                               In the

    United States Court of Appeals
                 For the Seventh Circuit
                     ____________________
Nos. 15-1776
VINCE MULLINS, on behalf of
himself and all others similarly situated,
                                                   Plaintiff-Appellee.
                                 v.

DIRECT DIGITAL, LLC, a Delaware Limited
Liability Company,
                                               Defendant-Appellant.
                     ____________________

         Appeal from the United States District Court for the
           Northern District of Illinois, Eastern Division.
            No. 13 CV 1829 — Charles R. Norgle, Judge.
                     ____________________

        ARGUED JUNE 3, 2015 — DECIDED JULY 28, 2015

   Before BAUER, KANNE, and HAMILTON, Circuit Judges.
    HAMILTON, Circuit Judge. We agreed to hear this appeal
under Federal Rule of Civil Procedure 23(f), which permits
interlocutory review of orders granting or denying class ac-
tion certification, to address whether Rule 23(b)(3) imposes a
heightened “ascertainability” requirement as the Third Cir-
cuit and some district courts have held recently. See, e.g.,
Carrera v. Bayer Corp., 727 F.3d 300 (3d Cir. 2013). In this case,
2                                                    No. 15-1776

the plaintiff alleges consumer fraud by the seller of a dietary
supplement, and the district court certified a plaintiff class.
The court found that the proposed class satisfies the explicit
requirements of Rule 23(a) and (b)(3), and the court rejected
defendant’s argument that Rule 23(b)(3) implies a height-
ened ascertainability requirement.
    We affirm. We and other courts have long recognized an
implicit requirement under Rule 23 that a class must be de-
fined clearly and that membership be defined by objective
criteria rather than by, for example, a class member’s state of
mind. In addressing this requirement, courts have some-
times used the term “ascertainability.” They have applied
this requirement to all class actions, regardless of whether
certification was sought under Rule 23(b)(1), (2), or (3). Class
definitions have failed this requirement when they were too
vague or subjective, or when class membership was defined
in terms of success on the merits (so-called “fail-safe” clas-
ses). This version of ascertainability is well-settled in our cir-
cuit, and this class satisfies it.
    More recently, however, some courts have raised the bar
for class actions under Rule 23(b)(3). Using the term “ascer-
tainability,” at times without recognizing the extension,
these courts have imposed a new requirement that plaintiffs
prove at the certification stage that there is a “reliable and
administratively feasible” way to identify all who fall within
the class definition. These courts have moved beyond exam-
ining the adequacy of the class definition itself to examine
the potential difficulty of identifying particular members of
the class and evaluating the validity of claims they might
eventually submit. See Byrd v. Aaron’s Inc., 784 F.3d 154, 168
No. 15-1776                                                  3

(3d Cir. 2015) (distinguishing between our circuit’s standard
and the Third Circuit’s ascertainability requirement).
    This heightened requirement has defeated certification,
especially in consumer class actions. See, e.g., Karhu v. Vital
Pharmaceuticals, Inc., — F. App’x —, 2015 WL 3560722, at *2–4
(11th Cir. June 9, 2015) (purchasers of dietary supplements);
Carrera, 727 F.3d at 307–12 (purchasers of dietary supple-
ments); Xavier v. Philip Morris USA Inc., 787 F. Supp. 2d 1075,
1089–90 (N.D. Cal. 2011) (Marlboro smokers); Weiner v. Snap-
ple Beverage Corp., No. 07 Civ. 8742(DLC), 2010 WL 3119452,
at *12–13 (S.D.N.Y. Aug. 5, 2010) (purchasers of Snapple bev-
erages). All of these classes would seem to have satisfied the
established meaning of “ascertainability.” See generally Myr-
iam Gilles, Class Dismissed: Contemporary Judicial Hostility to
Small-Claims Consumer Class Actions, 59 DePaul L. Rev. 305
(2010) (describing recent cases).
    We decline to follow this path and will stick with our set-
tled law. Nothing in Rule 23 mentions or implies this height-
ened requirement under Rule 23(b)(3), which has the effect
of skewing the balance that district courts must strike when
deciding whether to certify classes. The policy concerns mo-
tivating the heightened ascertainability requirement are bet-
ter addressed by applying carefully the explicit requirements
of Rule 23(a) and especially (b)(3). These existing require-
ments already address the balance of interests that Rule 23 is
designed to protect. A court must consider “the likely diffi-
culties in managing a class action,” but in doing so it must
balance countervailing interests to decide whether a class
action “is superior to other available methods for fairly and
efficiently adjudicating the controversy.” See Fed. R. Civ. P.
23(b)(3).
4                                                    No. 15-1776

    The heightened ascertainability requirement upsets this
balance. In effect, it gives one factor in the balance absolute
priority, with the effect of barring class actions where class
treatment is often most needed: in cases involving relatively
low-cost goods or services, where consumers are unlikely to
have documentary proof of purchase. These are cases where
the class device is often essential “to overcome the problem
that small recoveries do not provide the incentive for any
individual to bring a solo action prosecuting his or her
rights.” Amchem Products, Inc. v. Windsor, 521 U.S. 591, 617
(1997), quoting Mace v. Van Ru Credit Corp., 109 F.3d 338, 344
(7th Cir. 1997); see also Suchanek v. Sturm Foods, Inc., 764 F.3d
750, 760 (7th Cir. 2014) (reversing denial of class certification:
“a class action has to be unwieldy indeed before it can be
pronounced an inferior alternative—no matter how massive
the fraud or other wrongdoing that will go unpunished if
class treatment is denied—to no litigation at all”), quoting
Carnegie v. Household Int’l, Inc., 376 F.3d 656, 661 (7th Cir.
2004) (affirming certification of class with millions of mem-
bers).
I. Factual and Procedural Background
    Plaintiff Vince Mullins sued defendant Direct Digital,
LLC for fraudulently representing that its product, Instaflex
Joint Support, relieves joint discomfort. He alleges that
statements on the Instaflex labels and marketing materials—
“relieve discomfort,” “improve flexibility,” “increase mobili-
ty,” “support cartilage repair,” “scientifically formulated,”
and “clinically tested for maximum effectiveness”—are
fraudulent because the primary ingredient in the supple-
ment (glucosamine sulfate) is nothing more than a sugar pill
and there is no scientific support for these claims. Mullins
No. 15-1776                                                                5

asserts that Direct Digital is liable for consumer fraud under
the Illinois Consumer Fraud and Deceptive Business Practic-
es Act, 815 ILCS 505/1 et seq., and similar consumer protec-
tion laws in nine other states.
    Mullins moved to certify a class of consumers “who pur-
chased Instaflex within the applicable statute of limitations
of the respective Class States for personal use until the date
notice is disseminated.” The district court certified the class
under Rule 23(b)(3).
    Direct Digital filed a petition for leave to appeal under
Rule 23(f) arguing that the district court abused its discretion
in certifying the class without first finding that the class was
“ascertainable.” Direct Digital also argued that the district
court erred by concluding that the efficacy of a health prod-
uct can qualify as a “common” question under Rule 23(a)(2).
We granted the Rule 23(f) petition primarily to address the
developing law of ascertainability, including among district
courts within this circuit. See Blair v. Equifax Check Services,
Inc., 181 F.3d 832, 835 (7th Cir. 1999) (granting an appeal is
appropriate to “facilitate the development of the law” gov-
erning class actions). 1

