14. A non-transitory computer-readable medium, comprising: a plurality of instructions which, when executed by one or more processors, cause the one or more processors to: receive a search query from a client device; identify documents as search results relevant to the search query, where each of the search results has a respective first score; rank the search results, based on the respective first scores, to form a ranked list of search results, where each of the search results has a respective position, within the ranked list, that is based on the ranking; determine a selection rate corresponding to each of the positions in the ranked list, a particular selection rate, corresponding to a particular position, being determined based on selections from the particular position, and the selection rate for one of the positions in the ranked list, being determined based on: selectively: obtaining information regarding an actual selection rate, corresponding to the one of the positions, from observed selections, or estimating a selection rate corresponding to the one of the positions, and using the actual selection rate or the estimated selection rate as the selection rate for the one of the positions; identify categories with which the search results are associated; generate second scores for the categories based on the selection rates, where the second score, for a particular one of the categories, is based on the selection rate corresponding to the position, in the ranked list, of one or more of the search results that are associated with the particular one of the categories, the second score, for one of the categories, being generated based on a combination of a plurality of selection rates corresponding to positions, in the ranked list, of a set of the search results that are associated with the one of the categories; select a plurality of the categories based on the second scores of the categories; select a plurality of the search results for the selected plurality of categories; generate a search result document that includes information regarding the selected plurality of categories and information regarding the selected plurality of search results; and provide the search result document for display on the client device.
14. A non-transitory computer-readable medium, comprising: a plurality of instructions which, when executed by one or more processors, cause the one or more processors to: receive a search query from a client device; identify documents as search results relevant to the search query, where each of the search results has a respective first score; rank the search results, based on the respective first scores, to form a ranked list of search results, where each of the search results has a respective position, within the ranked list, that is based on the ranking; determine a selection rate corresponding to each of the positions in the ranked list, a particular selection rate, corresponding to a particular position, being determined based on selections from the particular position, and the selection rate for one of the positions in the ranked list, being determined based on: selectively: obtaining information regarding an actual selection rate, corresponding to the one of the positions, from observed selections, or estimating a selection rate corresponding to the one of the positions, and using the actual selection rate or the estimated selection rate as the selection rate for the one of the positions; identify categories with which the search results are associated; generate second scores for the categories based on the selection rates, where the second score, for a particular one of the categories, is based on the selection rate corresponding to the position, in the ranked list, of one or more of the search results that are associated with the particular one of the categories, the second score, for one of the categories, being generated based on a combination of a plurality of selection rates corresponding to positions, in the ranked list, of a set of the search results that are associated with the one of the categories; select a plurality of the categories based on the second scores of the categories; select a plurality of the search results for the selected plurality of categories; generate a search result document that includes information regarding the selected plurality of categories and information regarding the selected plurality of search results; and provide the search result document for display on the client device. 21. The non-transitory computer-readable medium of claim 14 , where one or more instructions, of the plurality of instructions, to generate the search result document cause the one or more processors to: organize the selected plurality of categories within the search result document, where a first one of the selected plurality of categories is presented at a particular location within the search result document irrespective of the second score of the first one of the selected plurality of categories, and where a second one of the selected plurality of categories and a third one of the selected plurality of categories are presented, below the first one of the selected plurality of categories, based on the second scores of the second one of the selected plurality of categories and the third one of the selected plurality of categories.