Ultimate Guide to Federated Learning of Cohorts (FLoC) in 2024

As third-party cookies begin their departure from the digital advertising landscape, Google has put forth a potential replacement tracking technology called Federated Learning of Cohorts (FLoC). FLoC aims to preserve targeted advertising capabilities while also enhancing user privacy.

But how exactly does FLoC work, and what does its emergence mean for the future of digital marketing? As an industry expert with over 10 years of experience in web data extraction and analysis, I‘ll explore everything you need to know about FLoC in this comprehensive 4000+ word guide.

What is Federated Learning of Cohorts (FLoC)?

FLoC is a new approach to interest-based ad targeting that relies on federated learning principles. Federated learning allows machine learning models to be trained locally on user devices, without direct access to raw user data.

With FLoC, Google Chrome will use on-device processing to analyze a user‘s browsing history and create interest groups or "cohorts" of at least thousands of people with similar behavioral patterns. These cohorts, not individual users, are then made available to websites for ad targeting purposes.

So in essence, FLoC aims to deliver customized ad experiences based on interests, while limiting individual user tracking and keeping data processing on the user‘s device.

How is FLoC different from third-party cookies?

Third-party cookies have enabled advertisers to target ads and personalize experiences for individual users based on their browsing history across websites. However, third-party cookies directly expose the identities and interests of users in order to function.

In contrast, FLoC relies on federated learning to develop machine learning models locally on user devices. This means user data does not need to be directly collected and exposed to third parties. Only the high-level cohort IDs are shared for ad targeting, not granular individual interests.

So while third-party cookies power hyper-personalized ads, they lack protections for user privacy that emerging techniques like FLoC aim to provide. However, critics argue FLoC still enables tracking against user wishes.

How Does FLoC Work?

Now that we‘ve covered the high-level concepts, let‘s dig deeper into exactly how the FLoC algorithm functions:

The FLoC workflow involves four key steps:

  1. Analyzes browsing history: The FLoC algorithm studies website domains a user has visited over the past week to identify interests and patterns. No personal data leaves the user‘s browser during this on-device processing.

  2. Generates cohorts: Based on the locally analyzed browsing data, the algorithm groups users into cohorts of at least thousands of people who have exhibited similar interests or browsing histories.

  3. Assigns cohort IDs: Each cohort receives an anonymous ID number, which is refreshed by the algorithm each week to keep up with evolving user interests.

  4. Communicates cohort ID: When a user visits a website, the Chrome browser passes along the current cohort ID attached to that user.

  5. Targets ads: The website receives the user‘s cohort ID and can then display ads relevant to the entire group‘s collective interests.

Importantly, neither websites nor advertisers are able to access the granular browsing history data that was used to generate the cohorts. They only receive the high-level, anonymous cohort IDs.

FLoC process diagram

Diagram illustrating how the FLoC workflow functions

So in summary, the key privacy-preserving mechanisms of FLoC include:

  • On-device processing that keeps data localized
  • Anonymization via large cohorts rather than individual data
  • Limited data exposure beyond cohort IDs

However, some experts have questioned whether cohorts could be de-anonymized using fingerprinting techniques, reducing the privacy protections.

FLoC and Privacy

Because FLoC relies on grouping thousands of similar users rather than tracking individuals, Google claims it offers far greater privacy benefits compared to third-party cookies and other tracking techniques.

Some of the key privacy aspects Google highlights include:

  • No personal identifiers: Individual browsing history and interests are never directly exposed to third parties. Websites receive only the cohort IDs.

  • Data stays on-device: Browsing history processing happens locally via federated learning, rather than sending data to external servers.

  • Large cohort size: Minimum cohort size is thousands of users, limiting the granularity for ad targeting and obscuring individuals.

  • User control: Chrome users can opt-out of FLoC participation entirely if they do not wish to share even cohort-level data.

However, a number of privacy-focused organizations have raised concerns about potential linkage attacks that could connect cohort IDs with individual identity. They argue that the cohorts still share behavioral data without user consent.

