graywolf - 6:34 pm on Mar 31, 2005 (gmt 0)
 The dates that links appear can also be used to detect "spam," where owners of documents or their colleagues create links to their own document for the purpose of boosting the score assigned by a search engine. A typical, "legitimate" document attracts back links slowly. A large spike in the quantity of back links may signal a topical phenomenon (e.g., the CDC web site may develop many links quickly after an outbreak, such as SARS), or signal attempts to spam a search engine (to obtain a higher ranking and, thus, better placement in search results) by exchanging links, purchasing links, or gaining links from documents without editorial discretion on making links. Examples of documents that give links without editorial discretion include guest books, referrer logs, and "free for all" pages that let anyone add a link to a document.