                                In the

    United States Court of Appeals
                 For the Seventh Circuit
                     ____________________
No. 17-2956
TIMOTHY O’BRIEN, et al.,
                                                 Plaintiffs-Appellants,
                                  v.

CATERPILLAR INC.,
                                                   Defendant-Appellee.
                     ____________________

             Appeal from the United States District Court
        for the Northern District of Illinois, Eastern Division.
         No. 14-cv-7229 — Sharon Johnson Coleman, Judge.
                     ____________________

    ARGUED MARCH 28, 2018 — DECIDED AUGUST 20, 2018
               ____________________

   Before EASTERBROOK, KANNE, and SYKES, Circuit Judges.
    SYKES, Circuit Judge. For more than half a century,
Caterpillar Inc. paid unemployment benefits to laid-off em-
ployees at its manufacturing plant in Joliet, Illinois. This ar-
rangement lasted until Caterpillar and the local union agreed
to end the program in their 2012 collective-bargaining agree-
ment. In exchange for the elimination of the benefits, Cater-
pillar distributed $7.8 million to certain employees who had
participated in the plan. Retirement-eligible employees
2                                                   No. 17-2956

received a pro rata share if they agreed to retire. Those who
were ineligible to retire received the same pro rata share of the
fund but with no strings attached.
    Timothy O’Brien and 47 other retirement-eligible employ-
ees who refused to retire brought this lawsuit, alleging that
the liquidation plan violates the Age Discrimination in Em-
ployment Act of 1967 (“ADEA”), 29 U.S.C. §§ 621 et seq. The
district judge entered summary judgment for Caterpillar, and
we affirm. Though the liquidation plan has a disparate impact
on older workers, it was justified by several “reasonable fac-
tors other than age.” Id. § 623(f)(1). The plan achieved one of
Caterpillar’s long-standing financial objectives—namely, the
elimination of costly unemployment benefits. It also saved
money by incentivizing early retirement and reducing admin-
istrative expenses, and contributed to labor peace between
Caterpillar and the union.
                        I. Background
    Caterpillar, a Delaware Corporation headquartered in
Peoria, Illinois, operates a manufacturing plant in nearby
Joliet. Employees at the Joliet plant are represented in collec-
tive bargaining by a local lodge of the International Associa-
tion of Machinists and Aerospace Workers. Since the 1950s,
Caterpillar and the union have agreed to a series of collective-
bargaining agreements. Every agreement has included a pen-
sion plan and, until 2012, a supplemental unemployment ben-
efit plan (“unemployment plan”). Under the unemployment
plan, Caterpillar made monthly contributions into a trust
based on the number of hours employees worked and paid
benefits out of that fund to laid-off workers.
No. 17-2956                                                    3

    As early as 1999, Caterpillar made the elimination of the
unemployment plan an important financial objective in col-
lective bargaining. Because layoffs—and therefore, payouts—
were infrequent, Caterpillar believed that the tied-up capital
could be put to better use. Caterpillar also sought to reduce
expenses related to the administration of the plan. In particu-
lar, the plan’s administration system was outdated and would
soon require a costly investment. The union resisted full elim-
ination of the unemployment plan but made an important
concession in the 1999 agreement: newly hired employees
could no longer participate in the unemployment plan.
   Six years later Caterpillar extracted other concessions
from the union. The 2005 agreement created a two-tiered com-
pensation scale with all subsequently hired workers in the
lower tier. Moving forward, new hires earned approximately
$18 less per hour in wages and benefits compared with previ-
ously hired employees.
    During the 2012 agreement negotiations, Caterpillar’s first
proposed labor contract eliminated the unemployment plan.
As part of its proposal, Caterpillar offered to liquidate the un-
employment plan’s trust fund—which had grown to $7.8 mil-
lion—in pro rata shares to unemployment-plan participants
who were eligible to retire under the pension plan and agreed
to retire. Caterpillar’s proposal would therefore accomplish
two goals: first, it would incentivize union members to elimi-
nate the unemployment plan, and second, it would incentiv-
ize retirements, which would allow Caterpillar to hire
replacements subject to the reduced compensation package
created in the 2005 agreement.
   On April 18, 2012, the union formally rejected Caterpillar’s
proposal. O’Brien, who served on the union’s bargaining
4                                                   No. 17-2956

