Online advertisers lost hundreds of millions of dollars to click fraud in 2005 - and as a result lost some confidence as well and so reduced search spending with Google, Yahoo and others, according to a study by Outsell Inc., reports the San Francisco Chronicle. According to the survey of advertisers, on average 14.6 percent of all clicks are fraudulent, and 75 percent of advertisers say they have been victims of click fraud at one time or another.

The actual or perceived pervasiveness of click fraud has resulted in many online advertisers (27 percent) either reducing or stopping ad spend on paid-search advertising - to the tune of $500 million in U.S. revenue, according to Outsell. Another 10 percent say they intend to reduce spending.

Outsell found that 7 percent of advertisers request a refund, on average $9,507. Unsolicited refunds were paid to 4.2 percent of advertisers, with $9,444 coming from Google, and $4,068 from Yahoo, on average.