14. The system of claim 12 , the operations further comprising: in response to determining, by the one or more computers, that the number of the first links that are included in the documents in the set of documents and that (i) have anchor text that includes a term of the search query, and (ii) refer to the particular document does not exceed the predetermined threshold, using a second value to modify a quality score that is associated with the particular document, wherein the second value is different than the first value; and storing, by one or more processors, the modified quality score for use in ranking the particular document in response to subsequently received search queries.
14. The system of claim 12 , the operations further comprising: in response to determining, by the one or more computers, that the number of the first links that are included in the documents in the set of documents and that (i) have anchor text that includes a term of the search query, and (ii) refer to the particular document does not exceed the predetermined threshold, using a second value to modify a quality score that is associated with the particular document, wherein the second value is different than the first value; and storing, by one or more processors, the modified quality score for use in ranking the particular document in response to subsequently received search queries. 15. The system of claim 14 , wherein using the first value or the second value to modify a quality score that is associated with the particular document comprises: associating the first value or the second value with a particular group of documents associated with the particular document.