25. A non-transitory computer storage medium encoded with instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: determining a first count of user selections, received by a search engine, of search results that were presented in response to queries that are categorized as referring include a reference to a particular site; determining a second count of user selections, received by the search engine, of search results that identify resources in the particular site; and determining a site quality score for the particular site including computing a ratio of a numerator based on the first count and a denominator based on the second count, wherein (i) the numerator is based on the first count reduced by a threshold value which is a predetermined threshold value, (ii) the denominator is based on the second count raised to a power that is greater than zero and less than one, or (iii) both.
25. A non-transitory computer storage medium encoded with instructions that when executed by one or more computers cause the one or more computers to perform operations comprising: determining a first count of user selections, received by a search engine, of search results that were presented in response to queries that are categorized as referring include a reference to a particular site; determining a second count of user selections, received by the search engine, of search results that identify resources in the particular site; and determining a site quality score for the particular site including computing a ratio of a numerator based on the first count and a denominator based on the second count, wherein (i) the numerator is based on the first count reduced by a threshold value which is a predetermined threshold value, (ii) the denominator is based on the second count raised to a power that is greater than zero and less than one, or (iii) both. 30. The computer storage medium of claim 25 , wherein the numerator is a maximum of (a) a lower-bound value and (b) the first count reduced by a threshold value.