Generating a related set of documents for an initial set of documents

Methods, systems, and apparatus, including computer programs encoded on computer storage media, for identifying one or more second documents related to one or more first documents. Strength of relationship scores between candidate documents in a group of candidate documents and each first document are determined by aggregating user selection data for users, the user selection data indicating, for each user, 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. An aggregate strength of relationship score is calculated for each candidate document from the strength of relationship scores for the candidate document. Second documents are selected from the candidate documents according to the aggregate strength of relationship scores for the candidate documents.