[mexico city] Wal-Mart Stores resigned from Mexico's National Retailers Association to protest an ethics code the group approved Oct. 4 prohibiting price comparisons in ads by its members. The code stipulates that members avoid ads that could cause negative publicity for other retailers or the retail industry. One paragraph explicitly bans ad campaigns using price comparisons, key to the "everyday low prices" strategy of Wal-Mart, Mexico's largest retailer. Wal-Mart, whose Mexican agency is WPP Group's J. Walter Thompson Co., has made it clear in statements and newspaper inserts that the company will not follow the new code of practice. Elsewhere, Wal-Mart is moving the account for its 95 stores in Germany to Grey Global Group's Grey Worldwide, Duesseldorf, from Publicis Groupe's Publicis, Frankfurt.