San Francisco--Thirteen of the Internet's top providers of advertising services agreed Monday to form the Advertising Standards Alliance, San Francisco. The trade group will lead development of standard measurements for an Internet advertisement's frequency and the number of unique users reached. The proposed standard also defines the role of a seller in tracking the efficacy of an advertising campaign, and the role of the buyer. Initial members of the ASA, each agreeing to implement the standards by June 1, are: DoubleClick, AdForce, 24/7 Media, Flycast Communications Corp., Engage Technologies, Burst! Media, Microsoft bCentral, Real Media, Sabela Media, Adsmart and SmartAge.com.