1. A system for determining the quality of visitors to a website, comprising: a first web server to operate an advertising campaign; a second web server configured to provide one or more web pages to a visitor, the one or more web pages including an advertisement provided by the first web server, wherein the first web server is configured to store a value associated with the visitor when (a) the visitor clicks on the advertisement included in the one or more web pages of the second server, and (b) the visitor visits a predefined collection of web pages provided by the first web server.