1. A computer-implemented method of measuring a degree of association between a search string and a selectable item, the method comprising: 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 said electronic repository, said item selection event occurring after the search string submission event; identifying one 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 one or more intervening events and the search string submission event comprise a user event history sequence; calculating a weight value that represents a degree of association between the search string submission event and the item selection event, wherein calculating the weight value comprises assigning different decay values to different ones of said one or more intervening events from the user event history sequence, wherein the decay values are dependent upon a type of said intervening event, and using the decay values in combination to calculate said weight value; and using said weight value, in combination with weight values corresponding to other event sequences, to calculate a relationship value representing a degree to which the search string and the item are associated.