Google and Yahoo were sent reeling at a Search Engine Strategies session on click fraud, with search-marketing vendors holding search engines responsible for protecting advertisers, reports AdAge. Unless marketer-conversion data is combined with search-engine data, accurate accountings are not possible, vendors on the panel said. Search vendors - who said fraud accounts for 5-35 percent of clicks, depending on the industry, and cost marketers up to $1 billion a year - charged Yahoo and Google with underestimating fraudulent clicks.

Yahoo and Google have their own tracking and filtering systems and say when they detect fraud they refund money to clients; they also offer advertisers a means of reporting questionable clicks. But one panelist charged that although search engines say they protect their customers, they also say they don't have enough data to do so.

The search engines should also be transparent about the way they collect click data and how they score it, panelists said. Both Google and Yahoo do not explain how they determine whether clicks are fraudulent.