Some of the top concerns shared by privacy advocates include:

  • Fingerprinting risks: While anonymous, cohorts may still be small enough to identify individuals using fingerprinting techniques, reducing anonymity.

  • Consent issues: Users never directly opt-in to share their browsing data, reducing control.

  • Persistent tracking: FLoC enables persistent monitoring of user interests week after week, rather than one-time data sharing.

  • Data minimization: The breadth of browsing history analyzed by FLoC may go beyond what‘s necessary for ad relevancy.

So while FLoC does incorporate data protection mechanisms, privacy advocates argue it still enables tracking against an individual‘s wishes and falls short of ideals like data minimization. There are open questions about whether cohorts are strictly anonymous in practice.

The State of FLoC Today

Google first announced a prototype for FLoC back in 2021 as a potential successor to replace third-party cookies. After initial lab testing, Google decided to move forward with small-scale trials of FLoC through what it calls "origin trials" in Chrome.

These origin trials allow new web platform features like FLoC to be tested safely with a sample of Chrome users and websites. As of late 2022, FLoC remains enabled only for these limited origin trials, not the general public.

Here are some key facts about the current status of FLoC‘s rollout:

  • Limited to trials: FLoC is only active in Chrome for a very small percentage of users who have been selected for the origin trials.

  • Cohort analysis: In these small-scale trials, Google says over 33,000 FLoC cohorts have been analyzed and tested thus far.

  • Select trial regions: Testing is limited to certain countries, excluding the European Union over GDPR compliance concerns. The US, Canada, Brazil, India, and parts of Southeast Asia have been included.

  • Future remains uncertain: Google has shared no definitive timeline for when FLoC may leave testing and have an official, widespread launch.

So in summary, FLoC remains in an early, wait-and-see experimental phase as of late 2022. Its future is still uncertain, as privacy advocates, regulators, and the broader tech ecosystem continue to debate FLoC‘s merits and drawbacks.

Industry Interest and Concerns Around FLoC

As a major tracking technology change proposed by Google, the FLoC announcement has generated substantial interest across the digital marketing and advertising industries.

Looking at Google search trends, interest in "Federated Learning of Cohorts" spiked worldwide in 2021 as news of FLoC first emerged, and has remained high since:

FLoC Google search interest graph

Google search interest volume for "Federated Learning of Cohorts" over time, via Google Trends

This sustained interest reflects how pivotal of a change FLoC could represent for digital advertising targeting approaches. However, along with the intrigue, some substantial concerns have also been voiced by industry stakeholders:

  • Privacy: As covered earlier, privacy groups argue cohorts still enable tracking of users without transparent consent.

  • Consolidated power: Some in the ad industry contend FLoC hands even more control to Google, since Chrome would mediate targeting and data flows.

  • Ad performance: Unclear if cohort-based targeting can match the performance and relevance of individualized tracking.

  • Walled gardens: FLoC may further strengthen walled gardens like Google and Facebook while disadvantaging other players.

  • Regulatory compliance: Can cohort data processing comply with regulations like GDPR that apply outside the US?

So while interest is high, the broader ad tech ecosystem is still weighing the pros and cons of a FLoC-driven future. In particular, privacy implications and the increased dominance of Google‘s ad stack are top concerns.

FLoC and Data Regulations

A crucial area of debate is whether FLoC‘s data processing approach properly aligns with key global data privacy laws, such as the European Union‘s General Data Protection Regulation (GDPR).

GDPR sets strict standards for collecting, handling, and sharing personal data of EU citizens and residents. Some considerations around FLoC‘s regulatory alignment:

  • Local processing: Data stays on-device, which matches GDPR‘s data minimization principles.

  • Anonymization: Cohorts contain thousands of users, avoiding directly identifiable personal data at first glance.

  • Opt-out: Users can disable FLoC in Chrome, providing an opt-out mechanism as GDPR mandates.

  • Data protection: But are cohorts still considered identifiable personal data under GDPR definitions? This remains ambiguous.

  • Legal basis: Can Google establish legitimate interest as a lawful basis for processing under GDPR Article 6? Unclear.

