FOR PUBLICATION

UNITED STATES COURT OF APPEALS

FOR THE NINTH CIRCUIT

JUSTIN LYTLE and CHRISTINE 

MUSTHALER, 

Plaintiffs-Appellees, 

 v. 

NUTRAMAX LABORATORIES, 

INC. AND NUTRAMAX 

LABORATORIES VETERINARY 

SCIENCES, INC, 

Defendants-Appellants.

No. 22-55744

D.C. No.

5:19-cv-00835-

FMO-SP

ORDER AND 

AMENDED

OPINION

Appeal from the United States District Court

for the Central District of California

Fernando M. Olguin, District Judge, Presiding

Argued and Submitted October 18, 2023

Pasadena, California

Filed April 22, 2024

Amended August 23, 2024

Before: A. Wallace Tashima and Holly A. Thomas, Circuit 

Judges, and Jed S. Rakoff,

* District Judge.

* The Honorable Jed S. Rakoff, United States District Judge for the 

Southern District of New York, sitting by designation.
2 LYTLE V. NUTRAMAX LABORATORIES, INC.

Opinion by Judge Rakoff 

SUMMARY**

Class Action

The panel affirmed the district court’s grant of class 

certification in a consumer class action concerning the 

marketing of the pet health product Cosequin.

Plaintiffs claim that Nutramax violated the California 

Consumers Legal Remedies Act (“CLRA”) by marketing 

Cosequin as promoting healthy joints in dogs, when in fact 

Cosequin provides no such health benefit. The district court 

certified a class of California purchasers of certain Cosequin 

products who were exposed to the allegedly misleading 

statements.

Nutramax challenged the district court’s reliance upon 

the proposed damages model of Plaintiffs’ expert, Dr. JeanPierre Dubé, to find that common questions predominated as 

to injury. The panel held that, contrary to Nutramax’s 

contention, there was no general requirement that an expert 

actually apply to the proposed class an otherwise reliable 

damages model in order to demonstrate that damages are 

susceptible to common proof at the class certification 

stage. Rather, class certification plaintiffs may rely on an 

unexecuted damages model to show that damages are 

susceptible to common proof. The panel concluded that the 

** This summary constitutes no part of the opinion of the court. It has 

been prepared by court staff for the convenience of the reader.
LYTLE V. NUTRAMAX LABORATORIES, INC. 3

district court did not abuse its discretion in finding that Dr. 

Dubé’s proposed model was sufficiently sound and 

developed to satisfy this standard at the class certification 

stage. 

The panel rejected Nutramax’s contention that the 

district court incorrectly concluded that the element of 

reliance was susceptible to common proof. The district court 

properly found that classwide reliance may be established 

under the CLRA through proof that a misrepresentation is 

material. While the presumption of reliance is rebuttable, 

the district court did not abuse its discretion in concluding 

that Nutramax failed to rebut the presumption here.

COUNSEL

Mark R. Sigmon (argued), Milberg Coleman Bryson Phillips 

Grossman PLLC, Raleigh, North Carolina; Greg F. Coleman 

and Adam A. Edwards, Milberg Coleman Bryson Phillips 

Grossman PLLC, Knoxville, Tennessee; Michael H. Pearson 

and Daniel L. Warshaw, Pearson Simon & Warshaw LLP, 

Sherman Oaks, California; Matthew D. Schultz, Levin 

Papantonio Rafferty Proctor Buchanan O'Brien Barr & 

Mougey PA; Pensacola, Florida; for Plaintiffs-Appellees.

David R. Carpenter (argued), Nicole M. Baade, Sean A. 

Commons, and Amy P. Lally, Sidley Austin LLP, Los 

Angeles, California; Joshua A. Glikin, Shulman Rogers PA, 

Potomac, Maryland; for Defendants-Appellants.
4 LYTLE V. NUTRAMAX LABORATORIES, INC.

ORDER

The opinion filed on April 22, 2024, and appearing at 99 

F.4th 557, is hereby amended. The amended opinion will be 

filed concurrently with this order. 

With these amendments, the panel has unanimously 

voted to deny the petition for panel rehearing. Judge H.A. 

Thomas has voted to deny the petition for rehearing en banc, 

and Judge Tashima and Judge Rakoff so recommend. The 

full court was advised of the petition for rehearing en banc. 

A judge requested a vote on whether to rehear the matter en 

banc. The matter failed to receive a majority of the votes of 

the nonrecused active judges in favor of en banc 

consideration. Fed. R. App. P. 35(a). 

The petition for panel rehearing and the petition for 

rehearing en banc, Dkt. No. 52, are DENIED. 

OPINION

RAKOFF, District Judge:

This is a putative consumer class action concerning the 

marketing of the pet health product Cosequin. PlaintiffsAppellees claim that Nutramax Laboratories, Inc. and 

Nutramax Laboratories Veterinary Sciences, Inc. 

(collectively, “Nutramax”) violated the California 

Consumers Legal Remedies Act (“CLRA”), Cal. Civil Code 

§§ 1750–1784, by marketing Cosequin as promoting healthy 

joints in dogs, when in fact Cosequin provided no such 

health benefits. Below, the district court certified a class of 

California purchasers of certain Cosequin products who 
LYTLE V. NUTRAMAX LABORATORIES, INC. 5

were exposed to the allegedly misleading statements. 

Nutramax now appeals that grant of class certification on 

two grounds.

First, Nutramax challenges the district court’s reliance 

upon the proposed damages model of Plaintiffs’ expert, Dr. 

Jean-Pierre Dubé, to find that common questions 

predominated as to injury. Nutramax claims this was error 

because the proposed model had not actually been applied to 

the proposed class. We conclude that, contrary to 

Nutramax’s contention, there is no general requirement that 

an expert actually apply to the proposed class an otherwise 

reliable damages model in order to demonstrate that 

damages are susceptible to common proof at the class 

certification stage. Rather, we hold that class action plaintiffs 

may rely on a reliable though not-yet-executed damages 

model to demonstrate that damages are susceptible to 

common proof so long as the district court finds that the 

model is reliable and, if applied to the proposed class, will 

be able to calculate damages in a manner common to the 

class at trial. We further conclude that the district court did 

not abuse its discretion in finding Dr. Dubé’s proposed 

model was sufficiently sound and developed to satisfy this 

standard at the class certification stage.

Second, Nutramax contends that the district court 

incorrectly concluded that the element of reliance was 

susceptible to common proof. We disagree. The district court 

properly found that classwide reliance may be established 

under the CLRA through proof that a misrepresentation is 

material. While the presumption of reliance is rebuttable, the 

district court did not abuse its discretion in concluding 

Nutramax had failed to rebut the presumption here. 
6 LYTLE V. NUTRAMAX LABORATORIES, INC.

Accordingly, for the reasons set forth more fully below, 

we affirm the district court’s grant of class certification.

I.

Nutramax develops and sells pet health supplements. 

Plaintiffs-Appellees Justin Lytle and Christine Musthaler are 

two dog owners who purchased a product produced by 

Nutramax, Cosequin, for their dogs. In this action, Plaintiffs 

allege that Nutramax marketed Cosequin as a health 

supplement that would improve their dogs’ joints and

mobility when, in fact, there is no evidence that Cosequin 

provides any such health benefit. 

After the close of fact and expert discovery, Plaintiffs 

sought to certify the following class pursuant to Federal Rule 

of Civil Procedure (“FRCP”) 23(b)(3):

All persons residing in California who 

purchased during the limitations period the 

following canine Cosequin products for 

personal use: Cosequin DS Maximum 

Strength Chewable Tablets; Cosequin DS 

Maximum Strength Plus MSM Chewable 

Tablets; and Cosequin DS Maximum 

Strength Plus MSM Soft Chews.

Plaintiffs initially asserted that numerous statements 

made in marketing materials for Cosequin and/or on the 

packaging of the three products listed above were false and 

misleading. However, at class certification Plaintiffs 

narrowed their claims to four statements that appeared on 

Cosequin’s packaging:

(1): “Joint Health Supplement”; 
LYTLE V. NUTRAMAX LABORATORIES, INC. 7

(2): “Use Cosequin to help your pet Climb 

stairs, Rise and Jump!”; 

(3): “Supports Mobility for a Healthy 

Lifestyle”; and 

(4): “Mobility, Cartilage and Joint Health 

Support.” 

Plaintiffs argued that these statements were false and 

misleading because Cosequin does not, in fact, improve 

dogs’ joint health. According to Plaintiffs, the only two peerreviewed, double-blinded, randomized controlled trials that 

have been conducted on Cosequin’s efficacy have concluded 

that Cosequin confers no more benefit to canine joint health 

than a placebo. Plaintiffs’ veterinary expert Dr. Steven 

Budsberg opined that “the strongest available 

evidence . . . has consistently found no reliable evidence that 

Cosequin provides any efficacy in supporting, maintaining, 

or improving joint health,” and Plaintiffs’ biostatistics expert 

Dr. Richard Evans concluded that “there is no evidence that 

[glucosamine and chondroitin, the active ingredients in 

Cosequin, have] a greater prophylactic effect than placebo 

control on maintaining joint health in healthy pet dogs.” 

