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Google Scholar (GS) is a freely-accessible web search engine that indexes the full-text of scholarly literature across an array of publishing formats and disciplines. Released in beta in November 2004, the GS index includes most peer-reviewed online journals the world's largest scientific publisher. It is similar in function to the freely available Scirus from Elsevier, CiteSeer, and getCITED. It is also similar to the subscription-based tools, Elsevier's Scopus and Thomson ISI's Web of Science. GS nonetheless claims to cover more websites, journal sources and languages. Its advertising slogan - "Stand on the shoulders of giants" - is a nod to the scholars who have contributed to their fields over the centuries, providing the foundation for new intellectual achievements.
In terms of features, GS allows users to search for digital or physical copies of articles, whether they be online or in libraries .
Using its "group of" feature, it shows the various available links to the journal article. In the 2005 version, this feature provided a link both to subscription-access versions of the article and to free full text versions of articles; for most of 2006, it provided links to only the official versions. At present, in December 2006, it provides access to both published versions and on major open access repositories, but does still not cover individual university pages; access to such self-archived non-subscription versions is now provided by a link to Google, where one can find such open access articles.)
Through its "cited by" feature, GS provides access to citations of articles that have cited the article being viewed (see ). It is this feature in particular that provides the citation indexing previously only found in Scopus and Web of Science. Through its "Related articles" feature, GS presents a list of closely related articles, ranked primarily by how similar these articles are to the original result, but also taking into account the relevance of each paper. 
In 2006, in response to release of Microsoft's Windows Live Academic Search, a potential competitor for GS, a citation importing feature was implemented using bibliography managers (such as RefWorks, RefMan, EndNote, and BibTeX). Similar features are also part of other search engines, such as CiteSeer and Scirus, and will presumably be part of the final version of Microsoft's Windows Live Academic Search.
Some searchers consider GS of comparable quality and utility to commercial databases (e.g. ), even though its user-interface (UI) is still in beta. The reviews recognize that its "cited by" feature in particular poses serious competition to Scopus and Web of Knowledge. Many search experts suggest that its functionality is severely hampered by poor database design. For example, when searching articles based on publication dates, GS results, like Google results, are unreliable, even inaccurate. The number of articles found in some searches, for example, increases when limiting to a range of years (i.e. 2000-2006) instead of decreases. Some librarian critics have said that GS's counterintuitive and illogical presentation of results hinders its usefulness in academia.
A significant problem with GS is the secrecy about its coverage. Some publishers do not allow it to crawl their journals (as of December 2006 the absentees notably include Elsevier); it refuses to publish a list of scientific journals crawled; its frequency of updates is unknown. It is therefore impossible to know how current and/or exhaustive searches are in GS. Nonetheless, it allows easy access to published articles without the difficulties that are encountered in some of the most expensive commercial databases.
 See also
- Citation index
- Institute for Scientific Information's Web of Science
 External links
- Google Scholar website
- Google Scholar: The New Generation of Citation Indexes. Libri 55(4): 170-180.
- Article on citation analysis of an individual based on multiple citation indexes (March, 2006)
- Google Scholar Versus Metasearch Systems (March, 2006).
- Nature's news piece on 24 November 2004
- Nature's news piece on 1 December 2005
- Critical review by Peter Jacso, librarian, at his digital reference shelf. (Nov., 2004)
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