    1 Compare Jenkins v. White Castle Mgmt. Co., No. 12 CV 7273, 2015
WL 832409, at *3–4 (N.D. Ill. Feb. 25, 2015) (favorably citing Carrera and
denying certification), with Boundas v. Abercrombie & Fitch Stores, Inc., 280
F.R.D. 408, 417–18 (N.D. Ill. 2012) (rejecting stringent version of ascer-
tainability and certifying class); see also Balschmiter v. TD Auto Finance
LLC, 303 F.R.D. 508, 514 (E.D. Wis. 2014) (noting “a dearth of case law
from this circuit on the requirement” of ascertainability and discussing
Third Circuit precedent); Harris v. comScore, Inc., 292 F.R.D. 579, 587–88
(N.D. Ill. 2013) (favorably citing Third Circuit precedent adopting
heightened ascertainability but also the district court opinion in Carrera,
which was later vacated by the Third Circuit).
6                                                    No. 15-1776

    We review the grant or denial of a motion for class certi-
fication for an abuse of discretion, e.g., Harper v. Sheriff of
Cook County, 581 F.3d 511, 514 (7th Cir. 2009), but a decision
based on an erroneous view of the law, such as imposing a
new requirement under Rule 23(b)(3), is likely to be an abuse
of discretion. E.g., Ervin v. OS Restaurant Services, Inc., 632
F.3d 971, 976 (7th Cir. 2011) (“If, however, the district court
applies an incorrect legal rule as part of its decision, then the
framework within which it has applied its discretion is
flawed, and the decision must be set aside as an abuse.”).
II. Analysis
    A. The Established Meaning of “Ascertainability”
    We begin with the current state of the law in this circuit.
Rule 23 requires that a class be defined, and experience has
led courts to require that classes be defined clearly and
based on objective criteria. See William B. Rubenstein, New-
berg on Class Actions § 3:3 (5th ed. 2015); Joseph M. McLaugh-
lin, McLaughlin on Class Actions § 4:2 (11th ed. 2014); see, e.g.,
Matamoros v. Starbucks Corp., 699 F.3d 129, 139 (1st Cir. 2012);
Bakalar v. Vavra, 237 F.R.D. 59, 64 (S.D.N.Y. 2006); De-
Bremaecker v. Short, 433 F.2d 733, 734 (5th Cir. 1970) (per curi-
am). When courts wrote of this implicit requirement of “as-
certainability,” they trained their attention on the adequacy
of the class definition itself. They were not focused on
whether, given an adequate class definition, it would be dif-
ficult to identify particular members of the class.
    This “weak” version of ascertainability has long been the
law in this circuit. See Jamie S. v. Milwaukee Public Schools, 668
F.3d 481, 495 (7th Cir. 2012) (“It’s not hard to see how this
class lacks the definiteness required for class certification;
No. 15-1776                                                     7

there is no way to know or readily ascertain who is a mem-
ber of the class.”); Oshana v. Coca-Cola Co., 472 F.3d 506, 513
(7th Cir. 2006) (class definition “must be definite enough that
the class can be ascertained”); accord, Adashunas v. Negley,
626 F.2d 600, 604 (7th Cir. 1980) (“In summary, the proposed
class of plaintiffs is so highly diverse and so difficult to iden-
tify that it is not adequately defined or nearly ascertaina-
ble.”).
    The language of this well-settled requirement is suscepti-
ble to misinterpretation, though, which may explain some of
the doctrinal drift described below. To understand its estab-
lished meaning, it’s better to focus on the three common
problems that have caused plaintiffs to flunk this require-
ment.
    First, classes that are defined too vaguely fail to satisfy
the “clear definition” component. See, e.g., Young v. Nation-
wide Mutual Ins. Co., 693 F.3d 532, 538 (6th Cir. 2012) (“There
can be no class action if the proposed class is amorphous or
imprecise.” (citation and internal quotation marks omitted));
APB Associates, Inc. v. Bronco’s Saloon, Inc., 297 F.R.D. 302, 316
(E.D. Mich. 2013) (denying certification because proposed
class definition was too “imprecise and amorphous”); De-
Bremaecker, 433 F.2d at 734 (affirming denial of certification
for proposed class defined as residents “active in the ‘peace
movement’”); 7A Charles Alan Wright et al., Federal Practice
& Procedure § 1760 (3d ed. 2005) (collecting cases). Vagueness
is a problem because a court needs to be able to identify who
will receive notice, who will share in any recovery, and who
will be bound by a judgment. See Kent v. SunAmerica Life Ins.
Co., 190 F.R.D. 271, 278 (D. Mass. 2000). To avoid vagueness,
class definitions generally need to identify a particular
8                                                     No. 15-1776

group, harmed during a particular time frame, in a particu-
lar location, in a particular way. See McLaughlin on Class Ac-
tions § 4:2; see, e.g., Rodriguez v. Berrybrook Farms, Inc., 672 F.
Supp. 1009, 1012 (W.D. Mich. 1987) (granting certification
and noting the class definition specified “a group of agricul-
tural laborers during a specific time frame and at a specific
location who were harmed in a specific way”).
    Second, classes that are defined by subjective criteria,
such as by a person’s state of mind, fail the objectivity re-
quirement. E.g., Simer v. Rios, 661 F.2d 655, 669–70 (7th Cir.
1981) (affirming denial of certification of class of people who
felt discouraged from applying for government energy assis-
tance); Alliance to End Repression v. Rochford, 565 F.2d 975,
977–78 (7th Cir. 1977) (affirming certification of class defined
by actions of defendants rather than class members’ states of
mind); Harris v. General Development Corp., 127 F.R.D. 655, 659
(N.D. Ill. 1989) (denying class certification of proposed sub-
class defined by mental state: “The proposed class of persons
who allegedly were discouraged from applying at GDC is
too imprecise and speculative to be certified.”); 7A Wright et
al., Federal Practice & Procedure § 1760 (collecting cases).
Plaintiffs can generally avoid the subjectivity problem by de-
fining the class in terms of conduct (an objective fact) rather
than a state of mind. See, e.g., National Org. for Women, Inc. v.
Scheidler, 172 F.R.D. 351, 358–59 (N.D. Ill. 1997) (accepting
modified class definition so that “membership in the classes
sought to be certified is based exclusively on the defendants’
conduct with no particular state of mind required”); Newberg
on Class Actions § 3:5.
  Third, classes that are defined in terms of success on the
merits—so-called “fail-safe classes”—also are not properly
No. 15-1776                                                      9

defined. See In re Nexium Antitrust Litig., 777 F.3d 9, 22 (1st
Cir. 2015); Young, 693 F.3d at 538; Messner v. Northshore Univ.
HealthSystem, 669 F.3d 802, 825 (7th Cir. 2012); Randleman v.
Fidelity Nat’l Title Ins. Co., 646 F.3d 347, 352 (6th Cir. 2011);
but see In re Rodriguez, 695 F.3d 360, 369–70 (5th Cir. 2012)
(affirming fail-safe class certification). Defining the class in
terms of success on the merits is a problem because “a class
member either wins or, by virtue of losing, is defined out of
the class and is therefore not bound by the judgment.” Mess-
ner, 669 F.3d at 825. This raises an obvious fairness problem
for the defendant: the defendant is forced to defend against
the class, but if a plaintiff loses, she drops out and can sub-
ject the defendant to another round of litigation. See Erin L.
Geller, Note, The Fail-Safe Class as an Independent Bar to Class
Certification, 81 Fordham L. Rev. 2769 (2013). The key to
avoiding this problem is to define the class so that member-
ship does not depend on the liability of the defendant.
     The class definition in this case complies with this settled
law and avoids all of these problems. It is not vague. It iden-
tifies a particular group of individuals (purchasers of In-
staflex) harmed in a particular way (defrauded by labels and
marketing materials) during a specific period in particular
areas. The class definition also is not based on subjective cri-
teria. It focuses on the act of purchase and Direct Digital’s
conduct in labeling and advertising the product. It also does
not create a fail-safe class. If Direct Digital prevails, res judi-
cata will bar class members from re-litigating their claims.
    Direct Digital argues, however, that we should demand
more. It urges us to adopt a new component to the ascertain-
ability requirement that goes beyond the adequacy of the
class definition itself. Drawing on recent decisions by the
10                                                   No. 15-1776