Nutramax, for its part, presented contrary evidence about the 

products’ efficacy, including non-randomized control trials 

and the testimony of Nutramax’s own veterinary expert, Dr. 

Marcellin-Little. Nutramax further argues that the two 

studies relied on by Plaintiffs involved dogs with 

osteoarthritis, even though Cosequin is not marketed to treat 

that condition. The district court found that Plaintiffs had 

adequately demonstrated for class certification purposes the

reasonable likelihood that a jury could find that Cosequin 

provided no benefit to dogs’ joint health.
8 LYTLE V. NUTRAMAX LABORATORIES, INC.

The four challenged statements appeared on the labels of 

various Cosequin products in various combinations over 

time. Although the parties offer differing characterizations 

about the extent and significance of these variations, the 

parties agree that the first statement (“Joint Health 

Supplement”) appeared on all product labels or packages 

throughout the class period. Further, approximately 90% of 

sales during the class period—including those to the named 

plaintiffs—were attributable to products that contained the 

“Joint Health Supplement” statement. The district court 

ultimately found that the presence of the Joint Health 

Supplement statement on every challenged product was 

sufficient to support class certification. 

Before the district court, Nutramax raised a host of 

challenges to Plaintiffs’ class certification motion,

contesting each prerequisite of FRCP 23(a) and 23(b)(3). 

Nutramax also belatedly filed two evidentiary objections to 

Plaintiffs’ expert witnesses Bruce Silverman and Dr. Dubé. 

The district court rejected each of these objections and 

certified the class as requested by Plaintiffs. Nutramax then 

promptly sought and obtained permission to file the instant 

interlocutory appeal of the district court’s class certification 

order pursuant to FRCP 23(f). 

On appeal, Nutramax limits its challenge to just two of 

the district court’s findings: (1) that damages are susceptible 

to common proof, and (2) that causation/reliance is

susceptible to common proof. Nutramax also persists in its 

evidentiary objection to Dr. Dubé insofar as it relates to the 

first question of whether damages are susceptible to common 

proof. The parties’ briefing and the district court’s rulings on 

these two issues are briefly described below.
LYTLE V. NUTRAMAX LABORATORIES, INC. 9

A.

To establish damages on a classwide basis, Plaintiffs put 

forward the testimony of Dr. Dubé, a professor of marketing 

at the University of Chicago Booth School of Business who, 

as the district court found, has “extensive experience in 

marketing data and analytics.” To measure classwide 

damages, Dr. Dubé proposed to conduct a “conjoint survey”

(or “conjoint analysis”). Simply put, a conjoint survey 

allows a researcher to test the economic value a consumer 

places on a given product feature, such as a particular 

statement on a package, by showing the product to individual 

survey participants with and without certain features, and 

then using survey responses to calculate the economic value 

consumers place upon the feature. Dr. Dubé explained that, 

in conducting a conjoint analysis, a researcher is able to 

control for other variables such as package size and the 

competing products by modifying the underlying choicetasks presented to participants.

At the time of class certification, Dr. Dubé had not yet 

actually applied his conjoint analysis to a representative 

sample in the case, that is, he had not actually surveyed any 

class members or calculated what class members’ damages 

might be. Rather, in his report, Dr. Dubé opined that, 

“[b]ased on [his] analysis of market data and other marketing 

documents made available through discovery,” he believed

a conjoint analysis “is well-suited to the facts of this case and 

will successfully isolate the economic damages associated 

with the Challenged Claims.” Dr. Dubé acknowledged, 

however, that he would not know whether the class actually 

suffered any damages until he actually executed his survey. 

Nutramax’s class certification opposition criticized Dr. 

Dubé for failing to actually conduct (i.e., “execute”) the 
10 LYTLE V. NUTRAMAX LABORATORIES, INC.

survey and thus complete his analysis. Before the district 

court, Nutramax also argued that Dr. Dubé’s model was 

under-developed in other respects, although Nutramax

apparently did not argue this was a basis to exclude his 

testimony under Federal Rule of Evidence (“FRE”) 702 or 

FRCP 23, but instead argued that his opinion violated FRCP 

16, 26, 37 and the district court’s scheduling order. 

The district court rejected Nutramax’s argument that it 

could not rely on Dr. Dubé’s unexecuted damages model or 

that the model should be excluded under the standard set 

forth in Daubert v. Merrell Dow Pharmaceuticals, Inc., 509 

U.S. 579 (1993). The district court began by noting Dr. 

Dubé’s credentials, finding him to be well qualified for the 

purposes offered. The district court further observed that 

conjoint surveys such as that proposed by Dr. Dubé “are a 

well-established method for measuring class-wide damages 

in CLRA mislabeling cases,” and also noted that conjoint 

analyses previously prepared by Dr. Dubé had been upheld 

as reliable in similar cases. Most importantly, the court 

below, citing other district court precedent, held that “[a] 

plaintiff is not required to actually execute a proposed 

conjoint analysis to show that damages are capable of 

determination on a class-wide basis.” 

B.

Nutramax also opposed class certification on the ground 

that individual questions predominated with respect to the 

element of reliance because (1) substantial variation in the 

labels precluded a finding of predominance, (2) Plaintiffs 

had failed to show common exposure to the challenged 

statements, and (3) Plaintiffs had not shown the challenged 

statements factored into the typical consumer’s purchasing 

decision or would have been material to such consumers. 
LYTLE V. NUTRAMAX LABORATORIES, INC. 11

Although reliance and materiality are separate elements, 

under the CLRA, a plaintiff can create a presumption of

reliance by demonstrating a material misrepresentation was 

made to the entire class. See Stearns v. Ticketmaster Corp., 

655 F.3d 1013, 1022 (9th Cir. 2011). To establish materiality 

(and relatedly, reliance for class certification purposes), 

Plaintiffs put forward the expert testimony of Silverman, a 

former advertising executive, who testified that a reasonable 

consumer would find the product labels misleading. As 

noted, Plaintiffs also put forward two expert witnesses, Dr. 

Budsberg and Dr. Evans, to testify to the falsity of the 

challenged statements and demonstrate that Cosequin did 

not offer joint health benefits. Plaintiffs also argued that the 

survey conducted by Nutramax’s own expert, Dr. Carol 

Scott, actually supported a finding of materiality.1

In response, Nutramax put forward two experts. First, 

Nutramax put forward Dr. Scott to present the results of a 

consumer survey about the purchasing decisions of 

Cosequin customers. Dr. Scott’s survey found that 

purchasers “consulted a variety of information sources prior 

to purchasing [Cosequin] for the first time, with one’s 

veterinarian being most frequently mentioned (i.e., 57% of 

respondents . . . ), followed by website ratings or reviews 

1 In this regard, Plaintiffs homed in on one particular finding of Dr. 

Scott’s survey that they claimed actually supports a finding of 

materiality. Dr. Scott showed respondents a label from Product #2, 

containing only the Joint Health Supplement statement, and asked: 

“Based on this package, what do you think[] Cosequin will do for your 

dog? That is, why should you give your dog Cosequin?” The “most 

frequently given response” (by nearly 80% of respondents) was

“improve/help/maintain mobility, flexibility, joint health/support.” In 

the district court and on appeal, Plaintiffs argue that this finding by Dr. 

Scott further demonstrates that the challenged statements would have 

been misleading to a reasonable consumer. 
12 LYTLE V. NUTRAMAX LABORATORIES, INC.

(26.9% of respondents).” Only about a quarter of 

respondents identified the packaging as a source of 

information they consulted prior to deciding to purchase 

Cosequin for the first-time. Dr. Scott’s survey found that the 

most common motivations for purchasing Cosequin were 

advice from a veterinarian, advice from someone else, or 

research on websites, whereas only a small fraction of 

respondents cited the packaging for their motive. Second, 

Nutramax put forward Dr. Olivier Toubia, who conducted a 

consumer choice survey somewhat similar to that proposed 

by Dr. Dubé. Dr. Toubia showed one group of individuals 

the actual product packaging and another group a modified 

version that (supposedly) removed the challenged claims. 

The results of this study, according to Dr. Toubia, showed 

that removing the statements did not materially impact the 

price a consumer would be willing to pay.

The district court reviewed the evidence presented by the 

parties and ultimately credited the evidence presented by 

Plaintiffs that the challenged statements would be materially 

misleading to a reasonable consumer. In a footnote, the 

district court explained that it was “unpersuaded” by Dr. 

Scott’s and Dr. Toubia’s expert reports, noting that they 

contained “flaws that undercut their persuasiveness.” In 

particular, the district court cited the fact that Dr. Toubia’s 

survey failed to remove the “Joint Health Supplement” 

statement from the packaging, even though that was at the 

core of Plaintiffs’ claim. The district court did not explain 

the basis for its rejection of Dr. Scott’s survey, but ultimately 

concluded Nutramax’s evidence was “outweighed by the 

common evidence presented by plaintiffs.” 
LYTLE V. NUTRAMAX LABORATORIES, INC. 13

II.

