GA4 User ID and Cross-Device Tracking

Overview

User ID enables cross-device tracking for authenticated users and user properties allow tracking custom user attributes across all events.

When to Use This Skill

Invoke this skill when:

• Implementing User ID for logged-in users
• Setting up cross-device tracking
• Configuring user properties for custom attributes
• Setting Reporting Identity options
• Tracking user lifecycle across devices
• Implementing authentication-based analytics
• Setting user-scoped custom dimensions
• Working with gtag.js user property commands
• Debugging user identification issues
• Implementing cross-domain tracking with User ID
• Analyzing user journeys across multiple devices

Core Capabilities

User ID Implementation

What is User ID:

• Persistent identifier for authenticated users
• Enables cross-device tracking
• Requires explicit user opt-in
• Must not contain PII (hash if needed)

Implementation Methods:

Method 1: gtag.js

// Set User ID when user logs in
gtag('config', 'G-XXXXXXXXXX', {
  'user_id': 'USER_12345'
});

// Or use gtag('set')
gtag('set', 'user_id', 'USER_12345');

Method 2: GTM (Data Layer)

// Push User ID to dataLayer
dataLayer.push({
  'user_id': 'USER_12345'
});

In GTM:

1. Create Data Layer Variable: user_id
2. In GA4 Configuration tag, add parameter:
   • Parameter: user_id
   • Value: {{DL - User ID}}

Method 3: Measurement Protocol

{
  "client_id": "client_123",
  "user_id": "USER_12345",
  "events": [...]
}

Best Practices:

• Set when user logs in
• Clear when user logs out: gtag('set', 'user_id', null)
• Use internal IDs (not emails)
• Hash sensitive IDs
• Document User ID policy

Reporting Identity

Path: Admin → Data Settings → Data Collection → Reporting Identity

Options:

1. Blended (Recommended)

   • Uses User ID when available
   • Falls back to device ID + Google signals
   • Most complete user view

2. Observed

   • Uses User ID and device ID only
   • No Google signals
   • More privacy-focused

3. Device-based

   • Uses only device ID (client_id)
   • No cross-device tracking
   • Most restrictive

Recommendation: Use "Blended" for most comprehensive tracking

User Properties

What are User Properties:

• Custom attributes set at user level
• Persist across all events
• Max 25 properties per property
• Property names ≤24 characters
• Values ≤36 characters

Setting User Properties:

gtag.js:

gtag('set', 'user_properties', {
  'user_tier': 'premium',
  'account_age_days': 365,
  'preferred_category': 'electronics'
});

GTM (Data Layer):

dataLayer.push({
  'user_properties': {
    'user_tier': 'premium',
    'account_age_days': 365
  }
});

Measurement Protocol:

{
  "client_id": "client_123",
  "user_properties": {
    "user_tier": {
      "value": "premium"
    },
    "account_age_days": {
      "value": 365
    }
  },
  "events": [...]
}

Common User Properties:

• user_tier: "free", "premium", "enterprise"
• signup_date: "2024-01-15"
• subscription_status: "active", "trial", "cancelled"
• customer_ltv: Lifetime value bucket
• industry: User's industry
• company_size: Employee count range
• interests: Comma-separated interests

Avoid PII:

• ❌ email, name, phone_number, address
• ✅ email_domain, first_name_initial, user_segment

Custom Dimensions for Users

Creating User-Scoped Custom Dimension:

1. Admin → Custom Definitions → Custom Dimensions
2. Click Create custom dimension
3. Dimension name: User Tier
4. Scope: User
5. User property: user_tier
6. Save

Using in Reports:

• Add as dimension in Explorations
• Filter/segment by custom dimension
• Analyze user cohorts

Cross-Device Tracking

Requirements:

• User ID implemented
• Google signals enabled (for Blended identity)
• Users opt in to personalization

How it Works:

1. User visits on mobile (logged in) → user_id set
2. User visits on desktop (logged in) → same user_id
3. GA4 stitches sessions together
4. Reports show unified user journey

Verifying Cross-Device:

1. Enable DebugView
2. Log in on Device 1 with debug_mode=true
3. Note user_id parameter in events
4. Log in on Device 2 with same account
5. Verify same user_id in events
6. Check reports for unified sessions

Cross-Domain Tracking

Setup for User ID Across Domains:

gtag.js Configuration:

gtag('config', 'G-XXXXXXXXXX', {
  'linker': {
    'domains': ['example.com', 'shop.example.com', 'blog.example.com']
  }
});

What This Does:

• Passes client_id between domains via URL parameter
• Preserves user_id if set
• Maintains session continuity

GTM Setup:

1. In GA4 Configuration tag
2. Expand Configuration Settings
3. Add parameter:
   • linker: {"domains": ["example.com", "shop.example.com"]}

Testing:

1. Visit domain1.com
2. Click link to domain2.com
3. Verify _gl parameter in URL
4. Check session continues in GA4

User Data Deletion

Deleting User Data by User ID:

Path: Admin → Data Deletion Requests

Process:

1. Click Create deletion request
2. Select parameter: User ID
3. Enter User ID to delete
4. Choose deletion scope (all data or date range)
5. Submit request

Processing:

• Takes ~72 hours to complete
• Deletes all events for that User ID
• Cannot be undone

Use Cases:

• GDPR/CCPA compliance requests
• User account deletion
• Data cleanup

Testing User ID

Workflow:

1. Implement User ID (gtag or GTM)
2. Enable DebugView
3. Test Login:
   • Before login: Check events have only client_id
   • After login: Verify user_id appears in event details
4. Test Logout:
   • After logout: Verify user_id cleared/null
5. Test Cross-Device:
   • Login on Device 1
   • Login on Device 2 with same account
   • Verify same user_id

DebugView Verification:

• Click any event
• Expand event details
• Check "User ID" field populated

Reports Verification:

• Wait 24-48 hours
• Admin → DebugView or Realtime
• Check user count vs sessions
• Lower user count = User ID working (same user, multiple sessions)

Integration with Other Skills

• ga4-setup - Initial property setup for User ID
• ga4-gtag-implementation - Implementing User ID via gtag.js
• ga4-gtm-integration - Implementing User ID via GTM
• ga4-custom-dimensions - Creating user-scoped dimensions
• ga4-audiences - Building audiences based on User ID
• ga4-privacy-compliance - User data deletion and compliance
• ga4-debugview - Testing User ID implementation

References

• references/user-id-implementation.md - Complete User ID setup guide
• references/user-properties-guide.md - User properties reference and examples
• references/cross-device-tracking.md - Cross-device implementation and testing
• references/reporting-identity.md - Reporting identity options explained

Quick Reference

Set User ID (gtag.js):

gtag('config', 'G-XXXXXXXXXX', {'user_id': 'USER_123'});

Set User Properties (gtag.js):

gtag('set', 'user_properties', {
  'user_tier': 'premium'
});

Clear User ID (gtag.js):

gtag('set', 'user_id', null);

Limits:

• 25 user properties per property
• User property name ≤24 characters
• User property value ≤36 characters
• User ID must not contain PII