committee, commented that the proposal “short changed” the
unemployment-plan participants who were not eligible to re-
tire. Caterpillar quickly proposed a new labor contract but did
not address the union’s concerns about how the trust would
be liquidated. The union rejected the new proposal and stated
that it would not eliminate the unemployment plan.
   On April 24 Caterpillar presented a modified proposal
that altered how the trust would be liquidated. The modified
proposal distributed the funds in equal shares to (1) unem-
ployment-plan participants who were eligible to retire and
agreed to retire; and (2) unemployment-plan participants
who were not eligible to retire. Caterpillar’s negotiators
hoped that this change would broaden the appeal of the pro-
posal. The union rejected the modified proposal, however,
and reiterated in “very strong language” that it would not
agree to eliminate the unemployment plan.
     The collective-bargaining agreement expired on May 1,
and the union went on strike. A three-month standoff ensued,
during which the parties met with a federal mediator three
times. The union blinked first: on June 27 it proposed an over-
all labor contract that eliminated the unemployment plan and
liquidated the funds in accordance with the modified pro-
posal. On August 14 the union and Caterpillar agreed to a ten-
tative agreement that included this concession. The Joliet
bargaining unit subsequently ratified the agreement. At that
time there were 269 employees who participated in the unem-
ployment plan. Of those, 184 were eligible to retire, and 136
opted to retire to collect a share of the fund. Caterpillar ulti-
mately issued a pro rata share of $37,836.51 to each employee
entitled to a distribution.
No. 17-2956                                                  5

    The plaintiffs are the 48 retirement-eligible employees
who opted not to retire and as a result did not receive a dis-
tribution. Their lawsuit alleges that Caterpillar’s liquidation
of the trust had a disparate impact on older workers and ac-
cordingly violated the ADEA. In support of their claim, the
plaintiffs presented an expert report by Whitman Soule, who
analyzed the ages and retirement eligibility of the unemploy-
ment-plan participants. Soule took two approaches in his
analysis. First, he compared the ages of retirement-eligible
and nonretirement-eligible participants. On average the re-
tirement-eligible employees were 61.57 years old, and nonre-
tirement-eligible employees were 47.81 years old. Second,
Soule performed a “tabular analysis” comparing the percent-
age of employees eligible for retirement at each age. He con-
cluded that “the retirement eligible employees are
substantially and significantly older than the employees who
were not eligible for retirement.”
    Caterpillar retained Dr. Jonathan Guryan to review
Soule’s expert report. Dr. Guryan opined that Soule’s report
is “wholly unnecessary as the relationship between age and
retirement eligibility is definitional: it is defined in para-
graphs (a) and (b) of subsection 4.1 of the Supplemental
Agreement Relating to [the] Non-Contributory Pension Plan
dated August 17, 2012.” The plan provides that participating
employees become retirement eligible by meeting one of the
following criteria:
      (a) the employee reached age 65 with 5 or more
      years of credited service, including at least one
      hour of credited service on or after December 1,
      1989;
6                                                     No. 17-2956

       (b) the employee reached age 60 with 10 or more
       years of credited service;
       (c) the employee reached age 55 and his/her age
       plus years of credited service totaled at least 85;
       or
       (d) the employee at any age accumulated 30 or
       more years of credited service.
Three of these factors depend explicitly on age and the fourth
depends on years of service, which is correlated with age.
    Caterpillar moved for summary judgment, and the judge
granted the motion. She held that the plaintiffs could not
make out a prima facie case of age discrimination because
(1) the unemployment-plan liquidation was a one-time deci-
sion that did not constitute a practice or policy and (2) the liq-
uidation treated workers differently based on retirement
eligibility, not age. In the alternative she held that any dispar-
ate impact on older workers was based on a reasonable factor
other than age under § 623(f)(1). In particular, she noted that
Caterpillar saved money and created a retirement incentive
so that it could hire cheaper labor.
                         II. Discussion
    We review a summary judgment de novo, construing the
evidence and drawing inferences in the plaintiffs’ favor. Indi-
anapolis Airport Auth. v. Travelers Prop. Cas. Co. of Am., 849 F.3d
355, 361 (7th Cir. 2017). We may affirm on any ground sup-
ported in the record so long as it was adequately addressed
below and the plaintiffs had an opportunity to contest the is-
sue. W. Side Salvage, Inc. v. RSUI Indem. Co., 878 F.3d 219, 222
(7th Cir. 2017).
No. 17-2956                                                      7