Third Circuit, Direct Digital argues that class certification
should be denied if the plaintiff fails to show a reliable and
administratively feasible way to determine whether a partic-
ular person is a member of the class. And, Direct Digital con-
tinues, affidavits from putative class members are insuffi-
cient as a matter of law to satisfy this requirement.
    In support of this argument, Direct Digital asserts that
the only method of identifying class members here is by af-
fidavit from the putative class members themselves. That
remains to be seen. We do not know yet what sales and cus-
tomer records Direct Digital has. We assume for purposes of
this decision that Direct Digital will have no records for a
large number of retail customers. We also assume that many
consumers of Instaflex are unlikely to have kept their re-
ceipts since it’s a relatively inexpensive consumer good.
     B. The Recent Expansion of “Ascertainability”
    To understand the genesis of Direct Digital’s argument,
we briefly summarize the law of the Third Circuit, which has
adopted this more stringent version of ascertainability. The
Third Circuit’s innovation began with Marcus v. BMW of
North America, LLC, 687 F.3d 583 (3d Cir. 2012), where the
court vacated certification of a poorly defined class. The de-
cisive portion of the opinion, id. at 592–94, certainly seems
sound, but the opinion went on to caution that on remand, if
defendants’ records would not identify class members, the
district court should not approve a method relying on “po-
tential class members’ say so,” and the opinion said that reli-
ance on class members’ affidavits might not be “proper or
just,” id. at 594 (internal quotation marks omitted). The opin-
ion did not explain this new requirement other than to cite
an easily distinguishable district court decision.
No. 15-1776                                                              11

    Since Marcus, the court has applied this heightened ascer-
tainability requirement in several more cases: Hayes v. Wal-
Mart Stores, Inc., 725 F.3d 349, 354–56 (3d Cir. 2013); Carrera v.
Bayer Corp., 727 F.3d 300, 305–12 (3d Cir. 2013); Grandalski v.
Quest Diagnostics Inc., 767 F.3d 175, 184–85 (3d Cir. 2014);
Shelton v. Bledsoe, 775 F.3d 554, 559–63 (3d Cir. 2015); Byrd v.
Aaron’s Inc., 784 F.3d 154, 161–71 (3d Cir. 2015). As the re-
quirement has evolved, several members of the court have
expressed doubts about the expanding ascertainability doc-
trine. See Byrd, 784 F.3d at 172–77 (Rendell, J., concurring);
Carrera v. Bayer Corp., No. 12–2621, 2014 WL 3887938, at *1–3
(3d Cir. May 2, 2014) (Ambro, J., dissenting from denial of
rehearing en banc). 2
    As it stands now, the Third Circuit’s test for ascertainabil-
ity has two prongs: (1) the class must be “defined with refer-
ence to objective criteria” (consistent with long-established
law discussed above), and (2) there must be “a reliable and
administratively feasible mechanism for determining wheth-

    2 The Eleventh Circuit recently applied a fairly strong version of an
ascertainability requirement in a non-precedential decision, Karhu v. Vital
Pharmaceuticals, Inc., — F. App’x —, 2015 WL 3560722, at *2–4 (11th Cir.
June 9, 2015) (unpublished). Some courts have followed the Third Cir-
cuit’s innovation. See, e.g., Jenkins v. White Castle Mgmt. Co., No. 12 CV
7273, 2015 WL 832409, at *3–4 (N.D. Ill. Feb. 25, 2015); Jones v. ConAgra
Foods, Inc., No. C 12–01633 CRB, 2014 WL 2702726, at *8–11 (N.D. Cal.
June 13, 2014), appeal docketed, No. 14–16327; Sethavanish v. ZonePerfect
Nutrition Co., No. 12–2907–SC, 2014 WL 580696, at *5–6 (N.D. Cal. Feb.
13, 2014). Others have rejected it. See, e.g., Daniels v. Hollister Co., 113
A.3d 796, 798–803 (N.J. App. 2015); Rahman v. Mott’s LLP, No. 13–cv–
03482–SI, 2014 WL 6815779, at *4 (N.D. Cal. Dec. 3, 2014); Lilly v. Jamba
Juice Co., No. 13–cv–02998–JST, 2014 WL 4652283, at *4–6 (N.D. Cal. Sept.
18, 2014); In re ConAgra Foods, Inc., 302 F.R.D. 537, 565–67 (C.D. Cal.
2014).
12                                                  No. 15-1776

er putative class members fall within the class definition.”
Byrd, 784 F.3d at 163, quoting Carrera, 727 F.3d at 355; see al-
so Shelton, 775 F.3d at 560 (making clear that “the question of
ascertainability” is separate from “the question of whether
the class was properly defined”).
    This second requirement sounds sensible at first glance.
Who could reasonably argue that a plaintiff should be al-
lowed to certify a class whose members are impossible to
identify? In practice, however, some courts have used this
requirement to erect a nearly insurmountable hurdle at the
class certification stage in situations where a class action is
the only viable way to pursue valid but small individual
claims.
     The demands of this heightened requirement are most
apparent from the Third Circuit’s discussion of self-
identification by affidavit. It has said that affidavits from pu-
tative class members cannot satisfy the stringent ascertaina-
bility requirement. See Carrera, 727 F.3d at 308–12 (remand-
ing to give plaintiff “another opportunity to satisfy the ascer-
tainability requirement” but rejecting plaintiff’s attempt to
use affidavits from class members to show their purchases of
weight loss supplement); Hayes, 725 F.3d at 356 (“But the na-
ture or thoroughness of a defendant’s recordkeeping does
not alter the plaintiff’s burden to fulfill Rule 23’s require-
ments.”); Marcus, 687 F.3d at 594 (“We caution, however,
against approving a method that would amount to no more
than ascertaining by potential class members’ say so.”). Di-
rect Digital urges us to adopt this rule and to reverse the cer-
tification order here because the only method for identifying
class members proposed by Mullins in the district court was
self-identification by affidavit.
No. 15-1776                                                  13

    We decline to do so. The Third Circuit’s approach in Car-
rera, which is at this point the high-water mark of its devel-
oping ascertainability doctrine, goes much further than the
established meaning of ascertainability and in our view mis-
reads Rule 23. Carrera and cases like it have given four policy
reasons for requiring more than affidavits from putative
class members. We address each one below and find them
unpersuasive.
    In general, we think imposing this stringent version of
ascertainability does not further any interest of Rule 23 that
is not already adequately protected by the Rule’s explicit re-
quirements. On the other side of the balance, the costs of im-
posing the requirement are substantial. The stringent version
of ascertainability effectively bars low-value consumer class
actions, at least where plaintiffs do not have documentary
proof of purchases, and sometimes even when they do. Ac-
cordingly, we conclude that the district court here did not
abuse its discretion by deferring until later in the litigation
decisions about more detailed aspects of ascertainability and
the management of any claims process. At bottom, the dis-
trict court was correct not to let a quest for perfect treatment
of one issue become a reason to deny class certification and
with it the hope of any effective relief at all.
    We now turn to the policy concerns identified by the
courts that have embraced this heightened ascertainability
requirement. The policy concerns are substantial and legiti-
mate, but we do not believe they justify the new require-
ment. As will become clear, we agree in essence with Judge
Rendell’s concurring opinion in Byrd, 784 F.3d at 172–77,
which urged “retreat from [the] heightened ascertainability
14                                                 No. 15-1776

