1. A computer-implemented method of determining a degree of association between a search string and a selectable item, the method comprising: as implemented by one or more computing devices configured with specific executable instructions, identifying a search string submission event in which a user submitted a search string to search an electronic repository; identifying an item selection event in which the user selected an item represented in the electronic repository, the item selection event occurring after the search string submission event; identifying two or more intervening events initiated by the user subsequent to the search string submission event but prior to the item selection event, wherein the item selection event, the two or more intervening events and the search string submission event comprise a user event history sequence; assigning a decay value to each of the two or more intervening events from the user event history sequence, wherein different decay values are assigned to different ones of the two or more intervening events; calculating a weight value that represents a degree of association between the search string submission event and the item selection event, wherein the weight value is calculated based at least in part on the two or more intervening events and the decay value assigned to each of the two or more intervening events, wherein the decay value assigned to each of the two or more intervening events serves to reduce the weight value; and calculating a relationship value representing a degree to which the search string and the item are associated, wherein the relationship value is calculated based at least in part on the weight value and at least one other weight value corresponding to a different user event history sequence.