A method for tracking a cumulative number of unique visitors to different to objects. The method includes maintaining a data structure that allows for a direct determination of the cumulative number of different visitors exposed to the different objects; establishing a connection with a visitor when the visitor requests to visit an object; examining a cookie stored in the visitor during the established connection to determine whether the visitor previously visited the currently requested object; and updating the data structure in the database and an overlap code contained in the cookie based on the examination results, wherein objects are different websites displaying a same set of advertisements of a same campaign.