Abstract: A system estimates the part of a current content page that is currently visible on a user screen. A movement detection unit detects movement of the content page within the user's browser, and an estimator infers whether a particular content item of interest is within the currently visible area. The movement detection unit in one version detects operating system directives to find out whether the user is scrolling etc through the page. Alternatively an identifiable point is placed on the content item and continual measurement is made between identifiable point in the content and a point in the currently visible area. The system is useful for providing statistical data about the effectiveness of advertisement placement within web pages.