Privacy Sandbox: What Is the Topics API?

Roni Gordon, Senior principal software engineer
In preparation for the deprecation of third-party cookies in Chrome, Google is rolling out a set of APIs called Privacy Sandbox. One of those, the Topics API, proposes to preserve consumer privacy while also allowing marketers to serve relevant ads and publishers to monetize their content. In this video, Roni Gordon, senior principal software engineer at Index, explains how the Topics API works.

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Interest-based advertising with the Topics API

The Privacy Sandbox Topics API replaces the FLoC (Federated Learning of Cohorts) proposal and will reflect a consumer’s interests rather than placing them into interest-based cohorts. Concerns about FLoC’s privacy safeguards and compatibility with GDPR led Google to develop this new proposal. 

With Topics, a consumer’s browsing history is no longer shared across the web, as it is today with third-party cookies. Instead, the browser assigns a consumer’s device to a few interest-based topics based on their recent browsing history. Those categories can then be used to serve an ad that’s relevant to the consumer.

So let’s say, based on your browsing history, Chrome can tell that you seem to really be into music, football, and traveling. Maybe you’re always checking football scores, or you love to daydream by looking at flights to Tokyo.

Chrome then decides which topics to place your device into, and makes those topics available to enable interest-based advertising.

However, no data about you or your specific browsing history is ever presented to outside parties—it all stays in your browser, on your device. This protects consumer privacy while still enabling addressability and relevant advertising.

How the Topics API works

Let’s go step by step to show you how Topics works.          

  1. First, a consumer spends some time online and visits various websites.
  2. Topics assigns each website a contextual label. For example, may have the label “travel,” or may be assigned the “football” label. 
  3. The browser calculates the most frequently visited topics from a consumer’s browsing history over a set period, currently three weeks, and stores them in the consumer’s browser.
  4. When the consumer visits a site that serves ads, that publisher’s SSP, who must already be observing Topics on the site, will receive a selection of the topics for that consumer. The selected topics remain the same for the three-week period, even if the SSP runs another auction for the same consumer on this site. There’s also a small chance a random topic will be included to further protect privacy.
  5. The SSP can then run an ad auction and share the selected topics with DSPs to help them choose a more relevant bid. 
  6. Finally, the winning ad is displayed to the consumer. For reporting attribution, SSPs can share the topics associated with ad impressions with publishers and DSPs

The Topics API taxonomy

Just how many topics are there and how granular do these interests get? 

Currently, the taxonomy includes about 470 topics. Google expects the final number to be between a few hundred and a few thousand.

Topics uses a hierarchical taxonomy: for example, Arts & Entertainment gets more detailed with Music & Audio, then a level deeper with Soul. Chrome’s goal is that the taxonomy becomes a resource maintained by trusted ecosystem contributors. The taxonomy needs to be small enough that many consumers’ browsers will be associated with each topic, so you can’t have a topic that’s so narrow that you could identify somebody with a high degree of certainty.

To further preserve privacy, Topics avoids sensitive categories, such as ethnicity or sexual orientation, and creates groups that are public and human-curated.

Consumers also have full transparency; they’ll be able to see what topics they belong to, remove themselves from them, or opt out entirely.

Implications for addressability in advertising

So, what are the implications of the Topics API? For consumers, this is a much more privacy-centric way to enable relevant advertising compared to third-party cookies and FLoC, which is a win. 

For publishers and marketers, there are still open questions around how performant Topics will be. Starting to test this solution at scale will quickly address any ambiguity.

What’s the timeline, and how can you get started?

Testing is underway, and will continue to ramp up through Q1 2024 as Google removes third-party cookies on 1% of traffic. 

We suggest publishers work with their SSPs to begin testing now. Publishers will also need to update to the latest version of Prebid that natively supports Topics.

We encourage DSPs and marketers to begin testing the Topics API within their ad buys while third-party cookies are still enabled. It’ll be easier to compare existing audience segments to the topics Chrome sends and fully understand the value of the signal and return on ad spend—for example, are consumers who are interested in travel more likely to purchase plane tickets? This feels like a safe assumption, but one that you can prove by ingesting and analyzing this data directly.

Our team is here to help you understand and support the Topics API, and evolve your addressability strategy as we sprint toward a world without third-party cookies. 

Learn more about choosing the right addressability solutions for your business.

Thank you to Josh Prismon and Mike McNeeley, who also contributed to this video. 

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