What else did the study discover?
Clicks believed by advertisers to be fraudulent: 14.6 percent
Money paid by advertisers for bogus clicks: $800 million (2005)
Advertisers who said they were victims of click fraud: 75 percent
Advertisers who said they reduced click-based advertising or plan to: 37 percent
Revenue lost by Google, Yahoo and other Web sites, as a result: $500 million
Advertisers who request refunds because of fraud: 7 percent
Average refund: $9,507
Outsell appears to be a legit market research firm with no hidden agendas on click-fraud (unlike many report publishers) other than to sell you their report.
Hat-tip to Barry.