Just how accurate is the user data being traded by advertisers, agencies and data firms in the $700bn global digital advertising system? The former Chief Privacy Officer of UM in the US, Arielle Garcia, is exasperated - it’s garbage she says and to prove it Garcia recently accessed her profile from an ad tech vendor and found she was in “500 different audience segments across seven different data brokers, and what I saw was just a bunch of contradictory, useless, garbage data.” I.e. she was both a man and a woman, worked in food service, agriculture, a defense contractor, an engineer and was simultaneously below the poverty threshold and classified as high income. “They're selling this garbage data back and forth to one another,” she says, allowing for a host of data “premiums” to be applied by various intermediaries in the process of executing digital advertising campaigns.    “Marketers have been tricked into believing that precision and personalisation equals performance,” she says. "There's so much that's wrong with that, and the inaccuracy of the data is only one piece of that fallacy.” Meanwhile, she says big agency groups have lost their way. “It’s not just principle media models (aka arbitrage) that are problematic, but the fact most of them are incentivised to hit targets by the likes of Google and it distorts the market - “it's literally about their objectivity,” she says.  Agencies are not the only ones. “Google has captured the trade associations … they buy their way into every room.” If trade associations are saying “nothing to see here … of course that’s going to lull marketers into a false sense of security,” she says. Garcia argues Google’s manoeuvres with PMax – it’s AI-powered “just trust us” media placement product for it’s owned media assets like YouTube – “gives Google the ability to opaquely use their black box algorithms to move money wherever they want”, which could prove handy ahead of impending antitrust trials. Meanwhile, AI Overviews in search will “massacre traffic” for publishers. She says publishers must stop forcing people to log in and refocus on quality. For marketers, Garcia urges a “reorientation around people” and “prioritising quality over the illusion of precision.” How? “Demand transparency … there cannot be a market for black box products”, she says. Plus hold agencies accountable. “No one messes with the client that audits.” For everybody else in industry, Garcia has one ask – work out if YouTube is “covertly tracking people.” How? “Let's find out what X-Goog-Visitor-Id is.” Nobody seems to know. But that may change.   See omnystudio.com/listener for privacy information.