How do you measure the success of a new Web feature? This is a critical issue for Vanguard Group, since the mutual fund company does about 80% of its interactions over the Web. So when it adds a feature, Vanguard tracks how many people start using the feature, how many get through the process, and how many drop out and where. The company also tracks what percentage of time a single tool gets used by site visitors. The data's used to head off problems but also to decide where a tool gets placed on the site.