Latent Semantic Indexing (LSI):

LSI is since quite recently (mid 2007) a part of Google ’s algorithm for indexing and ranking web pages.

The basic concept is that to evaluate the relevancy of a website for a specific user search phrase Google doesn’t only focus on this one keyword anymore, i.e. whether it appears in the headline , meta tags, anchor texts and with a certain frequency in the text, but also if semantically related words are part of the text.

To judge this factor Google has compiled a LSI database by comparing different documents containing the same keywords and noting what other words are occurring. Documents which have not only the keyphrase but also a lot of other significant words in common are considered as semantically close and relevant. Pages which only contain the search phrase itself but none or very little of the other keywords which appear in Google’s LSI data base as related to this phrase, are considered as less relevant and therefore won’t rank as high.

End 2006 there was a readjustment of website rankings in Google. And without Google giving a clear statement as to the reasons many attributed this to a heavier weighting of LSI compared to other ranking factors.

The consequences for a webmaster would be that optimising a page for a specific keyword would not only require to stuff this keyword in the various page tags, headlines as well as in the content with a certain frequency, but also to make sure that semantically related phrases appear in the content.

Example: You try to optimise a page for the keyword “home theater”. Google’s LSI data base might indicate that most documents containing “home theater” also contain the phrases “HDTV”, “TV”, “cinema”, “resolution”, “Dolby Surround” etc. If your page contains none or only very few of these related terms it will not rank very high for “home theater” no matter how well optimised it is for this term.

[Latent comes from Latin latere = to lie hidden;

Semantic comes from Greek semantikos = significant, from semainein = to show, signify, indicate by a sign, sema = sign;

Index comes from Latin index = forefinger, sign, pointer, list (e.g. of a book’s content); later use as a verb meaning “to compile an index”]

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