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Its index now provides access to more than 20 billion items comprising 19.2 billion Web documents, 1.6 billion images, and over 50 million audio and video files.

Yahoo! adds that it does not usually disclose the size of its index 'since we've always said that size is only one dimension of the quality of a search engine'. This time around it clearly could not resist the opportunity for trumpet blowing.

Google's homepage states that it is searching a tad over 8 billion Web pages.

But,

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as Yahoo! acknowledges, sheer weight of numbers does not tell the full story. According to a recent report by Dogpile and researchers from the University of Pittsburgh and Penn State University results from four different search engines - Ask Jeeves, Google, MSN and Yahoo! - using the same terms can vary hugely.

Looking at more than 485,000 sets of first page results, the researchers discovered that 84.9 per cent of results are unique to one search engine and only 1.1 per cent appeared on all four.

Although the survey is an obvious marketing tool for Dogpile - a meta search engine which combines results from all the individual engines, and which recently added MSN to its roster - the results still suggest that relying on a single search site will not get you all the results that are relevant to your query.

More information on the survey can be found in this Search Engine Watch article, with links to the complete results.