a method for uniquely identifying an initial exposure of a visitor to an advertisement campaign is provided . the method includes causing a web browser to call an overlap server upon serving of an advertisement of the advertisement campaign ; storing , by the overlap server , an overlap data structure in any one of the web browser and a web server , wherein the overlap data structure is coded to identify the advertisement campaign , a host website through which the visitor is exposed to the advertisement campaign , and an indication as to whether the advertisement campaign was previously visited by the visitor of the web browser ; and processing , by the overlap server , the overlap data structure to determine the initial exposure of the visitor to the advertisement of the advertisement campaign and the host website through which the visitor was exposed to the advertisement campaign .