Reporting identity

GA4 can recognise a person in several ways, called identity spaces:

• User-ID: an ID you send (user_id in the config or event) when someone is logged in. It is the only space that joins the same person across browsers and devices.
• Device ID: the _ga cookie's client ID on the web, or the app-instance ID on mobile. The default when nothing else is available.
• Modelled data: when users decline cookies under consent mode, GA4 can model their behaviour from consented users.

Until February 2024 Google signals (data from signed-in Google users who allow ads personalisation) was also part of identity. It no longer is: Google signals is still used for demographics, interests and remarketing audiences, but it does not stitch users for reporting any more.

In Admin > Reporting identity you pick how these spaces are combined:

• Blended: User-ID, then device ID, then modelled data. Default.
• Observed: User-ID, then device ID. No modelling.
• Device-based: device ID only; ignores User-ID.

The choice affects reports only, not collection, so you can switch back and forth: nothing is lost. Why switch? Because any identity space that could reveal individuals makes GA4 apply data thresholding (hiding small rows). If you see the thresholding warning icon in a report and need the full rows, temporarily switch to Device-based.

User-ID must not be personally identifiable (no emails), must be the same string everywhere, and user counts fall when you add it because cross-device duplicates collapse. That drop is correct, not a bug.