For instance, stores can make you sign in with your email address before you can use their free Wi-Fi, which lets them tie your MAC Address with your email address. Then, stores can link your in-store activity with your online activity. For instance, if you were browsing for scarves on macys.com, you could, in theory, get a coupon for scarves as

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There are some limits and safeguards on all this data. Technically, a user can reset their assigned advertising ID number (though few people do so—or even know they have one). And users do have some control over what they share, via their app settings. If consumers don’t allow the app they’re using to access GPS, the ad exchange can’t pull the

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