Success rate is defined by how many search queries with a search engine lead to an actual visit to a website.
Yahoo searches were the most efficient, at 81.36 percent, although Yahoo is now considered part of Bing.
Whilst Yahoo and Microsoft are more efficient, Experian believes all three search engines can improve: "The share of unsuccessful searches highlights the opportunity for both the search engines and marketers to evaluate the search engine results pages to ensure that searchers are finding relevant information."
Experian also notes that Google's share of the search market fell to 66.05 percent while Bing-powered searches (Yahoo + Bing) increased, to 28.05 percent.
For more info, check here: Experian Hitwise reports Google share of searches at 66 percent in July 2011