   The Supreme Court has held that the ADEA encompasses
disparate-impact liability. Smith v. City of Jackson, 544 U.S. 228,
243 (2005) (interpreting 29 U.S.C. § 623(a)(2)). Section
623(a)(2) makes it unlawful for an employer “to limit, segre-
gate, or classify his employees in any way which would de-
prive or tend to deprive any individual of employment
opportunities or otherwise adversely affect his status as an
employee, because of such individual’s age.” Unlike claims of
disparate treatment, a disparate-impact claim does not re-
quire proof of discriminatory intent.
    To state a prima facie disparate-impact claim, the plaintiffs
must (1) identify a specific employment practice and (2) offer
statistical evidence demonstrating that the identified practice
caused a significant age-based disparity. Smith, 544 U.S. at
241. If the plaintiffs establish a prima facie case, Caterpillar
may affirmatively defend by showing that the identified prac-
tice or policy is based on a reasonable factor other than age.
See § 623(f)(1).
A. Specific Employment Practice or Policy
   The plaintiffs must “isolat[e] and identify[] the specific em-
ployment practices that are allegedly responsible for any ob-
served statistical disparities.” Smith, 544 U.S. at 241 (quoting
Wards Cove Packing Co. v. Atonio, 490 U.S. 642, 656 (1989)). This
requirement safeguards employers against liability “for the
myriad of innocent causes that may lead to statistical imbal-
ances.” Id. (internal quotation marks omitted).
    Caterpillar asserts that the cumulative effect of an “assort-
ment of documents” does not constitute a single policy and
that any age disparity is attributable to the definitions for re-
tirement eligibility and for participation in the
8                                                     No. 17-2956

unemployment plan. Both definitions predated the liquida-
tion plan and neither depends solely on age. In support of this
argument, Caterpillar cites the Supreme Court’s decisions in
Smith v. City of Jackson and Wards Cove Packing Co. v. Atonio,
but both cases are easily distinguishable.
    In Smith a city overhauled its police pay scale to match that
of surrounding localities. The plaintiffs showed that the pay
plan was “relatively less generous to older workers” but
failed to identify any “test, requirement or practice” that
caused the disparate impact. 544 U.S. at 241. Wards Cove pre-
sented a different problem. There the plaintiffs identified sev-
eral employment practices—nepotism, a lack of objective
hiring criteria, separate hiring channels, and rehire prefer-
ences—that possibly contributed to a disparate racial impact
in violation of Title VII. 490 U.S. at 647–48. Their speculation
as to possible causes was not enough; they failed to “specifi-
cally show[] that each challenged practice [had] a significantly
disparate impact on employment opportunities.” Id. at 657.
Here the plaintiffs have specifically identified a cause for the
disparate impact: Caterpillar’s decision to condition benefits
on retirement eligibility, a status strongly correlated with age.
No more is required.
    Caterpillar also contends that the liquidation plan is a
“one-off event” that does not constitute an actionable practice
or policy. See, e.g., Tex. Dep’t of Housing & Cmty. Affairs v. In-
clusive Cmtys. Project, Inc., 135 S. Ct. 2507, 2523 (2015) (A “one-
time decision” to construct a new building in one location
“may not be a policy at all.”). In the employment context, we
have recognized that a single, isolated decision to hire or fire
an employee may not amount to a policy in the absence of
other evidence. See Bennett v. Roberts, 295 F.3d 687, 698 (7th
No. 17-2956                                                     9

