In an experiment, the NYT bought and displayed ads targeting 16 profile attributes to reveal what advertisers think they know about users seeing targeted ads (Stuart A. Thompson/New York Times)

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Opinion The Privacy Project These Ads Think They Know You More from The Privacy Project » Illustration by Alexis Beauclair <!– What is this ad? What data are we using? Who else can do this? –>

Nearly every ad you see online is tailored just for you. These digital ads are powered by vast, hidden datasets that allow advertisers to make eerily accurate guesses about who you are, where you’ve been, how you feel and what you might do next.

While targeted ads may be familiar by now, how they work — and the power they have — often seems invisible.

We decided to lift the veil on this part of the internet economy, so we bought some ad space. We picked 16 categories (like registered Democrats or people trying to lose weight) and targeted ads at people in them. But instead of trying to sell cars or prescription drugs, we