Before a class may be certified, the district court must 

conduct a “rigorous analysis” to determine if the 

prerequisites of FRCP 23 have been satisfied. Olean 

Wholesale Grocery Coop., Inc. v. Bumble Bee Foods LLC, 

31 F.4th 651, 664 (9th Cir. 2022) (en banc). The plaintiffs 

must “actually prove—not simply plead—that their 

proposed class satisfies each requirement of Rule 23.” 

Halliburton Co. v. Erica P. John Fund, Inc., 573 U.S. 258, 

275 (2014). The plaintiffs bear “the burden of establishing 

that the prerequisites of Rule 23 are satisfied by a 

preponderance of the evidence.” Olean, 31 F.4th at 665.

At issue here is the predominance requirement: that 

“questions of law or fact common to class members 

predominate over any questions affecting only individual 

members.” FRCP 23(b)(3). This requirement presupposes 

satisfaction of the commonality requirement of FRCP 

23(a)(2), which itself tests “the capacity of a classwide 

proceeding to generate common answers apt to drive the 

resolution of the litigation.” Alcantar v. Hobart Serv., 800 

F.3d 1047, 1052 (9th Cir. 2015) (quoting Wal-Mart Stores, 

Inc. v. Dukes, 564 U.S. 338, 350 (2011)). But the 

predominance inquiry goes further and “asks whether the 

common, aggregation-enabling, issues in the case are more 

prevalent or important than the non-common, aggregationdefeating, individual issues.” Tyson Foods, Inc. v. 

Bouaphakeo, 577 U.S. 442, 453 (2016).

At the same time, it is critical to keep in mind that class 

certification is different from summary judgment. “A 

court . . . is merely to decide [whether a class action is] a 

suitable method of adjudicating the case.” Edwards v. First 

Am. Corp., 798 F.3d 1172, 1178 (9th Cir. 2015). With 
14 LYTLE V. NUTRAMAX LABORATORIES, INC.

respect to the predominance inquiry specifically, a district 

court must evaluate “‘the method or methods by which 

plaintiffs propose to use the [class-wide] evidence to prove’ 

the common question in one stroke.” Olean, 31 F.4th at 666 

(quoting In re Hydrogen Peroxide Antitrust Litig., 552 F.3d 

305, 312 (3d Cir. 2008)). “In determining whether the 

‘common question’ prerequisite is met, a district court is 

limited to resolving whether the evidence establishes that a 

common question is capable of class-wide resolution, not 

whether the evidence in fact establishes that plaintiffs would 

win at trial.” Olean, 31 F.4th at 666–67.

We review the “decision to certify a class and any 

particular underlying Rule 23 determination involving a 

discretionary determination” for an abuse of discretion. 

Olean, 31 F.4th at 663 (cleaned up). We review de novo “the 

district court’s determination of underlying legal questions.” 

Id. “A district court applying the correct legal standard 

abuses its discretion only if it (1) relies on an improper 

factor, (2) omits a substantial factor, or (3) commits a clear 

error of judgment in weighing the correct mix of factors.” 

Sali v. Corona Reg’l Med. Ctr., 909 F.3d 996, 1004 (9th Cir. 

2018) (cleaned up). We review evidentiary rulings for an 

abuse of discretion. City of Pomona v. SQM N. Am. Corp., 

750 F.3d 1036, 1043 (9th Cir. 2014). We review factual 

findings underlying a class certification ruling for clear 

error. Sali, 909 F.3d at 1002. We review the question of 

whether an expert’s damages model “is capable of showing 

class-wide impact, thus satisfying one of the prerequisites of 

Rule 23(b)(3) of the Federal Rules of Civil Procedure, for an 

abuse of discretion.” Olean, 31 F.4th at 663. 
LYTLE V. NUTRAMAX LABORATORIES, INC. 15

III.

A.

Nutramax’s principal argument on appeal is that the 

“rigorous analysis” required by FRCP 23 categorically 

prohibits a class-action plaintiff from relying on an 

unexecuted damages model to demonstrate predominance

(at least where that model is the only evidence of classwide 

injury).2 In addressing this argument, the parties focus on the 

statement in the Supreme Court’s decision in Comcast Corp. 

v. Behrend—repeated in our en banc decision in Olean 

Wholesale Grocery Cooperative, Inc. v. Bumble Bee Foods 

LLC—that a class action plaintiff must “establish[] that 

damages are capable of measurement on a classwide basis.” 

569 U.S. 27, 34 (2013) (emphasis added). Plaintiffs contend 

that this phrase sanctions precisely what Dr. Dubé has done

here: submit an expert report opining that damages can be

measured on a classwide basis and setting forth a reliable 

method for doing so. Nutramax, by contrast, argues that to 

satisfy the predominance requirement, “[P]laintiffs must 

proffer admissible, affirmative evidence that classwide 

injury and damages in fact are capable of classwide 

measurement.” In Nutramax’s view, the only way to carry 

this burden is to actually put forward common evidence 

showing that classwide damages exist. 

2 On appeal, the parties both appear to assume that the predominance 

requirement would be satisfied only if damages are capable of 

measurement on a classwide basis. For purposes of this appeal, we adopt 

this assumption. We note, however, that we have held that individual 

questions of damages do not necessarily defeat class certification, as the 

district court here expressly acknowledged. See Vaquero v. Ashley 

Furniture Indus., Inc., 824 F.3d 1150, 1155 (9th Cir. 2016); Pulaski & 

Middleman, LLC v. Google, Inc., 802 F.3d 979, 987 (9th Cir. 2015).
16 LYTLE V. NUTRAMAX LABORATORIES, INC.

Nutramax’s argument rests upon a misapprehension of

the temporal focus of the class certification inquiry. As 

explained below, class action plaintiffs are not required to 

actually prove their case through common proof at the class 

certification stage. Rather, plaintiffs must show that they will 

be able to prove their case through common proof at trial. 

Given, moreover, that the Federal Rules contemplate that 

certification will be made “[a]t an early practicable time,”

FRCP 23(c)(1)(A), we see no reason why plaintiffs may not, 

in appropriate circumstances, satisfy this burden through a 

proffer of a reliable method of obtaining evidence that will 

come into existence once a damages model is executed, even 

when the results are not yet available at the class certification 

stage. We thus hold that class action plaintiffs may rely on 

an unexecuted damages model to demonstrate that damages 

are susceptible to common proof so long as the district court 

finds, by a preponderance of the evidence, that the model 

will be able to reliably calculate damages in a manner 

common to the class at trial.

Contrary to Nutramax’s contention, there is no 

requirement that the evidence relied upon by Plaintiffs to 

support class certification be presented in an admissible form 

at the class certification stage. See Sali, 909 F.3d at 1004 

(“Inadmissibility alone is not a proper basis to reject 

evidence submitted in support of class certification.”); B.K. 

v. Snyder, 922 F.3d 957, 974 (9th Cir. 2019) (“At this 

‘tentative, preliminary, and limited’ stage we have held 

strictly admissible evidence is not required, and we have 

indicated that plaintiffs can meet their evidentiary burden in 

part through allegations when the allegations are detailed 

and supported by additional materials” (internal citation 

omitted)). As we explained in Sali v. Corona Regional 

Medical Center, “an inquiry into the evidence’s ultimate 
LYTLE V. NUTRAMAX LABORATORIES, INC. 17

admissibility should go to the weight that evidence is given 

at the class certification stage.” 909 F.3d at 1006.3 Of course, 

if it is unlikely that a particular piece of common proof will 

be available or admissible at trial, that possibility weighs 

against a finding that common questions (and common 

answers) will predominate. But “[n]either the possibility that 

a plaintiff will be unable to prove his allegations, nor the 

possibility that the later course of the suit might 

unforeseeably prove the original decision to certify the class 

wrong, is a basis for declining to certify a class which 

apparently satisfies” Rule 23. Id. at 1004–05 (quoting 

Blackie v. Barrack, 524 F.2d 891, 901 (9th Cir. 1975)).

Nor is there a requirement that class action plaintiffs

actually prove that classwide damages exist in order to 

obtain class certification. Rather, we have repeatedly found 

class treatment to be appropriate, in analogous contexts, 

based upon a showing that damages could be calculated on 

a classwide basis, even where such calculations have not yet 

been performed. See Just Film, Inc. v. Buono, 847 F.3d 1108,

1121 (9th Cir. 2017) (“At this stage, Plaintiffs need only 

show that such damages can be determined without 

excessive difficulty and attributed to their theory of liability, 

and have proposed as much here.”); Lambert v. 

Nutraceutical Corp., 870 F.3d 1170, 1182 (9th Cir. 2017) 

(“Uncertainty regarding class members’ damages does not 

prevent certification of a class as long as a valid method has 

been proposed for calculating those damages.”), rev’d and 

remanded on other grounds, 139 S. Ct. 710 (2019); Leyva v. 

3 The district court correctly cited Sali for this exact principle. Nutramax

inexplicably asserts the district court committed legal error by doing so

without ever discussing Sali or explaining why, in Nutramax’s view, the 

rule announced in that case is inapplicable. 
18 LYTLE V. NUTRAMAX LABORATORIES, INC.