requirement in favor of following the historical meaning of
ascertainability under Rule 23,” id. at 177.
       1. Administrative Convenience
    Some courts have argued that imposing a stringent ver-
sion of ascertainability “eliminates serious administrative
burdens that are incongruous with the efficiencies expected
in a class action by insisting on the easy identification of
class members.” Marcus, 687 F.3d at 593 (citation and internal
quotation marks omitted). It does this by ensuring that the
court will be able to identify class members without “exten-
sive and individualized fact-finding or mini-trials.” Carrera,
727 F.3d at 307 (citation and internal quotation marks omit-
ted).
   This concern about administrative inconvenience is better
addressed by the explicit requirements of Rule 23(b)(3),
which requires that the class device be “superior to other
available methods for fairly and efficiently adjudicating the
controversy.” One relevant factor is “the likely difficulties in
managing a class action.” Fed. R. Civ. P. 23(b)(3)(D).
    The superiority requirement of Rule 23(b)(3) is clarified
by substantial case law. See 7AA Wright et al., Federal Practice
& Procedure §§ 1779, 1780. Imposing a stringent version of
ascertainability because of concerns about administrative in-
convenience renders the manageability criterion of the supe-
riority requirement superfluous. See Daniel Luks, Note, As-
certainability in the Third Circuit: Name That Class Member, 82
Fordham L. Rev. 2359, 2395 (2014). It also conflicts with the
well-settled presumption that courts should not refuse to
certify a class merely on the basis of manageability concerns.
See, e.g., In re Visa Check/MasterMoney Antitrust Litig., 280
No. 15-1776                                                    15

F.3d 124, 140 (2d Cir. 2001) (Sotomayor, J.) (noting that fail-
ure to certify a class action under Rule 23(b)(3) solely on
manageability grounds is generally disfavored), overruled on
other grounds by In re IPO, 471 F.3d 24 (2d Cir. 2006); accord,
Byrd, 784 F.3d at 175 (Rendell, J., concurring) (“Imposing a
proof-of-purchase requirement does nothing to ensure the
manageability of a class or the ‘efficiencies’ of the class action
mechanism; rather, it obstructs certification by assuming that
hypothetical roadblocks will exist at the claims administra-
tion stage of the proceedings.”).
    A reader might fairly ask whether there is any practical
difference between addressing administrative inconvenience
as a matter of ascertainability versus as a matter of superiori-
ty. In fact, there is. When administrative inconvenience is
addressed as a matter of ascertainability, courts tend to look
at the problem in a vacuum, considering only the adminis-
trative costs and headaches of proceeding as a class action.
See, e.g., Sethavanish, 2014 WL 580696, at *6 (purchasers of
“all natural” nutrition bars sold through retailers; denying
class certification solely on the ground of ascertainability
without addressing other available methods for adjudicating
the controversy). But when courts approach the issue as part
of a careful application of Rule 23(b)(3)’s superiority stand-
ard, they must recognize both the costs and benefits of the
class device. See 7AA Wright et al., Federal Practice & Proce-
dure § 1780 (“Viewing the potential administrative difficul-
ties from a comparative perspective seems sound and a deci-
sion against class-action treatment should be rendered only
when the ministerial efforts simply will not produce corre-
sponding efficiencies. In no event should the court use the
possibility of becoming involved with the administration of
16                                                  No. 15-1776

a complex lawsuit as a justification for evading the responsi-
bilities imposed by Rule 23.”).
    Rule 23(b)(3)’s superiority requirement, unlike the free-
standing ascertainability requirement, is comparative: the
court must assess efficiency with an eye toward “other avail-
able methods.” In many cases where the heightened ascer-
tainability requirement will be hardest to satisfy, there realis-
tically is no other alternative to class treatment. See id. (“If
judicial management of a class action … will reap the re-
wards of efficiency and economy for the entire system that
the drafters of the federal rule envisioned, then the individ-
ual judge should undertake the task. Ironically, those Rule
23(b)(3) actions requiring the most management may yield
the greatest pay-off in terms of effective dispute resolu-
tion.”); cf. Schleicher v. Wendt, 618 F.3d 679, 686–87 (7th Cir.
2010) (rejecting defendant’s invitation to “tighten” Rule 23
requirements for class certification and noting that doing so
would make certification impossible in many securities
fraud cases).
    This does not mean, of course, that district courts should
automatically certify classes in these difficult cases. But it
does mean that before refusing to certify a class that meets
the requirements of Rule 23(a), the district court should con-
sider the alternatives as Rule 23(b)(3) instructs rather than
denying certification because it may be challenging to identi-
fy particular class members. District courts have considera-
ble experience with and flexibility in engineering solutions
to difficult problems of case management.
    In addition, a district judge has discretion to (and we
think normally should) wait and see how serious the prob-
lem may turn out to be after settlement or judgment, when
No. 15-1776                                                    17

much more may be known about available records, response
rates, and other relevant factors. And if a problem is truly
insoluble, the court may decertify the class at a later stage of
the litigation. See Carnegie v. Household Int’l, Inc., 376 F.3d
656, 661 (7th Cir. 2004).
    If faced with what appear to be unusually difficult man-
ageability problems at the certification stage, district courts
have discretion to insist on details of the plaintiff’s plan for
notifying the class and managing the action. In conducting
this inquiry, district courts should consider also whether the
administrative burdens can be eased by the procedures set
out in Rule 23(c) and (d). See, e.g., Bobbitt v. Academy of Court
Reporting, Inc., 252 F.R.D. 327, 344–45 (E.D. Mich. 2008)
(granting class certification despite potential manageability
problems and noting options “a special master, representa-
tive trials, or other means” to manage the problems).
    Under this comparative framework, refusing to certify on
manageability grounds alone should be the last resort. See
Carnegie, 376 F.3d at 661 (“a class action has to be unwieldy
indeed before it can be pronounced an inferior alternative—
no matter how massive the fraud or other wrongdoing that
will go unpunished if class treatment is denied—to no litiga-
tion at all”), quoted in Suchanek, 764 F.3d at 760. In all events,
deciding whether and when to insist on details, and how
many details, are matters for the sound discretion of district
judges who have so much first-hand experience managing
class actions.
    On the other hand, if courts look only at the cost-side of
the equation and fail to consider administrative solutions
like those available under Rule 23(c) and (d), courts will err
systematically against certification. See Geoffrey C. Shaw,
18                                                  No. 15-1776

Note, Class Ascertainability, 124 Yale L.J. 2354, 2396–99 (2015)
(explaining why addressing issue of manageability under
umbrella of superiority is preferable to addressing it as a
matter of ascertainability). The stringent version of ascer-
tainability invites precisely this type of systemic error.
       2. Unfairness to Absent Class Members
    Courts also have asserted that the heightened ascertaina-
bility requirement is needed to protect absent class mem-
bers. If the identities of absent class members cannot be as-
certained, the argument goes, it is unfair to bind them by the
judicial proceeding. See Carrera, 727 F.3d at 307; Marcus, 687
F.3d at 593. A central premise of this argument is that class
members must receive actual notice of the class action so
that they do not lose their opt-out rights.
     We believe that premise is mistaken. For Rule 23(b)(3)
classes, Rule 23(c)(2)(B) requires the “best notice that is prac-
ticable under the circumstances, including individual notice
to all members who can be identified through reasonable ef-
fort.” The rule does not insist on actual notice to all class
members in all cases. It recognizes it might be impossible to
identify some class members for purposes of actual notice.
See Shaw, 124 Yale L.J. at 2367–69. While actual individual
notice may be the ideal, due process does not always require
it. See Mirfasihi v. Fleet Mortgage Corp., 356 F.3d 781, 786 (7th
Cir. 2004) (rejecting requirement of individual notice); Juris v.
Inamed Corp., 685 F.3d 1294, 1321 (11th Cir. 2012) (noting that
“even in Rule 23(b)(3) class actions, due process does not re-
quire that class members actually receive notice” and collect-
ing cases); accord, Phillips Petroleum Co. v. Shutts, 472 U.S.
797, 811–12 (1985); Mullane v. Central Hanover Bank & Trust
Co., 339 U.S. 306, 314–15 (1950).
No. 15-1776                                                    19

