8. A system comprising: a memory device for storing instructions executable by a processor; a processor for executing instructions stored in the memory, at least a portion of those instructions representing an item listing presentation management module, which, when executed by the processor, is to process a query to i) 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, ii) obtain 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, iii) identify up to a predetermined number of leaf-level categories with click probability scores exceeding a threshold score, iv) for each of the identified leaf-level categories, calculate 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 iv) cause a search results page to be presented 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.
8. A system comprising: a memory device for storing instructions executable by a processor; a processor for executing instructions stored in the memory, at least a portion of those instructions representing an item listing presentation management module, which, when executed by the processor, is to process a query to i) 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, ii) obtain 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, iii) identify up to a predetermined number of leaf-level categories with click probability scores exceeding a threshold score, iv) for each of the identified leaf-level categories, calculate 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 iv) cause a search results page to be presented 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. 11. The system of claim 8 , wherein the item listing presentation management module is to derive the category boost score for each identified leaf-level category based in part on the click probability score of each identified leaf-level category.