Cir. 2002) (holding that an employer’s rejection of an individ-
ual job applicant is not a policy); Ilhardt v. Sara Lee Corp., 118
F.3d 1151, 1156–57 (7th Cir. 1997) (holding that the elimina-
tion of a single employee’s part-time position is not a policy).
In contrast, a policy likely exists where employees “can show
significant disparities stemming from a single decision.”
Council 31, AFSCME v. Ward, 978 F.2d 373, 378 (7th Cir. 1992).
Though the liquidation plan is a single event, it applies the
same rules to hundreds of employees and causes significant
age-based disparities between workers. It is an actionable pol-
icy.
B. Evidence of Disparate Impact
    Caterpillar also argues that the plaintiffs have failed to
demonstrate that the liquidation plan has an age-related dis-
parate impact. A disparate-impact claim involves “employ-
ment practices that are facially neutral in their treatment of
different groups but that in fact fall more harshly on one
group than another.” Hazen Paper Co. v. Biggins, 507 U.S. 604,
609 (1993) (quotation marks omitted). The plaintiffs must
demonstrate a “significant” age-based disparity to establish a
prima facie case. Karlo v. Pittsburgh Glass Works, LLC, 849 F.3d
61, 69 (3d Cir. 2017).
    Under the pension plan, retirement eligibility depends on
a combination of age and a factor correlated with age (cred-
ited service). By definition, therefore, the terms of the liquida-
tion plan have a disparate impact on older workers. This is
borne out by the statistics: the mean age of the retirement-eli-
gible employees is 61.57 compared with a mean age of 47.81
for those ineligible to retire. The liquidation plan has an espe-
cially severe impact on employees aged 55 and older. Only
1.4% of employees under the age of 55 were eligible for
10                                                    No. 17-2956

retirement compared with 93.4% of those 55 and older. In
light of the substantial statistical disparities between the ages
of retirement-eligible and nonretirement-eligible employees,
the plaintiffs have established a prima facie disparate-impact
case.
     Caterpillar responds that the disparate-impact analysis is
not so simple, and that the ADEA does not extend to “sub-
group” claims and thus the relevant statistical comparison
should be between employees 40 years and older and em-
ployees under 40. Caterpillar also argues that retirement-eli-
gible employees are not similarly situated to those who are
not eligible to retire. Finally, Caterpillar asserts that the plain-
tiffs cannot maintain an ADEA claim where the disparate im-
pact resulted from “the valid criterion of retirement
eligibility.” These arguments are flawed for reasons we dis-
cuss below.
     1. Subgroup Claims
     To prove a disparate-impact claim, the plaintiffs must
show that Caterpillar’s liquidation plan has a disparate im-
pact on them “because of” their membership in a protected
group. Watson v. Fort Worth Bank & Tr., 487 U.S. 977, 994
(1988) (discussing a Title VII disparate-impact claim). Under
the ADEA, the protected group includes “individuals who are
at least 40 years of age.” 29 U.S.C. § 631(a). Caterpillar makes
a perfunctory argument that the proper statistical comparison
is between those who are in the statutorily protected group
(i.e., employees 40 years and older) and those who are not.
   O’Connor v. Consolidated Coin Caterers Corp., 517 U.S. 308
(1996), forecloses this argument. In O’Connor a 56-year-old
employee brought a disparate-treatment claim when he was
No. 17-2956                                                     11

replaced by a 40-year-old. Id. at 309–10. The Court found his
claim cognizable, reasoning that the ADEA’s
       language does not ban discrimination against
       employees because they are aged 40 or older; it
       bans discrimination against employees because
       of their age, but limits the protected class to
       those who are 40 or older. The fact that one per-
       son in the protected class has lost out to another
       person in the protected class is thus irrelevant[]
       so long as he has lost out because of his age.
Id. at 312. The disparate-treatment and disparate-impact pro-
visions are identically phrased with each requiring discrimi-
nation “because of such individual’s age.” See 29 U.S.C. §
623(a)(1) (disparate treatment); id. § 623(a)(2) (disparate im-
pact). The Court’s reasoning therefore applies with equal
force to disparate-impact claims. See Karlo, 849 F.3d at 71
(reaching the same conclusion).
   2. Relevant Statistical Comparison
    Caterpillar next argues that statistical comparisons be-
tween dissimilar groups cannot support a prima facie dispar-
ate-impact claim. For support Caterpillar cites Wards Cove
where the Court rejected a comparison between the racial
composition of a fish company’s skilled workers—including
accountants, doctors, and engineers—and its unskilled can-
nery laborers. 490 U.S. at 651–52. The Court called the com-
parison “nonsensical” given the differently constituted labor
pools. Id. at 615. Caterpillar asserts that this case is analogous:
only retirement-eligible workers are susceptible to retirement
12                                                 No. 17-2956

