indefinitely. The district court thought that the inevitable differences in consumers’ experiences with IKO’s tiles prevent class certification under the language we have quoted.
The court read Comcast Corp. v. Behrend, — U.S. —, 133 S.Ct. 1426, 185 L.Ed.2d 515 (2013), and Wal-Mart Stores, Inc. v. Dukes, — U.S. —, 131 S.Ct. 2541, 180 L.Ed.2d 374 (2011), to require proof “that the plaintiffs will experience a common damage and that their claimed damages are not disparate.” 2014 U.S. Dist. Lexis 80243 at 9. Elsewhere the district court wrote that “commonality of damages” is essential. Id. at *13, 20, 23. If this is right, then class actions about consumer products are impossible, and our post-Comcast decision in Butler v. Sears, Roebuck & Co., 727 F.3d 796 (7th Cir.2013), must be wrong. Pella Corp. v. Saltzman, 606 F.3d 391 (7th Cir.2010), which approved class treatment of litigation asserting defects in casement windows, another product used in home construction or renovation, also would be wrongly decided under the district court’s reading of Comcast and Wal-Mart. Indeed, we could not affirm the district court’s decision in this case without overruling Pella,* so close are the circumstances of the two home-product-defect suits.
Yet Wal-Mart has nothing to do with commonality of damages. It dealt instead with the need for conduct common to members of the class, and it concerned Rule 23(a)(2) rather than Rule 23(b)(3). Plaintiffs in Wah-Mart contended that discretionary acts by managers of more than 2,000 local stores produced discriminatory effects. When writing that commonality under Rule 23(a)(2) requires proof of the same injury, the Court observed that each store was managed independently; it held that when multiple managers exercise discretion, conditions at different stores do not present a common question. See also Bolden v. Walsh Construction Co., 688 F.3d 893 (7th Cir.2012). In that situation damages differ, to be sure, but only because the underlying conduct differs. In a suit alleging a defect common to all instances of a consumer product, however, the conduct does not differ.
Comcast, by contrast, does discuss the role of injury under Rule 23(b)(3), though not in the way the district court thought. Plaintiffs filed an antitrust suit and specified four theories of liability. The district judge certified a class limited to one of these four. The plaintiffs’ damages expert, however, estimated harm starting with the assumption that all four theories had been established. The Court held that this made class treatment inappropriate: without a theory of loss that matched the theory of liability, the class could not get anywhere.
That would be equally true in a suit with just one plaintiff. In antitrust law, damages are limited to the sort of injury that flows from unlawful conduct. See, e.g., Brunswick Corp. v. Pueblo Bowl-O-Mat, Inc., 429 U.S. 477, 97 S.Ct. 690, 50 L.Ed.2d 701 (1977). Competition creates benefits for consumers and harm for producers at the same time, while monopoly causes harm to consumers and some producers. It is essential to distinguish the encouraged injuries (to producers, from competition) from the forbidden ones (to consumers, from monopoly). That requires matching the theory of liability to the theory of damages. Comcast explained: “The first step in a damages study is the translation of the legal theory of the harmful event into an analysis of the economic impact of that event.” 133 S.Ct. at 1435, quoting from Federal Judicial Center, Reference Manual on Scientific Evidence 432 (3d ed.2011) (emphasis added by Comcast ).