Medline Indus. Inc., 716 F.3d 510, 514 (9th Cir. 2013) 

(finding evidence “that damages could feasibly and 

efficiently be calculated once the common liability questions 

are adjudicated” was sufficient to satisfy predominance). For 

example, in Lambert v. Nutraceutical Corp., we reversed a 

district court’s denial of class certification and concluded the 

plaintiffs had satisfied the predominance requirement by 

proposing a valid damages model, even where the plaintiffs 

did not yet have all of the data necessary to perform their 

damages calculations. 870 F.3d at 1183–84. We explained 

that the “precise [data] is unnecessary for class certification” 

because “the question is only whether [plaintiff] has 

presented a workable method.” Id. at 1184.

Requiring that class action plaintiffs actually prove 

classwide injury at this stage would improperly conflate the 

class certification inquiry with the merits. To be sure, courts

may not avoid resolving questions pertinent to class 

certification merely because they overlap with the merits of 

the plaintiffs’ case, see Wal-Mart, 564 U.S. at 351; Comcast, 

569 U.S. at 33–34, but this does not “grant[]

courts . . . license to engage in free-ranging merits inquiries 

at the certification stage.” Amgen Inc. v. Conn. Ret. Plans & 

Tr. Funds, 568 U.S. 455, 466 (2013). Rather, such “[m]erits 

questions may be considered to the extent—but only to the 

extent—that they are relevant to determining whether the 

Rule 23 prerequisites for class certification are satisfied.” Id. 

While acknowledging this rule, Nutramax argues that the

theoretical possibility Dr. Dubé’s model, once executed, 

might show no injury at all “goes to the heart of the Rule 23 

inquiry.” But Nutramax fails to convincingly explain why 

this is so. The focus of the predominance inquiry “is whether 

the method of proof would apply in common to all class 

members,” “not whether the method of proof would or could 
LYTLE V. NUTRAMAX LABORATORIES, INC. 19

prevail.” In re Celexa & Lexapro Mktg. & Sales Pracs. 

Litig., 915 F.3d 1, 12 (1st Cir. 2019); see Stockwell v. City & 

Cnty. of San Francisco, 749 F.3d 1107, 1112 (9th Cir. 2014) 

(“[W]hether class members could actually prevail on the 

merits of their claims is not a proper inquiry in determining 

the preliminary question whether common questions exist.” 

(internal quotation marks omitted)). As the Supreme Court 

explained in Tyson Foods, Inc. v. Bouaphakeo, 

“[w]hen . . . ‘the concern about the proposed class is not that 

it exhibits some fatal dissimilarity but, rather, a fatal 

similarity—[an alleged] failure of proof as to an element of 

the plaintiffs’ cause of action—courts should engage that 

question as a matter of summary judgment, not class 

certification.’” 577 U.S. at 457 (quoting Richard A. 

Nagareda, Class Certification in the Age of Aggregate Proof, 

84 N.Y.U. L. Rev. 97, 107 (2009));see also Amgen, 568 U.S.

at 468 (“A failure of proof on the common question of 

materiality ends the litigation and thus will never cause 

individual questions of reliance or anything else to 

overwhelm questions common to the class.”); Alcantar, 800 

F.3d at 1053 (“A common contention need not be one that 

‘will be answered, on the merits, in favor of the class.’” 

(quoting Amgen, 568 U.S. at 459)). “To hold otherwise 

would turn class certification into a mini-trial, when the 

purpose of class certification is merely to select the method 

best suited to adjudication of the controversy fairly and 

efficiently.” Alcantar, 800 F.3d at 1053 (cleaned up).

The theoretical possibility that Dr. Dubé’s model, when 

executed, will reveal no damages thus does not undermine 

predominance, because that result would nonetheless be 
20 LYTLE V. NUTRAMAX LABORATORIES, INC.

common to the class.4 Nor does the possibility Dr. Dubé’s 

analysis might reveal damages with respect to some, but not 

all, of the challenged statements undermine predominance, 

because the very structure of the conjoint survey allows for 

an overcharge to be associated with each individual 

statement and label, allowing the amount each class member 

is entitled to recover to be easily assessed based solely on the 

product the class member purchased. And the possibility that 

an ascertainable portion of the class may be unable to 

recover—those not exposed to a statement with any 

attributable overcharge—does not in itself demonstrate class 

certification was improper. See Olean, 31 F.4th at 669, 680–

81 (holding that the possibility some class members suffered 

no injury does not, by itself, defeat class certification); Just 

Film, 847 F.3d at 1120 (“To gain class certification, 

Plaintiffs need to be able to allege that their damages arise 

from a course of conduct that impacted the class. But they 

need not show that each members’ damages from that 

conduct are identical.”). 

Nutramax’s argument in favor of its proposed 

categorical rule—requiring that a damages model always be 

executed prior to class certification—rests almost entirely 

upon its misinterpretation of the Supreme Court’s decision 

4 Nutramax argues that this is not so because “Plaintiffs’ complaint 

asserted claims based not only on the labels, but also representations on 

the Cosequin® website and in other media; and Plaintiffs pled alternative 

theories of damages beyond the price-premium theory.” But if that were 

sufficient to defeat predominance, the rule announced in Tyson Foods

would be rendered meaningless. In almost every case it will be possible 

to point to some individual proof that could substitute for the 

(purportedly) deficient aggregate proof. Here, however, Plaintiffs have 

narrowed the case to their challenge of these specific label claims using 

aggregate proof, and that is what the district court properly focused on.
LYTLE V. NUTRAMAX LABORATORIES, INC. 21

in Comcast Corp. v. Behrend, 569 U.S. 27 (2013), and our

recent en banc decision in Olean. We agree that these cases 

demand some assessment of the adequacy of Plaintiffs’ 

common proof at class certification, but nothing in these 

cases requires that Plaintiffs actually prove damages at the 

class certification stage or prohibits Plaintiffs from relying 

on an unexecuted but reliable damages model.

In Comcast, plaintiffs put forward four theories of 

liability, but the district court certified a class as to only one 

of them. 569 U.S. at 31. However, the plaintiffs’ damages 

expert—whose model provided the only means of proving 

classwide damages—conducted an indivisible analysis 

which “did not isolate damages resulting from any one [of 

the four] theor[ies] of [liability].” Id. at 32. On appeal, the 

defendants argued class certification was improper because 

plaintiffs had “failed to attribute damages” to the sole 

remaining theory in the case, but the Third Circuit affirmed 

the district court’s grant of class certification over this 

objection, reasoning that it was an improper “attac[k] on the 

merits of the methodology [that had] no place in the class 

certification inquiry.” Id.

The Supreme Court reversed. The Court explained that 

plaintiffs’ “model failed to measure damages resulting from 

the particular antitrust injury on which petitioners’ liability 

in this action is premised.” Id. at 36. This disconnect was 

fatal, because it meant plaintiffs could not “establish[] that 

damages are capable of measurement on a classwide basis.” 

Id. at 34. In other words, where an expert’s damages model 

is untethered from plaintiff’s theory of liability such that it 

has no possibility of demonstrating the amount of damages 

in a particular case, Comcast holds that a plaintiff may not 

rely upon it to show that damages are “capable of 

measurement on a classwide basis.” Id. Notably, the 
22 LYTLE V. NUTRAMAX LABORATORIES, INC.

plaintiffs in Comcast “never challenged” the “need to prove 

damages on a classwide basis” in order to demonstrate 

predominance. Id. at 42 (Ginsburg, J., dissenting).

Nutramax contends that, as in Comcast, permitting 

Plaintiffs here to rely on an unexecuted damages model 

would “reduce Rule 23(b)(3)’s predominance requirement to 

a nullity,” and would “not establish that the requirements of 

Rule 23 are satisfied ‘in fact.’” This is an overreading of 

Comcast. That decision has generally been construed to 

stand for the unremarkable proposition that “plaintiffs must 

be able to show that their damages stemmed from the 

defendant’s actions that created the legal liability.” Leyva v. 

Medline Indus. Inc., 716 F.3d 510, 514 (9th Cir. 2013). 

Indeed, even after Comcast, we have repeatedly reaffirmed 

that class treatment may be appropriate even where damages 

must be assessed on an individualized basis. See Vaquero v. 

Ashley Furniture Indus., Inc., 824 F.3d 1150, 1155 (9th Cir. 

2016); Pulaski & Middleman, LLC v. Google, Inc., 802 F.3d 

979, 987 (9th Cir. 2015). Contrary to Nutramax’s contention, 

the possibility that Dr. Dubé’s analysis might reveal 

damages with respect to some, but not all, of the challenged 

statements does not create “the problem that manifested in 

Comcast,” because, as explained above, the structure of the 

conjoint analysis allows a damages figure to be associated 

with each challenged statement. The same was not true in 

Comcast, where plaintiffs’ damages model, again, did not 

isolate damages between any of plaintiffs’ theories of 

liability. See Comcast, 569 U.S. at 32.

