1. A computer-implemented method for identifying one or more second documents related to one or more documents of a set of first documents, the method 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.