26. A computer-readable storage medium having instructions stored thereon that direct a computing system to, at least: submit a query to a plurality of search indexes, each search index associated with a relevance ranking function for ranking items according to an individual ranking scale, at least two of the individual ranking scales being different; queue a plurality of search index result sets received from the plurality of search indexes, each of the plurality of search index result sets being ordered lists identifying items related to the query and ranked according to one of the individual ranking scales; determine a query index association for each of the plurality of search indexes; and create a universal query result set that includes a list of items obtained from the plurality of search index result sets, the list being ordered based on at least a probability of those items satisfying the query, the list of items containing items obtained from different search index result sets, wherein the probability of the items of the list satisfying the query is determined at least in part by: computing an allocation score for each of the plurality of search indexes, the allocation score for each search index being computed based at least in part upon a relative number of search results in the corresponding search index result set; for each of the plurality of search index result sets, computing a universal item score for at least one item identified in the respective search index result set, the universal item score for each item being computed at least in part by normalizing the individual ranking scale for each of the plurality of search index result sets to a common ranking scale; and determining a probability of each item satisfying the query based at least upon a combination of the allocation score for the search index associated with the item and the universal item score for the item.
26. A computer-readable storage medium having instructions stored thereon that direct a computing system to, at least: submit a query to a plurality of search indexes, each search index associated with a relevance ranking function for ranking items according to an individual ranking scale, at least two of the individual ranking scales being different; queue a plurality of search index result sets received from the plurality of search indexes, each of the plurality of search index result sets being ordered lists identifying items related to the query and ranked according to one of the individual ranking scales; determine a query index association for each of the plurality of search indexes; and create a universal query result set that includes a list of items obtained from the plurality of search index result sets, the list being ordered based on at least a probability of those items satisfying the query, the list of items containing items obtained from different search index result sets, wherein the probability of the items of the list satisfying the query is determined at least in part by: computing an allocation score for each of the plurality of search indexes, the allocation score for each search index being computed based at least in part upon a relative number of search results in the corresponding search index result set; for each of the plurality of search index result sets, computing a universal item score for at least one item identified in the respective search index result set, the universal item score for each item being computed at least in part by normalizing the individual ranking scale for each of the plurality of search index result sets to a common ranking scale; and determining a probability of each item satisfying the query based at least upon a combination of the allocation score for the search index associated with the item and the universal item score for the item. 30. The computer-readable medium of claim 26 , wherein the instructions that direct the computing system to create a universal query result set further include instructions that direct the computing system to: compute the universal item score for an item listed in each of the plurality of search index result sets.