  • Minor protections: GDPR requires extra protections for children‘s data which may not be addressed by FLoC.

So at first pass, FLoC does appear to incorporate data protection mechanisms that partially align with GDPR. However, upon deeper analysis, its full compliance with GDPR‘s strict requirements around personal data handling remain doubtful.

For example, top EU data protection authorities contend that FLoC cohorts likely qualify as personal data that requires explicit opt-in consent under GDPR. This ambiguity around GDPR alignment is why Google has excluded EU countries from FLoC testing thus far. Without compliance confidence, launching FLoC in the EU could lead to substantial legal and regulatory risk.

The Future of FLoC in 2024 and Beyond

Because it remains in early limited testing, the future path and timeline for FLoC becoming mainstream on the public web is still highly speculative. Here I will outline a few potential scenarios that could play out in 2024 and subsequent years:

  • Broader rollout – If current FLoC trials generate positive results, Google may gradually expand it to a wider portion of Chrome browsers globally. However, privacy concerns and murky regulatory compliance could hinder rapid mainstream adoption.

  • Competitive response – If FLoC moves forward, other browser vendors like Apple, Mozilla, and Microsoft may feel pressure to follow suit with their own implementations of cohort-based tracking. Alternatively, they could resist joining FLoC as a differentiator.

  • Improved cohorts – Over time, advances in on-device federated learning could enable Google to create more nuanced cohorts by analyzing richer behavioral signals. This could lead to interest groups that are smaller and more personalized.

  • Integration with IDs – To enhance performance, cohorts could be combined with authenticated user IDs and Android Advertising IDs to enable cross-device targeting and attribution. However, this would conflict with Google‘s privacy claims.

  • Legal challenges – If scaled up, FLoC may face regulatory lawsuits from EU authorities related to GDPR violations, forcing Google to halt rollout or make changes.

  • User blocking expands – As more users become aware, many may proactively block FLoC tracking by using anti-tracking browser extensions or disabling Chrome‘s Privacy Sandbox. Widespread blocking could limit FLoC‘s viability.

Overall, I anticipate FLoC will continue facing an uncertain, uphill battle in 2024. But with the pressure to adapt a post-cookie tracking solution, Google will likely continue pushing forward with FLoC in some capacity, even if adoption is rocky.

Recommendations for Marketers and Publishers

For digital marketers, advertisers, and publishers, it is prudent to begin actively investigating and preparing for solutions like FLoC, regardless of the unclear timelines. Here are my top recommendations based on over a decade in web data analytics:

  • Closely monitor developments – Keep up-to-date on the latest FLoC announcements, trial results, and privacy concerns through news and industry forums. Be aware of changes.

  • Evaluate tech integrations – Assess how FLoC could integrate with your existing ad tech stack and analytics tools. Test integrations as possible to identify impacts.

  • Run controlled experiments – If you gain access to FLoC trials, run A/B tests to compare FLoC cohorts vs. cookie-based targeting for your campaigns.

  • Analyze FLoC alternatives – Research other emerging privacy-centric targeting techniques that go beyond FLoC, such as UID 2.0 and MAC addresses.

  • Prioritize first-party data – Reduce dependence on third-party data by improving your first-party data pipelines, identity resolution, and data science capabilities.

  • Enhance measurement – Explore alternate attribution and measurement approaches beyond cookies, such as data clean rooms and contextual methods.

While the post third-party cookie landscape will require adaptation, proactively preparing for changes like FLoC will help you stay ahead of the curve and build future-proof strategies.


Third-party cookies are fading into obscurity, and cohort-based tracking solutions like FLoC aim to fill the void. However, given the privacy concerns and regulatory uncertainty, FLoC still faces a long road ahead.

As an industry expert, I recommend digital marketers monitor FLoC developments closely, run robust experiments, evaluate alternative targeting tactics, and focus on fortifying first-party data strategies.

Though the exact future is unclear, staying vigilant and forward-thinking will enable your organization to navigate the cookieless advertising revolution smoothly and successfully.