incentives, so it wouldn’t make sense to compare them to em-
ployees who are ineligible to retire.
    But Caterpillar overlooks stark differences between this
case and Wards Cove. There are no inherent differences be-
tween the unemployment-plan participants who were eligible
to retire and those who were not. They occupied the same fac-
tory jobs, were subject to the same compensation plan, and
were entitled to the same benefits. Caterpillar simply offered
a worse deal to a similarly situated group of employees.
     3. The Court’s Pension Cases
    Caterpillar asserts that the Supreme Court has expressly
approved of classifications based on pension status. For sup-
port it cites two ADEA disparate-treatment cases. See Hazen
Paper, 507 U.S. at 611 (holding that an employer permissibly
terminated a 62-year-old to avoid paying pension benefits
that would have vested six months later); Ky. Ret. Sys. v.
EEOC, 554 U.S. 135, 143 (2008) (holding that disability benefits
that turned on pension eligibility were permissible).
Caterpillar explains that an action based purely on pension
status is not taken “because of such individual’s age,” so lia-
bility exists only where “pension status serve[s] as a proxy for
age.” Ky. Ret. Sys., 554 U.S. at 142 (internal quotation marks
omitted).
    But these disparate-treatment cases shed no light on the
scope of disparate-impact liability. The plaintiff in a dispar-
ate-treatment case must prove that age “actually motivated the
employer’s decision.” Id. at 141 (quotation marks omitted).
Because age and pension status are “analytically distinct,” an
employer may “take account of one while ignoring the other.”
Hazen Paper, 507 U.S. at 611. In contrast, a disparate-impact
No. 17-2956                                                               13

claim necessarily involves a policy—e.g., conditioning bene-
fits on pension status—that is motivated by a factor other than
age. As the Court explained in Meacham v. Knolls Atomic Power
Laboratory:
        The [reasonable factor other than age] defense
        in a disparate-impact case … is not focused on
        the asserted fact that a non-age factor was at
        work; we assume it was. The focus of the de-
        fense is that the factor relied upon was a “rea-
        sonable” one for the employer to be using.
554 U.S. 84, 96 (2008). We therefore turn to Caterpillar’s af-
firmative defense. 1
C. Reasonable Factor Other Than Age
    The plaintiffs have established a prima facie case, but Cat-
erpillar raises an affirmative defense that its liquidation plan
is based on a reasonable factor other than age. To prevail Cat-
erpillar must show that its less favorable treatment of retire-
ment-eligible employees was “reasonably designed to further
or achieve a legitimate business purpose and administered in
a way that reasonably achieves that purpose in light of the
particular facts and circumstances that were known, or
should have been known, to the employer.” 29 C.F.R. §

1 We note for completeness that the ADEA does contain certain pension-
related exceptions. See, e.g., 29 U.S.C. § 623(l)(1)(A)(i) (allowing pension
eligibility to turn on age); id. § 623(l)(2)(A) (allowing an employer to con-
sider age-related pension benefits in determining the level of severance
pay); id. § 623(l)(3) (allowing an employer to consider age-related pension
benefits in determining the level of long-term disability benefits); see also
Teufel v. N. Tr. Co., 887 F.3d 799, 802 (7th Cir. 2018) (explaining the rela-
tionship between the ADEA and pension plans). Caterpillar did not in-
voke any of these provisions.
14                                                    No. 17-2956

1625.7(e)(1). This is a “relatively light burden.” Karlo, 849 F.3d
at 80. “Unlike the [Title VII] business necessity test, which
asks whether there are other ways for the employer to achieve
its goals that do not result in a disparate impact on a protected
class, the reasonableness inquiry includes no such require-
ment.” Smith, 544 U.S. at 243.
    Caterpillar has satisfied its burden. The liquidation plan
was justified by a reasonable factor other than age—several,
actually. For over a decade, Caterpillar tried to eliminate the
unemployment plan to reduce labor costs at the Joliet plant.
Unemployment payments were infrequent, so Caterpillar
thought it could use the funds more efficiently. Caterpillar
also hoped to avoid the expense of upgrading the unemploy-
ment plan’s outdated administration system. Recognizing
that the union had resisted its prior attempts to eliminate the
plan, Caterpillar offered to distribute the trust’s $7.8 million
to union members to secure their support. Caterpillar’s pro-
posal was reasonably designed to achieve these cost-cutting
measures. See Allen v. Highlands Hosp. Corp., 545 F.3d 387, 392,
406 (6th Cir. 2008) (holding that the termination of “employ-
ees based on seniority to facilitate the hiring of new, less costly
employees” qualified as a reasonable factor other than age).
    Caterpillar’s decision to offer an inferior deal to retire-
ment-eligible employees is also valid. Voluntary retirement
incentives are permissible under the ADEA, see Henn v. Nat’l
Geographic Soc’y, 819 F.2d 824, 826–28 (7th Cir. 1987), and in
conducting the reasonableness inquiry, we do not consider
“whether there are other ways for the employer to achieve its
goals that do not result in a disparate impact on a protected
class,” Smith, 544 U.S. at 243. Caterpillar’s proposal killed two
No. 17-2956                                                   15