    When class members’ names and addresses are known or
knowable with reasonable effort, notice can be accomplished
by first-class mail. See, e.g., Eisen v. Carlisle & Jacquelin, 417
U.S. 156, 174–75 (1974). When that is not possible, courts
may use alternative means such as notice through third par-
ties, paid advertising, and/or posting in places frequented by
class members, all without offending due process. See
Hughes v. Kore of Indiana Enterprise, Inc., 731 F.3d 672, 676–77
(7th Cir. 2013). As long as the alternative means satisfy the
standard of Rule 23(b)(3), there is no due process violation.
See, e.g., Lilly v. Jamba Juice Co., No. 13–cv–02998–JST, 2014
WL 4652283, at *5 (N.D. Cal. Sept. 18, 2014) (rejecting notice
argument for same reason); Boundas v. Abercrombie & Fitch
Stores, Inc., 280 F.R.D. 408, 418 (N.D. Ill. 2012) (same). Due
process simply does not require the ability to identify all
members of the class at the certification stage.
    More broadly, the stringent version of ascertainability
loses sight of a critical feature of class actions for low-value
claims like this one. In these cases, “only a lunatic or a fanat-
ic” would litigate the claim individually, Carnegie v. House-
hold Int’l, Inc., 376 F.3d 656, 661 (7th Cir. 2004), so opt-out
rights are not likely to be exercised by anyone planning a
separate individual lawsuit. When this is true, it is particu-
larly important that the types of notice that courts require
correspond to the value of the absent class members’ inter-
ests. Cf. Mathews v. Eldridge, 424 U.S. 319, 334–35 (1976). That
is why in Hughes, for example, where each plaintiff’s claim
was valued at approximately $1,000 or less, we approved a
notice plan consisting of sticker notices on the defendant’s
two ATMs, publication of a notice in the primary local
newspaper, and notice on a website. Hughes, 731 F.3d at 676–
77. We did not insist on first-class mail even though the no-
20                                                   No. 15-1776

tice plan likely would not reach everyone in the class. We
approved the plan because the notice plan was “commensu-
rate with the stakes.” Id. at 676.
    The heightened ascertainability approach upsets this bal-
ance. It comes close to insisting on actual notice to protect
the interests of absent class members, yet overlooks the reali-
ty that without certification, putative class members with
valid claims would not recover anything at all. See Amchem,
521 U.S. at 617; Eisen, 417 U.S. at 161; Hughes, 731 F.3d at 677;
Butler v. Sears, Roebuck & Co., 727 F.3d 796, 798 (7th Cir. 2013);
see also, e.g., Ebin v. Kangadis Food Inc., 297 F.R.D. 561, 567
(S.D.N.Y. 2014) (“Against this background, the ascertainabil-
ity difficulties, while formidable, should not be made into a
device for defeating the action.”); Astiana v. Kashi Co., 291
F.R.D. 493, 500 (S.D. Cal. 2013) (“If class actions could be de-
feated because membership was difficult to ascertain at the
class certification stage, there would be no such thing as a
consumer class action.” (citation and internal quotation
marks omitted)). When it comes to protecting the interests of
absent class members, courts should not let the perfect be-
come the enemy of the good.
       3. Unfairness to Bona Fide Class Members
    The third concern offered to justify the heightened ascer-
tainability requirement is the interests of class members with
valid claims. Courts have expressed concern that if class
members are identified only by their own affidavits, indi-
viduals without a valid claim will submit erroneous or
fraudulent claims and dilute the share of recovery for true
class members. See Carrera, 727 F.3d at 310 (“It is unfair to
absent class members if there is a significant likelihood their
No. 15-1776                                                              21

recovery will be diluted by fraudulent or inaccurate
claims.”). 3
    Again, this concern about the danger of fraudulent or
mistaken claims is legitimate and understandable, especially
when contemplating the prospect that money might seem
available just for the asking. In the words of then-future
President John Adams, “it is prudent not to put virtue to too
serious a test.” 2 John Adams, The Works of John Adams, Sec-
ond President of the United States: Diary, with A Life of the Au-
thor, Notes & Illustrations 457 (Charles Francis Adams ed.
1850) (during 1775 debate on whether to open ports for trade
and the need for customs officials to regulate the ports).
    We see two problems with using these concerns to im-
pose the heightened ascertainability standard. First, in prac-
tice, the risk of dilution based on fraudulent or mistaken

    3 Bello v. Beam Global Spirits & Wine, Inc., No. 11–5149 (NLH/KMW),
2015 WL 3613723 (D.N.J. June 9, 2015), is a striking example of how de-
manding this approach has become, requiring something close to perfec-
tion in identifying class members. When the plaintiff first moved to certi-
fy a class of consumers who had purchased a beverage product, she at-
tempted to satisfy the ascertainability requirement with affidavits from
putative class members. The court, relying on the recent Third Circuit
cases, denied the motion without prejudice and gave her another oppor-
tunity to propose “a reliable and administratively feasible mechanism for
determining whether putative class members fall within the class defini-
tion.” Id. at *11, quoting Hayes, 725 F.3d at 355. The plaintiff renewed her
motion, this time proposing a detailed screening method to weed out
mistaken or fraudulent claims. See id. at *6–7 (describing three levels of
review). The court denied her renewed motion, holding that even this
screening method failed to satisfy Carrera’s heightened ascertainability
requirement. See id. at *11–14. At one point, the court wrote that even an
affidavit plus a receipt would not be enough to clear the ascertainability
hurdle. See id. at *12.
22                                                    No. 15-1776

claims seems low, perhaps to the point of being negligible.
We are aware of no empirical evidence that the risk of dilu-
tion caused by inaccurate or fraudulent claims in the typical
low-value consumer class action is significant. In most cases,
the expected recovery is so small that we question whether
many people would be willing to sign affidavits under pen-
alty of perjury saying that they purchased the good or ser-
vice. See Byrd, 784 F.3d at 175 (Rendell, J., concurring). In this
case, for example, the value of each claim is approximately
$70 (the retail price). Direct Digital has provided no evi-
dence, and we have found none, that claims of this magni-
tude have provoked the widespread submission of inaccu-
rate or fraudulent claims.
    We could be wrong, of course, about this empirical pre-
diction. Suppose people are more willing to file inaccurate or
fraudulent claims for low-value recoveries than we suspect.
Even then, the risk of dilution appears small because only a
tiny fraction of eligible claimants ever submit claims for
compensation in consumer class actions. See Christopher R.
Leslie, The Significance of Silence: Collective Action Problems and
Class Action Settlements, 59 Fla. L. Rev. 71, 119–20 (2007) (not-
ing that it is not unusual to have participation rates of 10 to
15 percent and examining more recent examples of rates
lower than 5 percent). Any participation rate less than 100
percent leaves unclaimed funds in the pot, whether it is a
judgment award or a settlement fund. When there are un-
claimed funds, the addition of a fraudulent or inaccurate
claim typically does not detract from a bona fide class mem-
ber’s recovery because the non-deserving claimant merely
takes from unclaimed funds, not the deserving class mem-
ber. It is of course theoretically possible that the total sum
claimed by non-deserving claimants exceeds the total
No. 15-1776                                                  23

