In keeping with its importance, law firms have been weighing in almost nonstop with corporate clients. Their overall verdict? The ruling likely will constrain future class-action cases, for one thing. "In vacating this morning what some regarded as history's largest business class action, the Supreme Court's landmark opinion in Wal-Mart Stores, Inc. v. Dukes considerably tightens the criteria for class certification in all would-be class actions," according to the .

But the legal briefings also point to the guidelines the ruling creates for plaintiffs.

Under Rule 23 of the Federal Rules of Civil Procedure, plaintiffs who want to proceed under a class action suit must show they share a common question of law or fact, as this brief by the law firm of . In the Wal-Mart case, the Supreme Court required the plaintiffs to show they suffered the same injury, and that the injury could be remedied in one stroke for the entire class.

The plaintiffs attempted to show this through the use of statistics. That is, the numbers would show that store managers across the country made pay and promotion decisions that negatively affected all women employees. "The Supreme Court ruled that the plaintiffs had not presented significant proof of their common question of fact or law," Sherman & Howard write. The plaintiffs couldn't point to anything the managers did in common to discriminate against Wal-Mart's women employees.

What's more, Wal-Mart's official policy prohibits discrimination, the Court noted. And, because the company allows individual managers leeway in hiring and promotion decisions, the plaintiffs would have to show "a common mode of exercising discretion that pervades the entire company," as this summary from the law firm of . The plaintiffs' statistical evidence did not suffice because it didn't preclude the existence of regional or local variations, Faegre & Benson explained. Their anecdotal evidence was too limited to prove a common, nationwide pattern of discrimination.