Nutramax’s reliance on Olean is similarly unavailing. In 

Olean, plaintiffs brought an antitrust class action against the 

major U.S. packaged tuna suppliers, alleging that they 

engaged in unlawful price fixing. 31 F.4th at 661–62. The 

appeal centered around whether the district court properly 
LYTLE V. NUTRAMAX LABORATORIES, INC. 23

granted class certification based upon plaintiffs’ expert 

evidence showing antitrust impact. Id. We analyzed each of 

the plaintiffs’ damages models and the defendants’ 

objections to those models, and ultimately concluded that the 

district court did not abuse its discretion in certifying the 

class. See id. at 670–85. 

Nothing in Olean requires that an expert actually execute 

a damages model before it can be relied on. In describing the 

legal standard, Olean made clear that the focus is on “‘the 

method or methods by which plaintiffs propose to use the 

[class-wide] evidence to prove’ the common question in one 

stroke,” and “a district court is limited to resolving whether 

the evidence establishes that a common question is capable

of class-wide resolution, not whether the evidence in fact 

establishes that plaintiffs would win at trial.” Olean, 31 F.4th 

at 666–67 (quoting In re Hydrogen Peroxide Antitrust Litig., 

552 F.3d at 313). While Olean suggested that a district court 

must “[w]eigh[] conflicting expert testimony” and 

“consider[] factors that may undercut the model’s 

reliability,” id. at 666, 683, this does not categorically 

require an expert to execute the expert’s model before it may 

be relied upon. Rather, as explained in more detail below, 

the fact that an expert’s model has not yet been executed is 

simply one factor that must be considered.

Nutramax cites snippets of Olean to argue that 

“[e]vidence is ‘capable of resolving a common issue’ where 

each class member could rely on it at trial and the evidence 

‘could reasonably sustain a jury verdict in favor of the 

plaintiffs, even though a jury could still decide that the 

evidence was not persuasive.’” But this suggests that Olean

required the same showing necessary to avoid summary 

judgment, which it plainly did not. Indeed, Nutramax’s

reading of the “could reasonably sustain a jury verdict” 
24 LYTLE V. NUTRAMAX LABORATORIES, INC.

language in Olean would mean that we took as mandatory

the Supreme Court’s suggestion that class certification might 

sometimes overlap with the merits inquiry and would make 

a determination of the merits required, which would be 

contrary to settled law. It is settled that the question at class 

certification is not whether plaintiffs have put forward 

evidence capable of sustaining a jury verdict, but rather 

whether plaintiffs have shown enough to satisfy FRCP 23. 

To require an actual weighing, at class certification, of 

whether plaintiffs’ evidence could sustain a jury verdict 

would collapse the class certification and summary 

judgment inquiries in precisely the manner Tyson Foods

warns against. 577 U.S. at 457 (noting that failures of proof 

that are common to the class should be engaged “as a matter 

of summary judgment, not class certification”); see also 

Miles v. Kirkland’s Stores Inc., 89 F.4th 1217, 1224 n.2 (9th 

Cir. 2024) (noting a “merits question that should be left for 

summary judgment or trial, not [resolved] at class 

certification”).

Finally, Nutramax contends that where it is uncertain 

that a damages model will show any injury at all, permitting 

a class to be certified and sending notice to class members 

would be an inefficient use of resources. Plaintiffs respond 

that executing a conjoint analysis is extremely expensive and 

time consuming, so that requiring Plaintiffs to do so before 

the precise contours of the class have been established would 

risk wasting resources, not save them. We regard these 

competing speculations as largely irrelevant to determining 

what Rule 23 does or does not require. And since the 

determination of class certification is largely within the 

discretion of the district court, it is worth noting that the vast 

majority of district courts in our circuit to consider the 

question have found that a damages expert need not fully 
LYTLE V. NUTRAMAX LABORATORIES, INC. 25

execute his or her proposed conjoint analysis before it can 

be relied upon at class certification. See, e.g., Gunaratna v. 

Dennis Gross Cosmetology LLC, 2023 WL 5505052, at *19 

(C.D. Cal. Apr. 4, 2023); Bailey v. Rite Aid Corp., 338 

F.R.D. 390, 408 n.14 (N.D. Cal. 2021); Testone v. Barlean’s 

Organic Oils, LLC, 2021 WL 4438391, at *17 (S.D. Cal. 

Sept. 28, 2021); Guido v. L’Oreal, USA, Inc., 2014 WL 

6603730, at *13–14 (C.D. Cal. July 24, 2014).5 This 

common practice weighs against imposing the categorical 

rule Nutramax requests.

5 While there are district court cases that have found a proposed conjoint 

analysis to be insufficiently detailed or thorough to support a finding of 

predominance, they also do not support the categorical rule Nutramax

proposes. Rather, they reflect the unremarkable proposition that an 

underdeveloped expert model is far less likely to be able to establish that 

a particular element is susceptible to common proof. See In re ConAgra 

Foods, Inc., 302 F.R.D. 537, 552 (C.D. Cal. 2014) (rejecting proposed 

conjoint survey where expert simply “offer[ed] a basic description of the 

manner in which hedonic regression and conjoint analysis operate, and 

assert[ed] that the exact specifications [the expert’s analysis would] use 

will be solidified as discovery progresses”); Miller v. Fuhu Inc., 2015 

WL 7776794, at *21-22 (C.D. Cal. Dec. 1, 2015) (concluding that

expert’s testimony that “it is possible and practical to design and 

conduct” a conjoint analysis was insufficient to establish predominance

where expert had not designed such a survey). Indeed, in ConAgra, the 

district court subsequently granted class certification based upon a 

“proposed conjoint analysis” once the expert had provided a more fully 

developed methodology. See In re ConAgra Foods, Inc., 90 F. Supp. 3d 

919, 954 (C.D. Cal. 2015); see also Vizcarra v. Unilever United States, 

Inc., 2023 WL 2364736, at *16–18 (N.D. Cal. Feb. 24, 2023) (noting the 

court had previously rejected un-executed conjoint analysis based on 

methodological flaws, but granting class certification based on revised 

analysis that corrected those flaws, even where the survey had not been 

fully executed).
26 LYTLE V. NUTRAMAX LABORATORIES, INC.

B.

Having concluded that there is no categorical prohibition 

on a district court relying on an unexecuted damages model 

to certify a class, we must nevertheless determine whether, 

on the facts of this case, it was error for the district court to 

grant certification. In particular, Nutramax argues that the 

district court erred in concluding that Dr. Dubé’s opinions 

were sufficiently reliable to satisfy FRE 702 or FRCP 23.

Nutramax argues that because Dr. Dubé had not yet 

obtained all of the data necessary to fully execute his model, 

the district court could not have found that his opinion 

satisfied the requirements of FRE 702 that his testimony be 

“based on sufficient facts or data.” Fed. R. Evid. 702(b). But 

here again, we think Nutramax confuses a class certification 

proceeding with a summary judgment motion. As applied to 

class certification where the issues are commonality and

predominance, the Rule 702(b) question concerns whether 

the data suffices to show that a common question 

predominates over individual issues, not whether the 

subsequently executed model applied to a more complete 

dataset would then meet the requirements of 702(b) as 

applied to a summary judgment motion. By the same token, 

we do not agree with Nutramax that the “rigorous analysis” 

required at the class certification stage means that every 

expert opinion offered at that stage must be subjected to a 

full evidentiary hearing to see if each such opinion meets the 

requirements of Daubert v. Merrell Dow Pharmaceuticals, 

Inc., 509 U.S. 579 (1993).

1.

The manner and extent to which the Daubert framework

applies at the class certification stage is an unsettled 

question. A leading treatise has suggested that there is at 
LYTLE V. NUTRAMAX LABORATORIES, INC. 27

least some divergence among the Circuits on this question, 

with some employing a “full” Daubert inquiry and others 

employing a more limited one. See 3 Newberg and 

Rubenstein on Class Actions § 7:24 (6th ed. 2022).6

Our own precedent has somewhat oscillated between 

these two approaches. In Ellis v. Costco Wholesale Corp., 

we affirmed a district court’s application of the Daubert

standard at the class certification stage. 657 F.3d 970, 982–

83 (9th Cir. 2011); see also 3 Newberg and Rubenstein on 

Class Actions § 7:24 (6th ed. 2022) (characterizing Ellis as 

adopting a full Daubert test). Similarly, in Olean, we stated

in dicta that “[i]n a class proceeding, defendants may 

challenge the reliability of an expert’s evidence under 

Daubert,” although the defendants in Olean did not actually 

raise a Daubert challenge. 31 F.4th at 665 n.7. In Sali v. 

