  Regulators and browsers increasingly restrict user tracking to protect users'
privacy online. In two large-scale empirical studies, we study the economic
implications for publishers relying on selling advertising space to finance
their content. In our first study, we draw on 42 million ad impressions from
111 publishers covering EU desktop browsing traffic in 2016. In our second
study, we use 218 million ad impressions from 10,526 publishers (i.e., apps)
covering EU and US mobile in-app browsing traffic in 2023. The two studies
differ in the share of trackable users (Study 1: 85%; Study 2: Apple: 17%,
Android: 91%). Still, we find similar average ad impression price decreases
(Study 1: 18% and Study 2: 23%) when user tracking is unavailable. More than
90% of the publishers realize lower prices when selling ad impressions for
untrackable users. Publishers offering content on sports, cars, lifestyle &
shopping, and news & information suffer the most. Premium publishers with
high-quality edited content and strong reputations, thematic-focused (niche)
publishers, and smaller publishers suffer less from the unavailability of user
tracking. In contrast, non-premium publishers with non-edited or user-generated
content, thematic-broad (general news) publishers, and larger publishers suffer
more. The availability of a user ID generates the highest value for publishers,
whereas collecting a user's browsing history, perceived as intrusive by most
users, generates only a small value for publishers. These results affirm that
ensuring user privacy online has substantial costs for online publishers, but
those costs differ across publishers and the type of collected data. This
article offers suggestions to reduce these costs.
