In one embodiment, an impression to be displayed in a placement of a web page is selected from candidate impressions that may be displayed in the placement. The candidate impression may be chosen based on behavioral data and a placement identifier particularly identifying the placement. The behavioral data may include categories of websites visited by an end-user on the client computer. The candidate impressions may be in learning modes, and the impression displayed in the placement may be selected based on its learning mode and revenue generation capability.