Corona Regional Medical Center, however, we noted that 

while “a district court should evaluate admissibility under 

the standard set forth in Daubert,” whether testimony is 

admissible under that standard is “not . . . dispositive,” but 

instead “should go to the weight that evidence is given at the 

class certification stage.” 909 F.3d at 1006. We cited with 

approval the Eighth Circuit’s decision in In re Zurn Pex 

Plumbing Products Liability Litigation, which is the leading 

decision endorsing a more limited Daubert inquiry. Id. at 

1004; see Cox v. Zurn Pex, Inc. (In re Zurn Pex Plumbing 

Prods. Liab Litig.), 644 F.3d 604, 613 (8th Cir. 2011) (“The 

main purpose of Daubert exclusion is to protect juries from 

being swayed by dubious scientific testimony. That interest 

6 The Supreme Court originally granted certiorari in Comcast to resolve 

this question, but ultimately resolved the case on other grounds once it 

became apparent the question was not properly presented. Comcast, 569 

U.S. at 39–40 (Ginsburg, J., dissenting).
28 LYTLE V. NUTRAMAX LABORATORIES, INC.

is not implicated at the class certification stage where the 

judge is the decision maker.”). 

We think that, at least for purposes of this case, the 

distinction between a “full” and “limited” Daubert inquiry is 

a function of what aspect of FRCP 23 is being addressed. 

Here, the question under FRE 702 is whether the model that 

the plaintiffs’ expert is offering on the issues of commonality 

and predominance is reliable for FRCP 23 purposes. 

Accordingly, such Daubert factors as peer review of the 

proffered model may be highly relevant, while others, such 

as known error rate, may be more applicable to the laterexecuted results of the test. Daubert itself stressed that its 

suggested factors were simply illustrative and needed to be 

applied flexibly, and this surely means applying them only 

to the extent helpful to the issue at hand. See Daubert, 509 

U.S. at 593–94 (noting that “[t]he inquiry envisioned by 

Rule 702 is . . . a flexible one” and disclaiming any intent “to 

set out a definitive checklist or test”).

Thus, for example, whether a “full” or “limited” Daubert

analysis should be applied may depend on the timing of the 

class certification decision. If discovery has closed and an 

expert’s analysis is complete and her tests fully executed, 

there may be no reason for a district court to delay its 

assessment of ultimate admissibility at trial. By contrast, 

where an expert’s model has yet to be fully developed, a 

district court is limited at class certification to making a 

predictive judgment about how likely it is the expert’s 

analysis will eventually bear fruit. This still requires 

determining whether the expert’s methodology is reliable, so 

that a limited Daubert analysis may be necessary, but the 

more full-blown Daubert assessment of the results of the 

application of the model would be premature. 
LYTLE V. NUTRAMAX LABORATORIES, INC. 29

Here, we are satisfied that the district court’s limited 

Daubert analysis was sufficient for the immediate purposes. 

As the district court expressly recognized, “the court 

considers only if expert evidence is useful in evaluating 

whether class certification requirements have been met,” and 

for that purpose a more limited Daubert inquiry may be 

sufficient. This is consistent with the Supreme Court’s 

general rule that “[m]erits questions may be considered to 

the extent—but only to the extent—that they are relevant to 

determining whether the Rule 23 prerequisites for class 

certification are satisfied.” Amgen, 568 U.S. at 466. 

In finding that the prerequisites of FRE 702 were met for 

FRCP 23 purposes, the district court relied on Dr. Dubé’s 

unchallenged credentials, Dr. Dubé’s review of 

documentary evidence and marketing data, the fact that 

“[c]onjoint surveys, like the one proposed by [Dr. Dubé], are 

a well-established method for measuring class-wide 

damages in CLRA mislabeling cases,” and the fact that Dr. 

Dubé had successfully performed conjoint analyses in prior 

cases similar to this. It is true Dr. Dubé has not collected all 

of the necessary data to perform his calculations in the 

instant case, but implicit in Dr. Dubé’s opinion—which the 

district court credited—is the conclusion that he would be 

able to obtain such information, and Nutramax offers no 

reason to think he would be unable to do so. Nor, as 

explained below, has Nutramax shown either that Dr. Dubé’s 

methodology is flawed or that there is a likelihood that he 

will improperly apply that method to the facts. In light of the 

foregoing, we cannot say that the district court abused its 

discretion in rejecting Nutramax’s Daubert challenge to Dr. 

Dubé’s opinion. 
30 LYTLE V. NUTRAMAX LABORATORIES, INC.

2.

Nutramax also argues that, even if an unexecuted 

damages model may in some circumstances support class 

certification, on the facts of this case Dr. Dubé’s model is 

too underdeveloped to satisfy the “rigorous analysis” 

required under FRCP 23. Nutramax contends that Dr. Dubé

has not designed the survey questionnaire, has not 

determined the precise demographic makeup of the 

individuals to be surveyed, has not selected all of the 

parameters for his model, and lacks certain data needed to 

finalize his calculations. Plaintiffs respond that Dr. Dubé has 

in fact fully designed the conjoint analysis and the 

methodology behind it, including by identifying the target 

population, analyzing economic data to determine the 

structure of the market, and specifying the mathematical 

analysis he will perform on the survey results. While 

Plaintiffs acknowledge Dr. Dubé has not yet programmed 

the survey (i.e., written the questions), they cite to Dr. 

Dubé’s testimony describing this as merely “an 

implementation detail,” and argue that it makes sense not to 

finalize the survey questions until the exact scope of the class 

is known. In short, according to Plaintiffs, the survey is fully 

designed and all that remains is for it to be executed.

As already discussed above, Plaintiffs may rely on an 

unexecuted damages model to demonstrate that damages are 

susceptible to common proof. To be sure, the fact the model 

has not been executed remains relevant. Olean makes clear 

that “[t]he determination whether expert evidence is capable 

of resolving a class-wide question in one stroke may include 

‘[w]eighing conflicting expert testimony’ and ‘[r]esolving 

expert disputes’ where necessary to ensure that Rule 

23(b)(3)’s requirements are met.” 31 F.4th at 666 (internal 

citation omitted) (quoting In re Hydrogen Peroxide, 552 
LYTLE V. NUTRAMAX LABORATORIES, INC. 31

F.3d at 323–24). This assessment of a “model’s reliability” 

required by Olean goes beyond the Daubert analysis, and the 

fact that an expert’s model is sufficiently reliable to meet the 

standard of FRE 702 as applied to a FRCP 23 determination 

may not be sufficient to satisfy the standard. See Ellis v. 

Costco Wholesale Corp., 657 F.3d 970, 982 (9th Cir. 2011) 

(suggesting FRCP 23 requires a distinct analysis beyond an 

assessment of admissibility under Daubert); Olean, 31 F.4th 

at 665–66 nn. 7, 9. Rather, the district court must also probe 

the likelihood that the model will be capable of generating 

common answers. A district court may not, however, 

“decline certification merely because it considers plaintiffs’ 

evidence relating to the common question to be unpersuasive 

and unlikely to succeed.” Id. at 667.7

In applying this test to an unexecuted damages model, 

the question a district court must ask is whether the model 

will likely be able to generate common answers at trial. The 

fact that a model is underdeveloped may weigh against a 

finding that it will provide a reliable form of proof. Merely 

gesturing at a model or describing a general method will not 

suffice to meet this standard. Rather, plaintiffs—or their 

expert—must chart out a path to obtain all necessary data 

7 Olean gave the following examples of expert evidence that, while 

otherwise admissible under Daubert, might be unable to generate

common answers: where “the expert evidence was inadequate to prove 

an element of the claim for the entire class [i.e., is not common to all];

where the damages evidence was not consistent with the plaintiffs’ 

theory of liability; where the evidence contained unsupported 

assumptions; or where the evidence demonstrated nonsensical results 

such as false positives, i.e., injury to class members who could not 

logically have been injured by a defendant’s conduct.” Id. at 666 n.9 

(internal citations omitted).
32 LYTLE V. NUTRAMAX LABORATORIES, INC.

and demonstrate that the proposed method will be viable as 

applied to the facts of a given case.8

Here, the district court recognized that Plaintiffs were 

required to “show that damages are capable of measurement 

on a class-wide basis,” while acknowledging they may do so 

without executing the model. On appeal, Nutramax raises a 

flurry of attacks on the reliability of Dr. Dubé’s model that 

were never presented to the district court. While Nutramax

referenced the underdeveloped nature of Dr. Dubé’s model 

throughout its briefing below, the only such argument it 

developed with any thoroughness was the contention that Dr. 

Dubé lacked critical data needed to complete his analysis.

Since we cannot say that the district court abused its 

discretion in failing to consider arguments with which it was 

not presented, see Van v. LLR, Inc., 61 F.4th 1053, 1066 n.9 

(9th Cir. 2023) (“An issue cannot form part of the district 

court’s class certification decision if it was never raised at 

the class certification stage.” (cleaned up)), we focus our 

analysis on those matters considered by the district court.9

8 Plaintiffs’ briefing on appeal seems to advance a rule that merely 

putting forward a viable method is sufficient. To the extent that is 

Plaintiffs’ position, we disagree. Even where a method is otherwise valid 

and reliable under FRE 702, it may nonetheless fail to produce common 

answers for any number of reasons, such as when the model does not 

apply in a manner common to the class. Hence, we underscore that the 

ultimate inquiry is whether a proposed model is likely to provide 

common answers at trial.

