The study, which analysed 5.5 billion ad impressions, found that one in 10 display ads were viewed by bots rather than real people.
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Ford, Pfizer and Nestlé were among the brands that took part in the investigation, which provides the clearest sign yet that fraud is rife in the fast-growing online advertising market.
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To take advantage of marketers, fraudsters create software that mimics the cursor movements and mouse clicks of humans, giving the impression that a person is visiting a website.
After installing the software on their own servers or on a hijacked computer, the fraudsters are able to direct the bots to visit certain websites. This allows them to profit from selling ads on their own sites or by driving fake traffic to a site of an affiliate.
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"A bot can look like a sports fan, someone with a six-figure income, someone interested in buying a car, or a grandparent looking for holiday gifts for grandchildren," the report said.
White Ops and the ANA worked with 36 of the association's member organisations to analyse digital advertising campaign traffic between August 1 and September 30.
The researchers inserted special software into the ad units of the advertisers and then applied proprietary analysis techniques, alongside data from third parties, to identify bots. Marketers rarely conduct such a detailed level of forensic analysis of their online advertising campaigns.
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The ANA estimated that advertisers will lose approximately $6.3 billion globally to bots in 2015, after applying the bot levels observed across its study to the estimated $40 billion spent globally on display ads and the estimated $8.3 billion spent globally on video ads.