Ad rotation - it is rare for web sites to display the same ads continuously

Ad rotation - it is rare for web sites to display the same ads continuously, primarily bei ause more revenue can be earned by changing them on a frequent or rotating them. There are a number of methods of rotation: (a) by user session - where the same ads are displayed while a user is on the site, (b) by page download - each time a user opens a page on the site a different ad from a series is displayed, or (c) by user profile (see contextual and behavioural targeting)