14. A system for identifying one or more second documents related to one or more documents of a set of first documents, the system comprising: one or more computers configured to perform operations comprising: determining a respective strength of relationship score between each candidate document in a group of candidate documents and each of the first documents by aggregating user selection data for multiple users, the first documents and the candidate documents being in a corpus of web documents, the user selection data indicating, for each of the multiple users, whether the user viewed the candidate document during a window of time after the first document is presented to the user on a search results web page in response to a query, wherein the strength of relationship score is a probability that the candidate document will be viewed given that the first document has been presented to a user on a search results web page in response to a query; calculating an aggregate strength of relationship score for each candidate document from the respective strength of relationship scores for the candidate document; and selecting the one or more second documents from the candidate documents according to the aggregate strength of relationship scores for the candidate documents.
14. A system for identifying one or more second documents related to one or more documents of a set of first documents, the system comprising: one or more computers configured to perform operations comprising: determining a respective strength of relationship score between each candidate document in a group of candidate documents and each of the first documents by aggregating user selection data for multiple users, the first documents and the candidate documents being in a corpus of web documents, the user selection data indicating, for each of the multiple users, whether the user viewed the candidate document during a window of time after the first document is presented to the user on a search results web page in response to a query, wherein the strength of relationship score is a probability that the candidate document will be viewed given that the first document has been presented to a user on a search results web page in response to a query; calculating an aggregate strength of relationship score for each candidate document from the respective strength of relationship scores for the candidate document; and selecting the one or more second documents from the candidate documents according to the aggregate strength of relationship scores for the candidate documents. 23. The system of claim 14 , further operable to perform operations comprising: receiving input from a second user indicating that one or more of the first documents are preferred documents; calculating a respective document weight for each of the preferred documents; and wherein calculating the aggregate strength of relationship score for each candidate document includes weighting the strength of relationship scores for the candidate document and each of the preferred documents by the respective document weight for the preferred document.