walkman - 4:13 pm on Oct 12, 2011 (gmt 0)
You cannot measure it, not even truly Google can since many searches are brand new and can't measure say people's reaction. Google can have their own criteria and show G as #1 in quality, Bing can have their own criteria and show Bing #1.
Then SERPs change every half an hour or whatever. The idea probably is that between ads and organic searches users find what they're looking for, more or less.