10. A system for determining a degree of association between a search string and an item, the system comprising: a data store configured to store user event history sequences that identify a number of user actions that occurred during user interaction with an electronic repository, wherein at least one of the user event history sequences comprises a search event in which a user submitted a request to search items in the electronic repository; and a computing device in communication with the data store, the computing device configured to: retrieve the at least one user event history sequence from the data store, wherein the at least one user event history sequence includes a search string submitted by the user in association with the search event and a subsequent item selection event, wherein the subsequent item selection event corresponds to a selection by the user of an item in the electronic repository, said subsequent item selection event occurring after the search event; determine a number of intervening events in the at least one user event history sequence that each occur after the search event and before the subsequent item selection event; and calculate an event weight that specifies an amount of weight to give to the item selection event for purposes of quantifying an association between the search string submitted by the user and the item selected in the subsequent item selection event, wherein the event weight is calculated based at least in part on the determined number of intervening events that occur after the search event and before the subsequent item selection event in the at least one user event history sequence; and based at least in part on the event weight, determine a relationship value representing a probability that a user who submits the search string will subsequently select the item.
12. The system of claim 10 , wherein, when the intervening events include an intervening search event, the event weight is calculated based at least in part on a degree of association between the search string and a second search string submitted in the intervening search event.