Mr. Liu's...office sent letters to the four biggest holding companies in the business -- Omnicom, Interpublic Group of Cos., Publicis Groupe and WPP...Citing little diversity in the advertising industry, the office asked for data classified by race, gender and job categories...Omnicom Group has resisted pressure to provide that data. A Publicis spokeswoman told Ad Age: "We are in a similar situation as Omnicom." WPP could not be reached for comment by press time. Companies outside the ad industry, such as Goldman Sachs and MetLife, have agreed to comply... the office is understood to be having some "dialogue" with Interpublic...[includes comments by Omnicom]