17. 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 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.
20. The computer storage medium of claim 17 , wherein a query is categorized as referring to the particular site when the query is a query that has been determined to be a navigational query to the particular site.