11. A method, performed by at least one computer system, of retrieving documents in response to a search query that includes one or more phrases, the method comprising: accessing an index for a corpus of documents, the corpus comprising documents having multiple instances, each instance of a particular document being acquired at a different date and a new instance being acquired when a change from a previous instance is detected, wherein each instance has an associated date range; selecting from the index a plurality of documents that are relevant to the search query; determining, using a processor of the at least one computer system, for each instance of a selected document, a relevance score based on relevance data valid for the instance of the selected document and its associated date range; selecting, for each document, the document instance with the highest relevance score; and ranking the selected document instances according to their relevance scores.
11. A method, performed by at least one computer system, of retrieving documents in response to a search query that includes one or more phrases, the method comprising: accessing an index for a corpus of documents, the corpus comprising documents having multiple instances, each instance of a particular document being acquired at a different date and a new instance being acquired when a change from a previous instance is detected, wherein each instance has an associated date range; selecting from the index a plurality of documents that are relevant to the search query; determining, using a processor of the at least one computer system, for each instance of a selected document, a relevance score based on relevance data valid for the instance of the selected document and its associated date range; selecting, for each document, the document instance with the highest relevance score; and ranking the selected document instances according to their relevance scores. 13. The method of claim 11 , wherein each instance of the document has an identifier and the index stores information for each instance using its respective identifier.