Earlier this week, Click Forensics stating that 17.1 percent of the clicks on pay-per-click (PPC) advertisements last year were fraudulent. The rate was 28.2 percent for search engine ad providers like Google AdSense and the Yahoo Publisher Network.

But Google puts the number of invalid clicks much lower. In a terse statement attributed only to a "spokesperson," Google says only that "These estimates continue to count clicks Google does not charge to advertisers as fraudulent, so they are not actually click fraud estimates. Furthermore, their estimates have never reflected the invalid click rates we see at Google, which we have previously stated have remained in the range of less than 10% of all clicks every quarter since we launched AdWords in 2002."

Google has criticized Click Forensics' numbers before, and gone into more detail. Two years ago, Google analyst Shuman Ghosemajumder on his private about the issue. "We did an analysis of Click Forensics and other click fraud consultants back in August 2006 to see why their numbers were so inflated ...We found serious flaws in their counting of clicks -- a more fundamental issue than their counting of click fraud."

He also argued that, because of these "basic counting mistakes," Click Forensics was "consistently classifying an advertiser's best users (the ones spending time browsing their site) as fraudulent."

On the other hand, at , Frank Watson skeptical of Google's skepticism. "I've been involved with the and know the people at Click Forensics, and I have to think Google doesn't want to admit to click fraud."