certification decisions, and their judgments are to be reviewed by the court of appeals only for abuse of discretion."); Brown I, 576 F.3d at 152; Thorn v. Jefferson-Pilot Life Ins. Co., 445 F.3d 311, 317 (4th Cir. 2006). A district court abuses its discretion when it materially misapplies the requirements of Rule 23. See Gunnells