amount of unclaimed funds, in which case there would be
dilution, but given the low participation rates actually ob-
served in the real world, this danger is not so great that it
justifies denying class certification altogether, at least with-
out empirical evidence supporting the fear. See Myriam
Gilles, Class Dismissed: Contemporary Judicial Hostility to
Small-Claims Consumer Class Actions, 59 DePaul L. Rev. 305,
315 (2010) (given actual claims rates in practice, “it is simply
not true that compensation of uninjured parties affects the
compensation interests of injured class members”). Carrera
and cases like it have given no reason to think otherwise.
    We recognize that the risk of mistaken or fraudulent
claims is not zero. But courts are not without tools to combat
this problem during the claims administration process. They
can rely, as they have for decades, on claim administrators,
various auditing processes, sampling for fraud detection, fol-
low-up notices to explain the claims process, and other tech-
niques tailored by the parties and the court to take into ac-
count the size of the claims, the cost of the techniques, and
an empirical assessment of the likelihood of fraud or inaccu-
racy. See Manual for Complex Litigation §§ 21.66–.661 (4th
ed. 2004); Newberg on Class Actions § 12:20; see also, e.g.,
Boundas v. Abercrombie & Fitch Stores, Inc., 280 F.R.D. 408, 417
(N.D. Ill. 2012) (affirming class certification where class in-
cluded individuals who threw away promotional gift cards
because they were told that the balances had been voided:
“anybody claiming class membership on that basis will be
required to submit an appropriate affidavit, which can be
evaluated during the claims administration process”). Rely-
ing on concerns about what are essentially claim administra-
tion issues to deny certification and to prevent any recovery
on valid claims upsets the balance a district judge must con-
24                                                    No. 15-1776

sider. In the face of such empirical uncertainty, a district
judge has discretion to say let’s wait until we know more and
see how big a problem this turns out to be.
    The second problem with this dilution argument is that
class certification provides the only meaningful possibility
for bona fide class members to recover anything at all. Keep
in mind what’s at stake. If the class is certified and fraudu-
lent or inaccurate claims actually cause dilution, then de-
serving class members still receive something. But if class
certification is denied, they will receive nothing, for they
would not have brought suit individually in the first place.
See Amchem, 521 U.S. at 617; Eisen, 417 U.S. at 161; Hughes,
731 F.3d at 677; Butler, 727 F.3d at 798. To deny class certifica-
tion based on fear of dilution would in effect deprive bona
fide class members of any recovery as a means to ensure
they do not recover too little.
    This stringent approach has far-reaching consequences,
too. By “focusing on making absolutely certain that compen-
sation is distributed only to those individuals who were ac-
tually harmed,” the heightened ascertainability requirement
“has ignored an equally important policy objective of class
actions: deterring and punishing corporate wrongdoing.”
Byrd, 784 F.3d at 175–76 (Rendell, J., concurring), discussing
Hughes, 731 F.3d at 677 (“A class action, like litigation in gen-
eral, has a deterrent as well as a compensatory objective.”).
Even if the risk of dilution is not trivial, refusing to certify on
this basis effectively immunizes defendants from liability
because they chose not to maintain records of the relevant
transactions. See Daniels v. Hollister Co., 113 A.3d 796, 801
(N.J. App. 2015) (“Ascertainability … is particularly mis-
guided when applied to a case where any difficulties en-
No. 15-1776                                                     25

countered in identifying class members are a consequence of
a defendant’s own acts or omissions. … Allowing a defend-
ant to escape responsibility for its alleged wrongdoing by
dint of its particular recordkeeping policies … is not in har-
mony with the principles governing class actions.”); Birch-
meier v. Caribbean Cruise Line, Inc., 302 F.R.D. 240, 250 (N.D.
Ill. 2014) (“Doing this—or declining to certify a class alto-
gether, as defendants propose—would create an incentive
for a person to violate the TCPA on a mass scale and keep no
records of its activity, knowing that it could avoid legal re-
sponsibility for the full scope of its illegal conduct.”); Lilly v.
Jamba Juice Co., No. 13–cv–02998–JST, 2014 WL 4652283, at *4
(N.D. Cal. Sept. 18, 2014) (“Adopting the Carrera approach
would have significant negative ramifications for the ability
to obtain redress for consumer injuries.”); Carrera v. Bayer
Corp., No. 12–2621, 2014 WL 3887938, at *3 (3d Cir. May 2,
2014) (Ambro, J., dissenting from denial of rehearing en
banc) (explaining that Carrera may have gone too far where
“a defendant’s lack of records and business practices make it
more difficult to ascertain the members of an otherwise ob-
jectively verifiable low-value class”).
    When faced with this counterargument, courts applying
the heightened ascertainability approach have tended to
emphasize that the plaintiff has the burden to satisfy Rule 23
and that the deterrence concern is therefore irrelevant. See,
e.g., Hayes v. Wal-Mart Stores, Inc., 725 F.3d 349, 356 (3d Cir.
2013) (“Rule 23’s requirements that the class be administra-
tively feasible to ascertain and sufficiently numerous to war-
rant class action treatment cannot be relaxed or adjusted on
the basis of Hayes’ assertion that Wal–Mart’s records are of
no help to him.”). With respect, that response begs an im-
portant question. Why are affidavits from putative class
26                                                No. 15-1776

members deemed insufficient as a matter of law to satisfy this
burden? In other words, no one disputes that the plaintiff
carries the burden; the decisive question is whether certain
evidence is sufficient to meet it. Cf. Carrera, 2014 WL
3887938, at *1 (Ambro, J., dissenting from denial of rehearing
en banc) (“Even if … the ability to identify class members is
a set piece for Rule 23 to work, how far we go in requiring
plaintiffs to prove that ability at the outset is exceptionally
important and requires a delicate balancing of interests.”).
    If not disputed, self-serving affidavits can support a de-
fendant’s motion for summary judgment, for example, and
defendants surely will be entitled to a fair opportunity to
challenge self-serving affidavits from plaintiffs. We are
aware of only one type of case in American law where the
testimony of one witness is legally insufficient to prove a
fact. See U.S. Const., Art. III, § 3 (“No person shall be con-
victed of treason unless on the testimony of two witnesses to
the same overt act, or on confession in open court.”). There is
no good reason to extend that rule to consumer class actions.
    Given the significant harm caused by immunizing cor-
porate misconduct, we believe a district judge has discretion
to allow class members to identify themselves with their
own testimony and to establish mechanisms to test those af-
fidavits as needed.
      4. Due Process Interest of the Defendant
    Finally, courts have said the heightened ascertainability
requirement is needed to protect a defendant’s due process
rights. Relying on cases about a defendant’s right to “present
every available defense,” e.g., Lindsey v. Normet, 405 U.S. 56,
66 (1972), these courts have argued that the defendant must
No. 15-1776                                                   27

