From the survey release:
The experiment was conducted in conjunction with Los Angeles-based Clicks2Customers.com and focused on three pay-per-click (PPC) campaigns running during a 10-day period in 2005. Duplicate clicks were determined by comparing IP addresses, language, browser settings, referring URL, time of click, operating system, browser plug-ins and country of origin.
“Our random sample of PPC campaigns uncovered as much as 29.5 percent PPC fraud and showed that Google was able to account for and credit only a tiny portion of those fraudulent charges,” McGlaughlin said. “Whether it is click fraud or the lesser known impression fraud, these fraudulent clicks can cause a lot of damage to advertisers because it drains their budgets. Companies should be aware of how big of a problem it really is and be equipped to more aptly detect it.”