There are many unknowns that make measuring click fraud hard, the researchers wrote. Ad networks do not know the false negative rate of their detection systems, or when they fail to detect a fraudulent click, which results in an underestimation of click fraud. Third-party analytics companies do not allow their systems to be scrutinized, which causes ad networks to claim they overestimate click fraud, according to the paper.
The researchers said they found "incontrovertible evidence of dubious behavior for around half of the search ad clicks and a third of the mobile ad clicks." Overall, around 22 percent of clicks on ads were fraudulent, Livic wrote.
Google and Facebook have periodically faced accusations that click fraud is more prevalent on their networks than the companies admit. Google says that less than 10 percent of clicks on AdWords, its search-engine based advertising product.
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