have a similar right to challenge the reliability of evidence
submitted to prove class membership. See Carrera, 727 F.3d
at 307 (“Ascertainability provides due process by requiring
that a defendant be able to test the reliability of the evidence
submitted to prove class membership.”); Marcus, 687 F.3d at
594 (“Forcing BMW and Bridgestone to accept as true absent
persons’ declarations that they are members of the class,
without further indicia of reliability, would have serious due
process implications.”).
    We agree with the due process premise but not the con-
clusion. A defendant has a due process right to challenge the
plaintiffs’ evidence at any stage of the case, including the
claims or damages stage. That does not mean a court cannot
rely on self-identifying affidavits, subject as needed to audits
and verification procedures and challenges, to identify class
members. To see why, separate the two claims about a de-
fendant’s interest. It is certainly true that a defendant has a
due process right not to pay in excess of its liability and to
present individualized defenses if those defenses affect its
liability. See Wal-Mart Stores, Inc. v. Dukes, 564 U.S. —, 131 S.
Ct. 2541, 2560–61 (2011). It does not follow that a defendant
has a due process right to a cost-effective procedure for chal-
lenging every individual claim to class membership. Cf.
American Express Co. v. Italian Colors Restaurant, 570 U.S. —,
133 S. Ct. 2304, 2309 (2013) (“the antitrust laws do not guar-
antee an affordable procedural path to the vindication of
every claim”). And we should not underestimate the ability
of district courts to develop effective auditing and screening
methods tailored to the individual case.
   Whether a defendant’s due process interest is violated
depends on the nature of the class action, the plaintiff’s theo-
28                                                 No. 15-1776

ry of recovery, and the defendant’s opportunity to contest
liability and the amount of damages it owes. The due pro-
cess question is not whether the identity of class members
can be ascertained with perfect accuracy at the certification
stage but whether the defendant will receive a fair oppor-
tunity to present its defenses when putative class members
actually come forward. A district court can tailor fair verifi-
cation procedures to the particular case, and a defendant
may need to decide how much it wants to invest in litigating
individual claims.
   To see why this due process argument does not justify
the heightened ascertainability requirement, consider three
types of class actions. The first type is where the total
amount of damages can be determined in the aggregate. A
leading treatise provides an example:
       Assume a class of employees has a $50 million
       pension fund with each employee’s share de-
       terminable only by a complex formula concern-
       ing age, years in service, retirement age, etc.
       Further assume that the fund’s trustee simply
       transfers the full $50 million to her own per-
       sonal account. In a case for conversion or
       fraud, the class would have to demonstrate
       damage to show liability. They could make that
       showing simply by demonstrating the aggre-
       gate damage the class has suffered—the
       amount the defendant converted. Individual
       damages could be worked out later or in sub-
       sequent proceedings.
Newberg on Class Actions § 12:2 (footnote omitted). In this sit-
uation, the identity of particular class members does not im-
No. 15-1776                                                      29

plicate the defendant’s due process interest at all. The addi-
tion or subtraction of individual class members affects nei-
ther the defendant’s liability nor the total amount of damag-
es it owes to the class. See, e.g., In re Urethane Antitrust Litig.,
768 F.3d 1245, 1269 (10th Cir. 2014) (rejecting Seventh
Amendment challenge to allocation of damages award
among class members because defendant “has no interest in
the method of distributing the aggregate damages award
among the class members”); In re Pharmaceutical Industry Av-
erage Wholesale Price Litig., 582 F.3d 156, 197–98 (1st Cir. 2009)
(rejecting due process challenge to entry of class-wide judg-
ment and award of aggregate damages); Allapattah Services,
Inc. v. Exxon Corp., 333 F.3d 1248, 1258 (11th Cir. 2003) (“[A]
defendant has no interest in how the class members appor-
tion and distribute a[n] [aggregate] damage [award] among
themselves.”), aff’d, 545 U.S. 546 (2005); Hilao v. Estate of Mar-
cos, 103 F.3d 767, 786 (9th Cir. 1996) (noting that defendant’s
interest is “only in the total amount of damages for which it
will be liable,” not “the identities of those receiving damage
awards”); Six (6) Mexican Workers v. Arizona Citrus Growers,
904 F.2d 1301, 1307 (9th Cir. 1990) (“Where the only question
is how to distribute the damages, the interests affected are
not the defendant’s but rather those of the silent class mem-
bers.”).
    The second type of class action is where the total amount
of damages cannot be determined in the aggregate, but there
is a common method of determining individual damages.
(Most consumer fraud class actions fit this model.) The same
treatise provides this example:
       Now assume that [the] same class of current
       employees is statutorily entitled to overtime
30                                                  No. 15-1776

       wages at time and a half after 40 hours
       work/week but that the defendant employer
       has never paid such overtime. In a case alleg-
       ing violation of the statute, it may be sufficient
       to demonstrate that the defendant failed to pay
       overtime without assessing a full aggregate li-
       ability. There would be a common method for
       showing individual damages—a simple formu-
       la could be applied to each class member’s em-
       ployment records—and that would be suffi-
       cient for the predominance and superiority re-
       quirements to be met.
Newberg on Class Actions § 12:2 (footnote omitted). In this sit-
uation, the defendant’s due process interest is implicated be-
cause the calculation of each class member’s damages affects
the total amount of damages it owes to the class. That’s why
the method of determining damages must match the plain-
tiff’s theory of liability and be sufficiently reliable. See Com-
cast Corp. v. Behrend, 569 U.S. —, 133 S. Ct. 1426, 1433 (2013).
It’s also why the defendant must be given the opportunity to
raise individual defenses and to challenge the calculation of
damages awards for particular class members. See Allapattah
Services, 333 F.3d at 1259.
    But neither of these requirements has any necessary con-
nection to the heightened ascertainability requirement.
Whether putative class members self-identify by affidavits
simply does not matter. Suppose an employee files an affi-
davit falsely claiming that she worked 60 hours a week when
in fact she worked only 50, or suppose a person files an affi-
davit falsely claiming to have been an employee. In either
case, so long as the defendant is given a fair opportunity to
No. 15-1776                                                     31

challenge the claim to class membership and to contest the
amount owed each claimant during the claims administra-
tion process, its due process rights have been protected.
    The third type of class action is where the defendant’s li-
ability can be determined on a class-wide basis, but aggre-
gate damages cannot be established and there is no common
method for determining individual damages. In this situa-
tion, courts often bifurcate the case into a liability phase and
a damages phase. See Butler v. Sears, Roebuck & Co., 727 F.3d
796, 800 (7th Cir. 2013) (“a class action limited to determin-
ing liability on a class-wide basis, with separate hearings to
determine—if liability is established—the damages of indi-
vidual class members, or homogeneous groups of class
members, is permitted by Rule 23(c)(4) and will often be the
sensible way to proceed”).
    It has long been recognized that the need for individual
damages determinations at this later stage of the litigation
does not itself justify the denial of certification. See Schleicher
v. Wendt, 618 F.3d 679, 685 (7th Cir. 2010) (“The possibility
that individual hearings will be required for some plaintiffs
to establish damages does not preclude certification.”); Pella
Corp. v. Saltzman, 606 F.3d 391, 394 (7th Cir. 2010) (per curi-
am); Arreola v. Godinez, 546 F.3d 788, 799–801 (7th Cir. 2008);
Carnegie v. Household Int’l, Inc., 376 F.3d 656, 661 (7th Cir.
2004). Here again, using the heightened ascertainability re-
quirement to deny class certification is not the only means,
or even the best means, to protect the defendant’s due pro-
cess rights.
   As long as the defendant is given the opportunity to chal-
lenge each class member’s claim to recovery during the
damages phase, the defendant’s due process rights are pro-
32                                                            No. 15-1776

tected. See Lilly v. Jamba Juice Co., No. 13–cv–02998–JST, 2014
WL 4652283, at *6 (N.D. Cal. Sept. 18, 2014) (“Defendants
would certainly be entitled to object to a process through
which a non-judicial administrator ‘ascertains’ each appli-
cant’s class membership on the basis of the applicants’ own
self-identification, gives a defendant no opportunity to chal-
lenge that determination, and then racks up the defendant’s
bill every time an individual submits a form.”); Johnson v.
General Mills, Inc., 276 F.R.D. 519, 524 (C.D. Cal. 2011) (“If Mr.
Johnson establishes liability for the class, Defendants may
challenge reliance and causation individually during a de-
termination of damages, after the issues that are common
have been litigated and resolved.”); Godec v. Bayer Corp., No.
1:10–CV–224, 2011 WL 5513202, at *7 (N.D. Ohio Nov. 11,
2011) (“In any event, to the extent Bayer has individualized
defenses, it is free to try those defenses against individual
claimants.”). 4
    In sum, the concern about protecting a defendant’s due
process rights does not justify the heightened ascertainabil-
ity requirement. In all cases, the defendant has a right not to
pay in excess of its liability and to present individual defens-
es, but both rights are protected by other features of the class
device and ordinary civil procedure. Carrera itself appeared