9 In particular, on appeal Nutramax relies heavily upon the rebuttal report 

of their expert, Dr. Toubia, to challenge the reliability of Dr. Dubé’s 

methodology. Had Dr. Toubia’s report been fairly presented to the 

district court as a basis for denying class certification, it might have been 

error for the district court to not address it. See Olean, 31 F.4th at 666 
LYTLE V. NUTRAMAX LABORATORIES, INC. 33

With respect to the data Dr. Dubé had not yet collected, 

the district court acknowledged defendant’s argument but 

credited Dr. Dubé’s implicit conclusion that he would be 

able to obtain such data prior to trial. Nutramax has not 

convincingly demonstrated that the district court erred in 

reaching this conclusion. 

Nutramax’s other attacks on Dr. Dubé’s methodology, to 

the extent they were presented to the district court, fare no 

better. As the district court observed, conjoint analysis is a 

well-accepted technique that is frequently used to establish 

damages in CLRA actions. See, e.g., Briseno v. ConAgra 

Foods, Inc., 674 F. App’x 654, 657 (9th Cir. 2017) 

(describing conjoint analysis as a “well-established damages 

model”); Hadley v. Kellogg Sales Co., 324 F. Supp. 3d 1084, 

1107 (N.D. Cal. 2018) (“[C]onjoint analysis is a wellaccepted economic methodology.” (quoting In re Dial 

Complete Mktg. & Sales Pracs. Litig., 320 F.R.D. 326, 331 

(D.N.H. 2017)). Where an expert’s proposed method is 

novel or untested, it makes sense to demand a greater degree 

of specificity and completeness before it is relied upon to 

certify a class. See In re New Motor Vehicles Canadian Exp. 

Antitrust Litig., 522 F.3d 6, 26–30 (1st Cir. 2008) (finding 

that “novelty and complexity of the theories advanced” by 

plaintiffs and their expert made certification of a class based 

upon incomplete model less appropriate). By contrast, here 

there is no dispute that a conjoint analysis is capable of 

measuring classwide damages, at least in the abstract, and 

(suggesting a district court must “[r]esolv[e] expert disputes” at class 

certification). However, our review of the record reveals that Nutramax 

never attempted to use Dr. Toubia’s rebuttal report to attack Dr. Dubé’s 

model, and instead cited Dr. Toubia’s rebuttal report only a single time, 

and for an entirely unrelated proposition.
34 LYTLE V. NUTRAMAX LABORATORIES, INC.

the only real question at this stage is whether Dr. Dubé will 

properly apply the method to the facts of the case.

Nutramax cites a variety of potential errors Dr. Dubé

might commit in executing his damages model. For example, 

Nutramax argues “the precise wording of a questionnaire is 

critical” and could “bias[] the results,” and that “assumptions 

underlying [his] economic model” may not account for realworld factors. While unanswered questions such as these, 

and the attendant possibility of errors, are certainly relevant, 

Nutramax offers no reason to think that Dr. Dubé will 

commit any of these errors. Dr. Dubé’s qualifications are 

undisputed, he has successfully conducted conjoint analyses 

in the past, and Dr. Dubé testified he did not “envision 

anything particularly unique about this survey.” The 

speculative possibility that Dr. Dubé might slip up in 

executing his model, standing alone, is insufficient to defeat 

class certification. See Sali, 909 F.3d at 1004–05 (“Neither 

the possibility that a plaintiff will be unable to prove his 

allegations, nor the possibility that the later course of the suit 

might unforeseeably prove the original decision to certify the 

class wrong, is a basis for declining to certify a class which 

apparently satisfies [Rule 23].” (quoting Blackie v. Barrack, 

524 F.2d 891, 901 (9th Cir. 1975)).10

Accordingly, the record was sufficient to support the 

district court’s conclusion that Dr. Dubé’s model is capable 

of showing damages on a class wide basis.

10 This is especially true here given that “[c]lass wide damages 

calculations under the . . . CLRA are particularly forgiving” and 

“require[] only that some reasonable basis of computation of damages be 

used.” Nguyen v. Nissan N. Am., Inc., 932 F.3d 811, 818 (9th Cir. 2019) 

(quoting Lambert, 870 F.3d at 1183).
LYTLE V. NUTRAMAX LABORATORIES, INC. 35

C.

As the above discussion makes clear, it is not required 

that a plaintiff’s expert must execute their damages model 

prior to class certification provided it is shown that the model 

provides a reliable and adequate method for calculating 

damages. We do, however, think it important to make clear 

that a plaintiff may not avoid ultimate scrutiny of the 

admissibility of their experts’ final opinions simply by 

declining to develop those opinions in advance of class 

certification. Accordingly, on remand, Nutramax must be 

given the opportunity in advance of trial to test the 

sufficiency and reliability of Dr. Dubé’s model once it has 

been fully executed, including through a motion for 

summary judgment and/or a renewed Daubert motion.

IV.

The second broad issue raised by Nutramax on appeal is 

whether the district court erred in concluding that common 

questions predominated with respect to the element of 

reliance. California’s CLRA prohibits “unfair methods of 

competition and unfair or deceptive acts or practices.” Cal. 

Civ. Code § 1770(a). To bring a CLRA claim, a plaintiff 

must show (1) the defendant engaged in deceptive conduct 

and (2) the deception caused plaintiff harm. Stearns, 655 

F.3d at 1022. However, under the CLRA, “[c]ausation, on a 

classwide basis, may be established by materiality. If the 

trial court finds that material misrepresentations have been 

made to the entire class, an inference of reliance arises as to 

the class.” Id. (quoting In re Vioxx Class Cases, 180 Cal. 

App. 4th 116, 129 (2009)). A misrepresentation is material 

“if a reasonable [person] would attach importance to its 

existence or nonexistence in determining his choice of action 

in the transaction in question[.]” Id. 
36 LYTLE V. NUTRAMAX LABORATORIES, INC.

Because materiality (and, hence, in this case reliance) 

may be proved by reference to an objective, reasonable 

consumer standard, reliance under the CLRA is generally 

susceptible to common proof. See Noel v. Thrifty Payless, 

Inc., 17 Cal. App. 5th 1315, 1334 (2017) (“When the 

consumer shows the complained-of misrepresentation would 

have been material to any reasonable person, he or she has 

carried the burden of showing actual reliance and causation 

of injury for each member of the class.”), rev’d on other 

grounds, 7 Cal. 5th 955 (2019); Stearns, 655 F.3d at 1022; 

Bradach v. Pharmavite, LLC, 735 F. App’x 251, 254 (9th 

Cir. 2018) (“Under California law, class members in CLRA

. . . actions are not required to prove their individual reliance 

on the allegedly misleading statements. Instead, the standard 

. . . is whether members of the public are likely to be 

deceived. For this reason, . . . [CLRA] claims are ideal for 

class certification because they will not require the court to 

investigate class members’ individual interaction with the 

product.” (internal quotation marks and citations omitted)).

However, while materiality can support an inference of 

reliance, that does not necessarily mean that the inference 

will hold as to the entire class, such that common questions 

predominate. “If the misrepresentation or omission is not 

material as to all class members, the issue of reliance ‘would 

vary from consumer to consumer’ and the class should not 

be certified.” Stearns, 665 F.3d at 1022–23 (quoting Vioxx, 

180 Cal. App. 4th at 129).

The district court cited the correct legal standard, noting 

that materiality can be used to establish reliance under the 

CLRA, while also acknowledging that the presumption is 

rebuttable insofar as “reliance would vary from consumer to 

consumer.” Nutramax nonetheless contendsthe district court 

committed legal error by finding “the ‘objective test’ used 
LYTLE V. NUTRAMAX LABORATORIES, INC. 37

for materiality rendered materiality and thus causation 

inherently a common issue.” Read in context, we do not 

think the district court applied an incorrect, irrebuttable 

presumption as Nutramax suggests. Rather, the observation

that materiality and causation are “inherently common 

issues” was made only after the district court considered the 

relevant evidence of materiality and concluded it supported 

an application of the presumption. We therefore review the 

district court’s findings on this issue for abuse of discretion. 

See Sali, 909 F.3d at 1002.

11

Plaintiffs presented ample evidence to show that the 

challenged statements would be materially misleading to a 

reasonable consumer. To demonstrate that a reasonable 

consumer would have understood the challenged statements

to have promised that the product would improve a dog’s

joint health, Plaintiffs cited the testimony of the named 

plaintiffs, the testimony of their advertising expert,

Silverman, and the survey results of Nutramax’s own expert,

Dr. Scott, which indicated that the near-universal 

understanding of Cosequin’s purpose was that it would 

“improve/help/maintain mobility, flexibility, joint 

health/support.” This understanding, caused by Nutramax’s

packaging statements, was, according to Plaintiffs’ experts, 

false and misleading because there is no evidence that 

Cosequin improves a dog’s joint health. The district court 

credited all of this evidence and expressly found that it 

“outweighed” the contrary evidence presented by 

defendants’ experts, including Dr. Scott. Nutramax has not 

11 We also do not think the district court committed legal error when it 

cited the rule that “a plaintiff is not required to show that the challenged 

statement is the ‘sole or even the decisive cause’ influencing the class 

members’ decisions to buy the challenged products.” As explained 

below, this statement is consistent with California law.
38 LYTLE V. NUTRAMAX LABORATORIES, INC.

shown that this conclusion was an abuse of discretion. The 

district court thus correctly found that Plaintiffs had 

demonstrated that the presumption of reliance applied.12

Nutramax nonetheless argues that it put forward 

sufficient evidence to rebut the presumption, and the district 

court erred in concluding otherwise. Nutramax points to 

what it describes as “overwhelming evidence that 

veterinarians frequently recommend joint supplements, 

including Cosequin.” Nutramax similarly cites the results of 

Dr. Scott’s survey, which suggests approximately half of 

survey respondents decided to purchase Cosequin before 

going to a physical store or website. According to Nutramax, 

all of this demonstrates that individualized assessments will 

be needed to determine whether any given class member 

actually relied on the label. 

