Elinor Mills: The Click fraud Index shows that the overall, industry-wide average click fraud rate is 13.7 percent. The click fraud rate at top-tier search engines such as Google and Yahoo is even less, at 12.1 percent, the data show. The rate rises to 21.3 percent at so-called Tier 2 search providers and 29.8 percent at Tier 3 search companies, according to the Index.

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