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They clearly have an estimate function
go to view/edit
then to Modify Price or Keywords
there you will find "Traffic Estimator"
that said, the numbers don't always pan out
I was pondering a big difference recently and came up with a rationalization that I pass on, for what it's worth. Aside from the difference in demographics between the two engines, and aside from the automated searching software that some people use (which can inflate the Overture figures), there's a difference in what the tools are reporting.
The Overture suggestion tool is reporting searches. Google's tool estimates AdWords clicks... so it's only partially popularity that's being reported. It may be that AdWords results are used more when Google (commercial) results aren't satisfactory to the user. This was consistent with the anomaly I was wondering about. Relatively many more Google AdWords clicks than what Overture results would suggest... in a search area that was muddied by the popularity of the term in several different fields.