Dogpile has published a paper that concludes searchers are not always finding what they are looking for. The paper, of course, is written to justify Dogpile's meta search model, but the numbers are very interesting.

Source: http://www.infospaceinc.com/onlineprod/Ove...rentResults.pdf

Some takeaways:

• The percent of total results unique to one search engine was established to be 88.3%.
• The percent of total results shared by any two search engines was established to be 8.9%.
• The percent of total results shared by three search engines was established to be 2.2%.
• The percent of total results shared by the top four search engines was established to be 0.6%.

Searching only one Web search engine may impede ability to find what is desired.
• By searching only Google a searcher can miss 72.7% of the Web’s best first page search
results.
• By searching only Yahoo! a searcher can miss 69.2% of the Web’s best first page search
results.
• By searching only Live a searcher can miss 69.9% of the Web’s best first page search
results.
• By searching only Ask a searcher can miss 73.0% of the Web’s best first page search
results.

Majority of all first results page results across top search engines are unique.
• On average, 69.6% of Google first page search results were unique to Google.
• On average, 79.4% of Yahoo! first page search results were unique to Yahoo!
• On average, 80.1% of Live first page search results were unique to Live.
• On average, 75.0% Ask first page search results were unique to Ask.

Search result ranking differs significantly across major search engines.
• Only 3.6% of the #1 ranked non-sponsored search results were the same across all
search engines for a given query, down from 7.0% in the July 2005 overlap study.
• The top four search engines do not agree on all three of the top non-sponsored search
results as no instances of agreement of all of the top three results were measured in the
data.
• More than one-third of the time (38.6%) the top search engines completely disagreed on
the top three non-sponsored search results.
• More than one-fourth of the time (26.1%) the top search engines completely disagreed
on the top five non-sponsored search results.

Yahoo! and Google have a low sponsored link overlap.
• Only 4.6% of Yahoo! and Google sponsored links overlap for a given query.
• For 22.8 % of all queries Google did not return a sponsored link where Yahoo! returned
one or more.
• For 9.9% of all queries Yahoo! did not return a sponsored link where Google returned
one or more.