Google has been using latent semantic indexing for a while, but recently increased its weighting. Latent Semantic indexing allows a search engine to determine what a page is about outside of specifically matching a search query text. In addition to recording which keywords a document contains, the method examines the document collection as a whole, to see which other documents contain some of those same words. LSI considers documents that have many words in common to semantically close, and ones with few words in common to be semantically distant. By placing additional weight on related words in content LSI has a net effect of lowering the value of pages which only match the specific term and do not back it up with related terms.
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