GA4’s Reporting Identity setting controls how users are identified and counted across devices and sessions. Choosing the wrong setting inflates or deflates your user counts, breaks audience syncing to Google Ads, and produces inconsistent reports. Learn more in our guide on GA4 Predictive Audiences.

The Three Reporting Identity Options

Found in Admin → Reporting Identity: Blended (recommended) uses User ID when available, falls back to Google Signals then device ID. Observed uses User ID then device ID only, no Google Signals. Device-based uses only the device ID (_ga cookie). Learn more in our guide on GA4 Consent Mode v2.

Blended Identity: Google Signals Cross-Device

Blended uses Google Signals to recognize signed-in Google users across devices. This produces lower user counts (10-30% reduction from deduplication), more accurate cross-device journeys, and enables cross-device remarketing audiences. The hidden cost: thresholding hides granular segment data when audience sizes are too small.

Observed Identity: Your Own User ID System

Observed uses your User ID for cross-device matching — no Google Signals. Best when you have a login system, want cross-device tracking based on your own identity, or operate in privacy-sensitive contexts. Anonymous users remain device-only.

Device-Based Identity: Simplest Option

Device-based treats each device as a separate user. Highest user counts, no thresholding, no cross-device insights. Comparable to Universal Analytics behavior. Best for sites where users rarely log in or use multiple devices.

Impact on Audience Syncing to Google Ads

Blended audiences include cross-device users — someone who added to cart on mobile is also targeted on desktop. Observed only cross-device-matches logged-in users. Device-based may include the same user twice (once per device) in remarketing lists, wasting ad spend.

User ID Implementation Requirements

For Observed or Blended to use your User IDs: pass user_id in gtag config or data layer on every page load for logged-in users (not just the login event). User ID must be consistent, must not be PII directly, and must be set before GA4 config fires.

Switching Identity Modes

Switching from Device-based to Blended drops user counts as deduplication kicks in. Historical data is reprocessed. Audience memberships may change. Document current user/session counts before switching and allow 1-3 days for reports to stabilize. For most businesses with a login system, Observed provides the best balance of accuracy and privacy control.