1. A computer-implemented method comprising: processing a query to identify, a set of item listings, each item listing associated with an item or service being offered for sale and assigned to a leaf-level category; obtaining for the query a click probability score for each leaf-level category to which an item listing satisfying the query has been assigned, the click probability score for a particular leaf-level category derived by dividing a number of clicks for the particular leaf-level category by the total number of clicks for all leaf-level categories to which an item listing satisfying the query has been assigned; identifying up to a predetermined number of leaf-level categories with click probability scores exceeding a threshold score; for each of the identified leaf-level categories, calculating a category boost score for use in determining the order in which the item listings are to be presented in a search results page; and presenting a search results page with the item listings ordered based in part on the category boost score for the leaf-level category, to which each item listing is assigned.
1. A computer-implemented method comprising: processing a query to identify, a set of item listings, each item listing associated with an item or service being offered for sale and assigned to a leaf-level category; obtaining for the query a click probability score for each leaf-level category to which an item listing satisfying the query has been assigned, the click probability score for a particular leaf-level category derived by dividing a number of clicks for the particular leaf-level category by the total number of clicks for all leaf-level categories to which an item listing satisfying the query has been assigned; identifying up to a predetermined number of leaf-level categories with click probability scores exceeding a threshold score; for each of the identified leaf-level categories, calculating a category boost score for use in determining the order in which the item listings are to be presented in a search results page; and presenting a search results page with the item listings ordered based in part on the category boost score for the leaf-level category, to which each item listing is assigned. 3. The computer-implemented method of claim 1 , wherein the threshold score is derived by dividing the click probability score of the leaf-level category with the highest click probability score by one less than the predetermined number.