33. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: determining a first count of unique queries, received by a search engine, that are categorized as referring to a particular site; determining a second count of unique queries, received by the search engine, that are associated with the particular site, wherein a query is associated with the particular site when the query is followed by a user selection of a search result that (a) was presented, by the search engine, in response to the query and (b) identifies a resource 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.
33. A system comprising: one or more computers and one or more storage devices storing instructions that are operable, when executed by the one or more computers, to cause the one or more computers to perform operations comprising: determining a first count of unique queries, received by a search engine, that are categorized as referring to a particular site; determining a second count of unique queries, received by the search engine, that are associated with the particular site, wherein a query is associated with the particular site when the query is followed by a user selection of a search result that (a) was presented, by the search engine, in response to the query and (b) identifies a resource 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. 34. The system of claim 33 , wherein a query is categorized as referring to the particular site when the query includes a site label identifying the particular site.