Recent estimates support the notion that a relatively small fraction of the information available on the Web is accessible through search engines. Furthermore, many search engines return irrelevant results.
Databases typically disclose their contents only in response to a query.
For databases accessible through the Web this means that their contents are ephemeral parts of the Web only being disclosed through a query. This makes it virtually impossible for search engines to index their contents. Researchers Steve Lawrence and Lee Giles have shown that search engines are not indexing even half of the information available on the Web. Their latest research is published in Nature , Vol. 400, July 1999, pp. While search engines are useful, there are a number of other types of resources that need to be considered when using the Internet for research.