15. The computer system of claim 12 , wherein the characteristics are selected among signals comprising: user location; user device information; publisher; local query time; local time of page request; page identity; content item properties; keyword in page request; keyvalue page visiting history; content item interaction history; and combinations thereof.
15. The computer system of claim 12 , wherein the characteristics are selected among signals comprising: user location; user device information; publisher; local query time; local time of page request; page identity; content item properties; keyword in page request; keyvalue page visiting history; content item interaction history; and combinations thereof. 16. The computer system of claim 15 , wherein training the predictive model includes dividing the online activity data into first and second portions and using the first portion to train the predictive model, and wherein identifying the cross product involves applying the trained model to the second portion.