Ps are consumers who purchased Wesson-brand cooking oil products labeled '100% Natural.' Ps argue that the '100% Natural' label is false or misleading because Wesson oils are made from bioengineered ingredients (genetically modified organisms, or GMOs) that Ps contend are 'not natural.' D manufactures, markets, distributes and sells Wesson products. Ps filed putative class actions asserting state-law claims against D in eleven states, and those cases were consolidated in this action. Ps moved to certify eleven classes; all persons who reside in the States of California, Colorado, Florida, Illinois, Indiana, Nebraska, New York, Ohio, Oregon, South Dakota, or Texas who have purchased Wesson Oils within the applicable statute of limitations periods. D opposed class certification on in that there would be no administratively feasible way to identify members of the proposed classes because consumers would not be able to reliably identify themselves as class members. The district court acknowledged that the Third Circuit and some district courts have refused certification in similar circumstances, but it declined to join in their reasoning. The district court held that, at the certification stage, it was sufficient that the class was defined by an objective criterion: whether class members purchased Wesson oil during the class period. D appealed.