1. A computer-implemented method for selectively accessing a document during a current crawl of a server computer, the document being identified by a document address specification, the document having been retrieved during a previous crawl, the method comprising: determining whether to access the document during the current crawl with the aid of a probabilistic model that is based on the probability that the document has changed since the previous crawl; and accessing the document if the determination produces an instruction indicative that the document at the document address specification should be accessed during the current crawl.