birds with one stone: it maintained the retirement incentives
and induced union members to sign off on the deal.
    The plaintiffs counter that Caterpillar’s use of retirement
eligibility was unreasonable for four reasons. First, they argue
that we should not consider Caterpillar’s rationale for elimi-
nating the unemployment plan. In their view we should con-
sider only whether it was reasonable for Caterpillar to modify
its proposal in a manner that discriminated against retire-
ment-eligible employees. See Romero v. Allstate Ins. Co., 251 F.
Supp. 3d 867, 886–87 (E.D. Pa. 2017). But we are required to
consider the “particular facts and circumstances surround-
ing” Caterpillar’s proposal, 29 C.F.R. § 1625.7(e)(1), which in-
clude Caterpillar’s long-standing desire to get rid of the
unemployment plan, its initial offer to distribute the funds
only to retirement-eligible employees, and its subsequent of-
fer to broaden the pool of beneficiaries. Moreover,
Caterpillar’s decision to treat retirement-eligible employees
less favorably allowed it to secure the union’s approval while
keeping the retirement incentives in place.
    Second, the plaintiffs argue that a desire to placate a union
in collective bargaining is an inherently suspect basis to favor
younger employees. They suggest that this rationale could
serve as a basis for discriminating against older employees in
collective bargaining. Such fears are overblown. If an em-
ployer seeks to discriminate on the basis of age, a plaintiff
may bring a disparate-treatment claim under § 623(a)(1). Fur-
ther, when an employer enters into an agreement that has a
disparate impact on older employees, the employer has the
burden to prove that the disparate impact resulted from a rea-
sonable factor other than age. Here Caterpillar needed the un-
ion’s agreement to achieve a legitimate business purpose. To
16                                                    No. 17-2956

the extent that the liquidation plan was reasonably designed
to further that objective, it is protected by the affirmative de-
fense.
    Third, the plaintiffs argue that a jury could find that the
modified liquidation proposal was not “reasonably de-
signed” or “administered” to secure a contract with the union.
Id. The thrust of their argument is that the union would have
readily agreed to eliminate the unemployment plan if the
funds were distributed equally to all participants. But the rea-
sonableness inquiry does not include a requirement that Cat-
erpillar could have achieved its goals in a nondiscriminatory
manner. Smith, 544 U.S. at 243.
     Along the same lines, the plaintiffs assert that Caterpillar’s
decision to modify the proposal made securing an agreement
with the union more difficult. They argue that the modified
proposal “introduced new objectionable elements” by dis-
criminating against retirement-eligible workers. The union’s
actions during the negotiations contradict this account. When
it first proposed to eliminate the unemployment plan, it in-
cluded the modified liquidation plan in its contract pro-
posal—not the original liquidation plan that Caterpillar had
favored.
    Finally, the plaintiffs argue that acquiescence by a union
in a civil-rights violation is not a defense for employers. See
e.g., EEOC v. Calumet County, 686 F.2d 1249, 1257 (7th Cir.
1982). This argument is a nonstarter. Caterpillar’s defense
does not turn on whether the union’s agreement insulates it
from ADEA liability.
   In sum, we hold that the plaintiffs have established a
prima facie case of disparate-impact age discrimination: the
No. 17-2956                                                  17

liquidation plan constitutes a specific policy and creates a
substantial age-based disparity between workers. But be-
cause the policy was based on several reasonable factors other
than age, Caterpillar has established its affirmative defense to
disparate-impact liability.
                                                     AFFIRMED.
