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The industry-average click-fraud rate was 16.25 percent in Q2, down from Q1's 16.3 percent but up from 2Q07's 15.8 percent, according to pay-per-click (PPC) figures from Click Forensics and click-fraud reporting service Click Fraud Index, MarketingCharts reports.

Other key findings from data reported for Q2:

• The average click-fraud rate of PPC advertisements appearing on search engine content networks, such as Google AdSense and the Yahoo Publisher Network and many smaller ones, was 27.6 percent. That's down from the 27.8 percent rate reported for Q1 and up from the 25.6 percent average click fraud rate reported for the year-earlier period.
• In Q2, traffic from botnets was for the first time responsible for more than 25 percent of all click-fraud traffic.
• The greatest percentage of click fraud in Q2 originating from countries outside North America came from China (4.3 percent), Russia (3.5 percent), and France (3.2 percent).

"Although click fraud rates were relatively unchanged in the second quarter, we found that the methods used to commit click fraud have become increasingly more sophisticated and difficult to detect," said Tom Cuthbert, president of Click Forensics.

"The threat from botnets is the biggest concern as they have grown to cause over one quarter of all click fraud. Online advertisers should be extra vigilant in watching for traffic from botnets in their search marketing campaigns."

About the data: The Click Fraud Index publishes data collected from the Click Fraud Network, which includes more than 4,000 online advertisers and agencies. Click fraud data is tracked and published on a quarterly basis for specific search providers, industries and trends.