The fact some class members considered sources of 

information other than the packaging in making their 

purchasing decisions does not necessarily undermine

reliance. To establish reliance under the CLRA, a 

misrepresentation need not be “the sole or even the decisive

cause of the injury-producing conduct.” Moore v. Mars 

Petcare US, Inc., 966 F.3d 1007, 1020 (9th Cir. 2020) 

(quoting Kwikset Corp. v. Superior Ct., 246 P.3d 877, 888 

(Cal. 2011)). For example, in Moore, we found that “[t]he 

fact that vets had prescribed each Plaintiff the pet food—

rather than each discovering the pet food on their own—

12 Nutramax argues that Plaintiffs’ evidence of falsity showed only that 

Cosequin was unable to treat dogs with arthritis, not that it was totally 

ineffective at promoting joint health generally, but Nutramax made this 

argument in the court below and it was expressly rejected by the district 

court, which credited Plaintiffs’ evidence that “Cosequin has no effect 

on joint health.” 
LYTLE V. NUTRAMAX LABORATORIES, INC. 39

[did] not negate the allegation of actual reliance because the 

prescription requirement and advertising need not be the sole 

or even the decisive cause of the purchase.” Id. at 1020–21.

In other words, Moore rejected the very argument now 

advanced by Nutramax, that there can be no reliance where 

a veterinarian recommends a product. 

Nutramax attempts to distinguish Moore on the grounds 

that it was decided on a motion to dismiss. However, at least 

one California appellate court has applied the rule beyond 

the motion to dismiss context. See Veera v. Banana 

Republic, LLC, 6 Cal. App. 5th 907, 919 (2016) (applying 

rule to deny motion for summary judgment). And the model 

jury instructions for a CLRA claim promulgated by the 

Judicial Council of California similarly indicate that, “[t]o 

prove reliance, [name of plaintiff] need only prove that the 

representation was a substantial factor in [his/her/nonbinary 

pronoun] decision” and “does not need to prove that it was 

the primary factor or the only factor in the decision.” Judicial 

Council of California, Model Civil Jury Instruction No. 

4700.13

13 The full instruction reads: 

[[Name of plaintiff]’s harm resulted from [name of 

defendant]’s conduct if [name of plaintiff] relied on 

[name of defendant]’s representation. To prove 

reliance, [name of plaintiff] need only prove that the 

representation was a substantial factor in 

[his/her/nonbinary pronoun] decision. 

[He/She/Nonbinary pronoun] does not need to prove 

that it was the primary factor or the only factor in the 

decision.
40 LYTLE V. NUTRAMAX LABORATORIES, INC.

To be sure, the fact that some consumers relied on other 

sources of information is relevant to the assessment of 

reliance. In an appropriate case, such evidence might be 

sufficient to demonstrate that the misrepresentation was not 

“a substantial factor” in a large percentage of consumers’ 

purchasing decisions. But we do not think it was an abuse of 

discretion for the district court to find to the contrary here. If 

one imagines a counterfactual where the packaging

contained no suggestion that Cosequin benefited a dog’s 

joint health, it is entirely plausible that no consumer would 

have chosen to purchase the product. See Alvarez v. NBTY, 

Inc., 331 F.R.D. 416, 423 (S.D. Cal. 2019) (“Without [the 

challenged] statement, no consumer would have a reason to 

purchase the Products and would otherwise be purchasing a 

random bottle of supplements without any knowledge of 

what benefit, if any, the supplements provided.”). Further, 

while one of Plaintiffs’ theories of causation is that they 

would not have purchased the products had they known the 

truth, another theory is that they paid more than they 

otherwise would have as a result of the misleading 

statements. See Hinojos v. Kohl’s Corp., 718 F.3d 1098, 

1108 (9th Cir. 2013) (“[T]he CLRA’s ‘any damage’ 

requirement is a capacious one that includes any pecuniary 

damage as well as opportunity costs and transaction costs 

that result when a consumer is misled by deceptive 

marketing practices.”). If, as Plaintiffs contend, Cosequin 

provided no benefits to joint health, it is still more plausible 

If [name of defendant]’s representation of fact was 

material, reliance may be inferred. A fact is material if 

a reasonable consumer would consider it important in 

deciding whether to buy or lease the [goods/services].]
LYTLE V. NUTRAMAX LABORATORIES, INC. 41

that the consumers would have paid less had they known the 

truth.

This construction of the reliance requirement comports 

with the objectives of the CLRA more generally. The 

California legislature has declared that the CLRA “shall be 

liberally construed and applied to promote its underlying 

purposes, which are to protect consumers against unfair and 

deceptive business practices and to provide efficient and 

economical procedures to secure such protection.” Cal. Civ. 

Code § 1760. Consistent with this remedial objective, we 

have suggested that CLRA claims “are ideal for class 

certification because they will not require the court to 

investigate class members’ individual interaction with the 

product.” Bradach, 735 F. App’x at 254 (quotation omitted). 

Nutramax’s cramped interpretation of the reliance 

requirement, permitting the presumption to be overcome 

based upon marginally different interactions with a product 

containing a label that is otherwise materially misleading, 

would undermine the presumption of reliance and the 

capacity to bring CLRA class actions.

None of the cases that Nutramax cites, finding that the 

presumption of reliance was overcome, is to the contrary.

For example, in Stearns v. Ticketmaster Corp., the plaintiffs 

sued Ticketmaster for creating a website that misled 

individuals who purchased tickets into inadvertently signing 

up for an unrelated monthly subscription. 655 F.3d at 1017. 

While the district court acknowledged that this practice 

likely violated the CLRA, it nevertheless found class 

certification inappropriate because the plaintiffs’ proposed 

class was too broad. Id. at 1024. We upheld this holding and 

explained that materiality was not uniform across the class 

because many class members would not have been deceived 
42 LYTLE V. NUTRAMAX LABORATORIES, INC.

by Ticketmaster’s marketing practice and had intentionally 

signed up for the subscription. Id.

Again, in In re Vioxx Class Cases, the “[p]laintiffs

suggest[ed] that Merck hid an increased risk of death, 

associated with Vioxx,” which plaintiffs used to support an 

inference of reliance. 180 Cal. App. 4th 116, 133 (2009)

(cleaned up). While a risk of death is material in the abstract, 

the trial court found class treatment inappropriate because 

defendants had introduced “overwhelming evidence” that 

materiality varied on an individual basis. Id. For example, 

the record contained undisputed evidence that the drug 

actually did not increase the risk of death for all of the class 

members and that some individuals continued taking the 

drug even after learning of the risks in light of its substantial 

countervailing benefits. Id. at 133–34. 

Finally, in Fairbanks v. Farmers New World Life Ins. 

Co., the plaintiffs alleged that defendant’s marketing 

practices surrounding “universal life insurance” were 

misleading because the marketing suggested the policies 

were “permanent” when in fact the policies were not 

permanent and were systematically underfunded. 197 Cal. 

App. 4th 544, 553 (2011). The court observed that plaintiffs’ 

class certification motion “assume[d] that anyone who 

purchases universal insurance does so because . . . a 

universal policy (if sufficiently funded) can be permanent,” 

when in fact the record showed there were many other 

reasons an individual might purchase a universal insurance 

policy unrelated to its supposed permanence. Id. at 565. 

Therefore, not all class members would find the 

misrepresentations about the policy being “permanent” 

material or misleading. 
LYTLE V. NUTRAMAX LABORATORIES, INC. 43

The common theme unifying each of these cases is that 

a sizable portion of the class either were not misled by the 

statements or would not have found the misrepresentations 

to be material had they known the truth. Here, by contrast, 

Dr. Scott’s own survey indicated that the near-universal 

reason class members purchased Cosequin was because it 

would “improve/help/maintain mobility, flexibility, joint 

health/support.” Indeed, it is difficult to see why else 

consumers would purchase this “joint health supplement” 

other than to improve their dog’s joint health. For purposes 

of class certification, Plaintiffs have adequately 

demonstrated that a reasonable consumer would have been 

misled into believing Cosequin would improve their dogs’ 

joint health, when, in fact, Cosequin provided no such 

benefits, and that this misrepresentation would have been 

material as to the entire class. The district court thus did not 

abuse its discretion in concluding reliance may be proven on 

a class wide basis.

V.

For the foregoing reasons, we affirm the district court’s 

grant of class certification.

AFFIRMED.