FLoC network opt-out

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This took me some time to understand as well, and I think I do understand now (but please correct me where I am wrong). The confusion is about “the calculations”.

There are two kinds of calculations here, and there are three ways for a site to “participate” in FLoC.

  1. The “global” algorithm that determines the (global) cohorts. Opt-out impossible. Indeed All sites with publicly routable IP addresses that the user visits when not in incognito mode will be included in the POC cohort calculation.

  2. The algorithm that determines the cohort for a specific user, based on their browsing habits. Opt-out based on header. A site should be able to declare that it does not want to be included in the user’s list of sites for cohort calculation. This can be accomplished via a new interest-cohort permissions policy. (taken from the same document as your quote)

  1. A site requesting the user specific cohort in order to get a targeted advertisement (or to misuse that information) using Javascript. The value is made available to websites via a new JavaScript API:
    cohort = await document.interestCohort();    .
    That API does not work on pages that opted out using the header in #2 and this is where a lot of confusion came from. Any frame that is not allowed interest-cohort permission will have a default value returned when they call document.interestCohort().

I cannot find a good source for this.

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