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We have many discussions in the past in our Keyword forum.
It is something that is very difficult to figure out because Overture's data isn't very accurate.
You can find many of the past posts here [searchengineworld.com].
As a rule of thumb, I usually take 40% of the Overture and base my analysis on that. For example, if Overture says that xyz product is searched 5,000 times in October, I would base my research on 2,000 searches.
I also cross reference a couple different programs if I want to get a more accurate estimate.
BrAsS_mOnKeY
Meanwhile I have had a look at Jacob Nielsen's figures on Useit where he lists the origins of searches to his site in March for each of the last 5 years. The most recent figures (March this year) indicate that the Overture partner sites represent 27% of all searches (to his site), Google 66% and the rest the other 6%. As a rough guide this seems to work, unless it is seriously skewed. Any comments?
(Hope this isn't old hat!)