ., 305 F.R.D. 197, 205 (E.D. Cal. 2015). The Ninth Circuit affirmed “for the reasons set forth” in the district court’s order. 690 F. App’x 526, 527 (9th Cir. 2017).