     4What we have said is consistent with Comcast Corp. v. Behrend, 569
U.S. —, 133 S. Ct. 1426 (2013), which held that class treatment is inap-
propriate where the class-wide measure of damages does not match the
plaintiff’s theory of liability. See Butler v. Sears, Roebuck & Co., 727 F.3d
796, 799–800 (7th Cir. 2013); see also In re Nexium Antitrust Litig., 777 F.3d
9, 18–19 (1st Cir. 2015); In re Deepwater Horizon, 739 F.3d 790, 817 (5th Cir.
2014); In re Whirlpool Corp. Front-Loading Washer Products Liability Litig.,
722 F.3d 838, 860–61 (6th Cir. 2013); Leyva v. Medline Industries Inc., 716
F.3d 510, 514 (9th Cir. 2013).
No. 15-1776                                                  33

to recognize this rejoinder, but it pivoted to the argument
discussed above about protecting absent class members. See
727 F.3d at 310 (“Because Bayer’s total liability cannot be so
affected by unreliable affidavits, Carrera argues Bayer lacks
an interest in challenging class membership. … But ascer-
tainability protects absent class members as well as defend-
ants, so Carrera’s focus on Bayer alone is misplaced.” (cita-
tion omitted)). Carrera gave no other reason to think the
heightened ascertainability requirement is needed to protect
a defendant’s due process rights. We can’t think of one either.
     Ultimately, we decline Direct Digital’s invitation to adopt
a heightened ascertainability requirement. Nothing in Rule
23 mentions or implies it, and we are not persuaded by the
policy concerns identified by other courts. Those concerns
are better addressed by a careful and balanced application of
the Rule 23(a) and (b)(3) requirements, keeping in mind un-
der Rule 23(b)(3) that the court must compare the available
alternatives to class action litigation. District courts should
continue to insist that the class definition satisfy the estab-
lished meaning of ascertainability by defining classes clearly
and with objective criteria. If a class is ascertainable in this
sense, courts should not decline certification merely because
the plaintiff’s proposed method for identifying class mem-
bers relies on affidavits. If the proposed class presents unu-
sually difficult manageability problems, district courts have
discretion to press the plaintiff for details about the plain-
tiff’s plan to identify class members. A plaintiff’s failure to
address the district court’s concerns adequately may well
cause the plaintiff to flunk the superiority requirement of
Rule 23(b)(3). But in conducting this analysis, the district
court should always keep in mind that the superiority
standard is comparative and that Rule 23(c) and (d) permit
34                                                 No. 15-1776

creative solutions to the administrative burdens of the class
device.
     C. Commonality
    Direct Digital’s other primary challenge to the district
court’s certification order relates to the commonality re-
quirement of Rule 23(a)(2). The district court found this re-
quirement satisfied by a preponderance of the evidence, see
Messner v. Northshore Univ. HealthSystem, 669 F.3d 802, 811
(7th Cir. 2012), explaining that whether Instaflex has been
clinically tested or scientifically formulated to relieve joint
pain, improve flexibility, increase mobility, and support car-
tilage repair are questions common to the class. [See R. 89 at
2, 3–4]
    Direct Digital argues that Mullins cannot satisfy the
commonality requirement because his suit alleges that In-
staflex is ineffective. The efficacy of a health product can
never form the basis of a common question, Direct Digital
argues, because efficacy depends on individual factors such
as the severity of the consumer’s pre-use medical condition,
the consumer’s pattern of use, and other potentially con-
founding variables such as the consumer’s overall health,
age, activity level, use of other drugs, and the like.
    Direct Digital’s objection fails because it has mischarac-
terized Mullins’s theory of liability. Mullins does not claim
that Instaflex was ineffective, ergo defendant is liable. He al-
leges that Direct Digital’s statements representing that In-
staflex has been “clinically tested” and “scientifically formu-
lated” to relieve joint discomfort, improve flexibility, in-
crease mobility, and repair cartilage are false or misleading
because they imply there was scientific support for these
No. 15-1776                                                    35

claims but in fact no reasonable scientific expert would con-
clude that glucosamine sulfate (the primary ingredient in the
supplement) has any positive effect on joint health. Mullins
alleges that these statements would have misled a reasonable
consumer. See Barbara’s Sales, Inc. v. Intel Corp., 879 N.E.2d
910, 925–27 (Ill. 2007) (reasonable consumer standard); ac-
cord, Suchanek v. Sturm Foods, Inc., 764 F.3d 750, 756–57 (7th
Cir. 2014) (discussing consumer fraud statutes in Illinois and
other states). As the district court correctly concluded, this
theory presents a common question: Were the statements
false or misleading? This is a “common contention” that is
“capable of classwide resolution” because the “determina-
tion of its truth or falsity will resolve an issue that is central
to the validity of each one of the claims in one stroke.” Wal-
Mart Stores, Inc. v. Dukes, 564 U.S. —, 131 S. Ct. 2541, 2551
(2011). Nothing more is required to satisfy Rule 23(a)(2).
    Of course the efficacy of the product can be relevant to
that determination. If consumers experience the reduction or
elimination of their symptoms, then that is evidence that the
supplement does in fact relieve joint discomfort consistent
with Direct Digital’s representations. But that’s not the focus
of Mullins’s theory of consumer fraud. What really matters
under his theory is whether there is any scientific support
for the assertions contained in the labels and advertising ma-
terials. In other words, Mullins’s claims do not rise or fall on
whether individual consumers experienced health benefits,
due to the placebo effect or otherwise. They rise or fall on
whether Direct Digital’s representations were deceptive. See
Suchanek, 764 F.3d at 756–57 (reversing district court’s order
denying class certification; commonality is satisfied where
plaintiff’s theory of liability turns on proving unfair or de-
ceptive marketing and packaging of consumer product).
36                                                  No. 15-1776

    That’s why even if Direct Digital were to prove that con-
sumers experienced less joint pain because of a placebo ef-
fect (a theory Direct Digital appears to embrace on appeal), it
could still be liable for consumer fraud. Consumers might
have paid more than they otherwise would have because of
the representations about clinical testing. Or they could have
decided not to seek out better therapeutic alternatives be-
cause they believed Instaflex was addressing their underly-
ing condition. See FTC v. QT, Inc., 512 F.3d 858, 862–63 (7th
Cir. 2008) (placebo effect is not a defense to consumer fraud
where defendant has made specific claims about intended
benefits; requiring truth in labeling leads to appropriate
prices and ensures that consumers do not forgo better alter-
natives in reliance on the placebo). At any rate, we express
no view on the merits of Mullins’s allegations. The key point
is that whether the representations were false or misleading
is a common question suitable for class treatment, even if In-
staflex relieved joint discomfort for some consumers.
III.   Conclusion
    Direct Digital raises a number of other, less developed
objections to the district court’s certification order. None of
these issues would have justified granting an appeal under
Rule 23(f), but we have considered them and find them
without merit. Direct Digital has not demonstrated that the
district court abused its discretion in certifying the class. The
order of the district court granting class certification is
AFFIRMED.
