practice of discrimination." Hazelwood Sch. Dist. v. United States, 433 U.S. 299, 307-08, 97 S. Ct. 2736, 53 L. Ed. 2d 768 (1977); accord Ardrey v. United Parcel Serv., 798 F.2d 679, 683 (4th Cir. 1986) . But see Warren v. Halstead Indus., Inc., 802 F.2d 746, 753 (4th Cir. 1986) ("[S]tatistics cannot alone prove t