19. A computer-readable storage medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the steps of: identifying a set of one or more terms related to a specific search query; wherein the set of one or more terms are not part of the specific search query; obtaining a plurality of search results for the specific search query; determining a first number of results of the plurality of search results; determining a second number of results for each term of the set of one or more terms, wherein the second number is the number of results that contain said each term; determining, for said each term, a third number of historical search queries, and adding said third number to a running total; wherein said third number is based on a count of those historical search queries that include both the specific search query and said each term; determining a first ratio of said third number to the running total; determining a second ratio of the second number to the first number; determining a relevance of a set of results; wherein the set of results includes the plurality of search results, and wherein determining the relevance of the set of results is based at least in part on: (a) said second ratio, for one or more related terms, of the second number to the first number; and (b) said first ratio, for one or more related terms, of said third number to the running total; determining that none of the results in the set of results contain a particular term from the set of one or more terms related to the specific search query; and determining, based at least in part on the ratio of said third number of the particular term to the running total, a loss in variety for the set of results.
19. A computer-readable storage medium carrying one or more sequences of instructions which, when executed by one or more processors, causes the one or more processors to perform the steps of: identifying a set of one or more terms related to a specific search query; wherein the set of one or more terms are not part of the specific search query; obtaining a plurality of search results for the specific search query; determining a first number of results of the plurality of search results; determining a second number of results for each term of the set of one or more terms, wherein the second number is the number of results that contain said each term; determining, for said each term, a third number of historical search queries, and adding said third number to a running total; wherein said third number is based on a count of those historical search queries that include both the specific search query and said each term; determining a first ratio of said third number to the running total; determining a second ratio of the second number to the first number; determining a relevance of a set of results; wherein the set of results includes the plurality of search results, and wherein determining the relevance of the set of results is based at least in part on: (a) said second ratio, for one or more related terms, of the second number to the first number; and (b) said first ratio, for one or more related terms, of said third number to the running total; determining that none of the results in the set of results contain a particular term from the set of one or more terms related to the specific search query; and determining, based at least in part on the ratio of said third number of the particular term to the running total, a loss in variety for the set of results. 21. The computer-readable medium of claim 19 , wherein determining the relevance of a set results includes: determining the relevance of each search result in the plurality of search results based on each term in the set of one or more terms; generating a total relevance for the set of results based on all the terms in the set of one or more terms; and the one or more sequences of instructions causes the one or more processors to further perform the steps of: generating a value representing the relevance of the set of results; and reporting the total relevance for the set of results.