1. A method comprising: receiving a request for an impression to be displayed in a placement of a web page; receiving a placement identifier particularly identifying the placement for the impression in the web page; receiving behavioral data indicative of an online history and previously received impressions; determining a plurality of candidate impressions that may be displayed in the placement of the web page, based, at least in part, on the behavioral data; determining a learning mode of each candidate impression, the learning mode of the each candidate impression being indicative of a number of times the each candidate impression has been served, wherein the learning mode is one of a plurality of learning modes, the plurality of learning modes comprising a first learning mode, a second learning mode, and a third learning mode, wherein the first learning mode includes candidate impressions that have been served less than or equal to a first predetermined threshold number of times, wherein the second learning mode includes candidate impressions that have been served more than the first predetermined threshold number of times and less than or equal to a second predetermined threshold number of times, and wherein the third learning mode includes candidate impressions that have been served more than the second predetermined threshold number of times; applying learning mode percentages to determine a retained learning mode from the plurality of learning modes; and selecting a selected impression from the plurality of candidate impressions that are in the retained learning mode.