“…Despite widespread concerns about online privacy and attention on the online advertising industry, there is one specific aspect of this ecosystem that remains opaque: what information is actually contained in advertising interest profiles, and are these profiles accurate? The extant literature presents a very complete picture of who is tracking users' online [7], [27], [9], [86], [71], [70], as well as how tracking is implemented (e.g., fingerprinting) [37], [78], [5], [57], [60], [72], [58], [1], [40], [28], but not necessarily the what. Controlled studies have shown that advertising platforms like Google do indeed draw inferences about users from tracking data [89], [20], [46], [47], but this does not address the broader question of what platforms actually know about users in practice.…”