[GA4] User-ID limits and caveats

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User-ID is an advanced feature that lets Analytics present a cross-platform, cross-device view of your users' behavior.

In this article:

User-ID values are not assigned by Analytics

Analytics cannot generate, assign, or manage the unique IDs that are assigned as User IDs. In a typical scenario, IDs can be generated through your authentication system, passed to an account when a user signs in, and then sent to Analytics. (You can, however, set up User-ID in different ways and are not limited to associating it with account sign-ins.)

Data collection begins after implementation

User-ID can only collect data after you’ve implemented the feature. Implementation includes generating unique IDs, assigning IDs to returning users, and sending IDs and all related engagement data to Analytics. Any data in your Analytics account collected and recorded prior to implementation cannot be reprocessed and associated with a User ID.

Guidelines and policies apply

The User-ID feature is built for use with Google Analytics technologies. All implementations must comply with the Analytics SDK / User-ID Feature Policy.

Opting-out of Analytics web collection

Website users that don’t want their data reported by Analytics can install the Analytics opt-out browser add-on, which instructs Analytics JavaScript to prohibit sending data to Analytics. This can affect data collection of Analytics technologies, including Google Analytics 4 properties and User-ID.

User-ID data can't be merged across properties

The User-ID feature can be implemented on any Google Analytics 4 property, but data collected in one property cannot be shared or mixed with data in another property.

User-ID processing of pre-login events

When Analytics collects a user ID after the first event in a session, Analytics only associates the initial event(s) within the same session (i.e., an event having the same session_id) with the user ID collected mid-session. There is no option to associate more or fewer events once a user ID is collected. When Analytics stops collecting a user ID mid-session, no further association is made between subsequent events and the user ID that was previously collected--you must continue to collect user IDs to maintain associations with on-going and subsequent events.

Remarketing audiences based on User-ID data are eligible for activation in Google Ads

If you collect User-ID data and share Remarketing audiences, those audiences are eligible to activate in Google Ads. When enabled, Analytics shares all device and user identifiers that are associated with shared audiences; in the case of user IDs, Analytics sends the last device identifier it saw for any given user ID.

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