13. A computer-readable storage medium having stored thereon instructions which, when executed by a processor, cause the processor to perform the operations of: retrieving a number of document identifiers, each document identifier identifying a corresponding document on a network; for each retrieved document identifier, determining a query-independent score indicative of a page rank of the corresponding document relative to other documents in a set of documents; determining a content change frequency of the corresponding document identifier by comparing information stored for successive downloads of the corresponding document, determining an age of the corresponding document, wherein the age is associated with the time of the last download of the corresponding document, and determining a first score for the document identifier that is a function of the query-independent score and the determined content change frequency and the determined age of the corresponding document; comparing the first score against a threshold value; and conditionally scheduling the document for indexing based on the result of the comparison.
16. A computer-readable storage medium of claim 13 , wherein determining the content change frequency for a respective document identifier comprises determining the content change frequency from a content checksum stored in a history log.