From Wal-Mart Stores, Inc. v. Dukes, 2011 U.S. LEXIS 4567 (June 20, 2011):

The crux of this case is commonality — the rule requiring a plaintiff to show that "there are questions of law or fact common to the class." Rule 23(a)(2).

Footnote 5. We have previously stated in this context that "[t]he commonality and typicality requirements of Rule 23(a) tend to merge. Both serve as guideposts for determining whether under the particular circumstances maintenance of a class action is economical and whether the named plaintiff's claim and the class claims are so interrelated that the interests of the class members will be fairly and adequately protected in their absence. Those requirements therefore also tend to merge with the adequacy-of-representation requirement, although the latter requirement also raises concerns about the competency of class counsel and conflicts of interest." General Telephone Co. of Southwest v. Falcon, 457 U.S. 147, 157-158, n. 13, 102 S. Ct. 2364, 72 L. Ed. 2d 740 (1982). In light of our disposition of the commonality question, however, it is unnecessary to resolve whether respondents have satisfied the typicality and adequate-representation requirements of Rule 23(a).

That language is easy to misread, since "[a]ny competently crafted class complaint literally raises common 'questions.'" Nagareda, Class Certification in the Age of Aggregate Proof, 84 N. Y. U. L. Rev. 97, 131-132 (2009). For example: Do all of us plaintiffs indeed work for Wal-Mart? Do our managers have discretion over pay? Is that an unlawful employment practice? What remedies should we get? Reciting these questions is not sufficient to obtain class certification. Commonality requires the plaintiff to demonstrate that the class members "have suffered the same injury," Falcon, supra, at 157, 102 S. Ct. 2364, 72 L. Ed. 2d 740. This does not mean merely that they have all suffered a violation of the same pro-vision of law. Title VII, for example, can be violated in many ways — by intentional discrimination, or by hiring and promotion criteria that result in disparate impact, and by the use of these practices on the part of many different superiors in a single company. Quite obviously, the mere claim by employees of the same company that they have suffered a Title VII injury, or even a disparate-impact Title VII injury, gives no cause to believe that all their claims can productively be litigated at once. Their claims must depend upon a common contention — for example, the assertion of discriminatory bias on the part of the same supervisor. That common contention, moreover, must be of such a nature that it is capable of classwide resolution — which means that determination of its truth or falsity will resolve an issue that is central to the validity of each one of the claims in one stroke.

"What matters to class certification . . . is not the raising of common 'questions' -- even in droves -- but, rather the capacity of a classwide proceeding to generate common answers apt to drive the resolution of the litigation. Dissimilarities within the proposed class are what have the potential to impede the generation of common answers." Nagareda, supra, at 132.

Rule 23 does not set forth a mere pleading standard. A party seeking class certification must affirmatively demonstrate his compliance with the Rule — that is, he must be prepared to prove that there are in fact sufficiently numerous parties, common questions of law or fact, etc. We recognized in Falcon that "sometimes it may be necessary for the court to probe behind the pleadings before coming to rest on the certification question," 457 U.S., at 160, 102 S. Ct. 2364, 72 L. Ed. 2d 740, and that certification is proper only if "the trial court is satisfied, after a rigorous analysis, that the prerequisites of Rule 23(a) have been satisfied," id., at 161, 102 S. Ct. 2364, 72 L. Ed. 2d 740; see id., at 160, 102 S. Ct. 2364, 72 L. Ed. 2d 740 ("[A]ctual, not presumed, conformance with Rule 23(a) remains . . . indispensable"). Frequently that "rigorous analysis" will entail some overlap with the merits of the plaintiff's underlying claim. That cannot be helped. "'[T]he class determination generally involves considerations that are enmeshed in the factual and legal issues comprising the plaintiff's cause of action.'" Falcon, supra, at 160, 102 S. Ct. 2364, 72 L. Ed. 2d 740 (quoting Coopers & Lybrand v. Livesay, 437 U.S. 463, 469, 98 S. Ct. 2454, 57 L. Ed. 2d 351 (1978); some internal quotation marks omitted). ***

Footnote 6. A statement in one of our prior cases, Eisen v. Carlisle & Jacquelin, 417 U.S. 156, 177, 94 S. Ct. 2140, 40 L. Ed. 2d 732 (1974), is sometimes mistakenly cited to the contrary: "We find nothing in either the language or history of Rule 23 that gives a court any authority to conduct a preliminary inquiry into the merits of a suit in order to determine whether it may be maintained as a class action." But in that case, the judge had conducted a preliminary inquiry into the merits of a suit, not in order to determine the propriety of certification under Rules 23(a) and (b) (he had already done that, see id., at 165, 94 S. Ct. 2140, 40 L. Ed. 2d 732), but in order to shift the cost of notice required by Rule 23(c)(2) from the plaintiff to the defendants. To the extent the quoted statement goes beyond the permissibility of a merits inquiry for any other pretrial purpose, it is the purest dictum and is contradicted by our other cases. ***

In this case, proof of commonality necessarily overlaps with respondents' merits contention that Wal-Mart engages in a pattern or practice of discrimination. 7 That is so because, in resolving an individual's Title VII claim, the crux of the inquiry is "the reason for a particular employment decision," Cooper v. Federal Reserve Bank of Richmond, 467 U.S. 867, 876, 104 S. Ct. 2794, 81 L. Ed. 2d 718 (1984). Here respondents wish to sue about literally millions of employment decisions at once.

Without some glue holding the alleged reasons for all those decisions together, it will be impossible to say that examination of all the class members' claims for relief will produce a common answer to the crucial question why was I disfavored.

*** The District Court concluded that Daubert did not apply to expert testimony at the certification stage of class-action proceedings. 222 F.R.D., at 191. We doubt that is so, but even if properly considered, Bielby's testimony does nothing to advance respondents' case. "[W]hether 0.5 percent or 95 percent of the employment decisions at Wal-Mart might be determined by stereotyped thinking" is the essential question on which respondents' theory of commonality depends. If Bielby admittedly has no answer to that question, we can safely disregard what he has to say. It is worlds away from "significant proof " that Wal-Mart "operated under a general policy of discrimination."