Cross-device attribution helps you understand user behavior when the same person interacts across mobile, desktop, and other platforms. By implementing User ID tracking, you create a unified view of the customer journey—leading to better personalization, attribution accuracy, and conversion analysis.

🎯 Why Use User ID?

• ✅ Recognize users across devices and browsers
• ✅ Combine anonymous and authenticated sessions
• ✅ Improve conversion funnel visibility
• ✅ Reduce inflated user counts
• ✅ Enable richer audience segmentation

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✅ Prerequisites

• GA4 property created
• Web GTM and Server-Side GTM containers
• Your platform supports user authentication (login)
• Server-side endpoint or backend access to enrich user ID
• Optionally: Consent Mode compliance

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🔧 Step-by-Step Guide

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🔹 Step 1: Make User ID Available on Login/Authenticated Pages

Expose the authenticated user ID from your server into the frontend via JavaScript or dataLayer.

Example (dataLayer push):

<script>
dataLayer.push({
  event: 'user_authenticated',
  user_id: 'USER_12345'
});
</script>

👉 Ideally fire this on all pages where the user is logged in (post-login, dashboard, checkout, etc.)

🔹 Step 2: Create a User ID Variable in Web GTM

1. Go to Variables > New
2. Type: Data Layer Variable
3. Name: user_id
4. Data Layer Variable Name: user_id
5. Save as: DLV - user_id

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🔹 Step 3: Pass User ID in GA4 Configuration Tag

Modify your GA4 Configuration Tag to include the user_id.

gtag('config', 'G-XXXXXXX', {
  user_id: 'USER_12345'
});

In GTM:

• Edit GA4 Config Tag
• Under Fields to Set:
  • Field Name: user_id
  • Value: {{DLV - user_id}}

✅ This automatically adds user_id to all GA4 events.

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🔹 Step 4: Forward User ID to Server-Side GTM

If you’re using Server-Side GTM, your GA4 Config tag should point to:

https://gtm.yourdomain.com

GA4 event hits sent to this endpoint will now include user_id.

🔹 Step 5: Capture User ID in Server-Side GTM

In ssGTM, extract user_id from GA4 Client request:

Create a Variable:

• Variable Type: Event Data
• Variable Name: user_id

return event.data.user_id || request.body?.events?.[0]?.user_id;

🔹 Step 6: Forward to GA4 (Server-Side Event Tag)

In ssGTM:

• Create a GA4 Event Tag
• Set user_id as a User Property

User Properties:

Property Name	Value
user_id	{{Event Data - user_id}}

✅ Also attach to custom events like purchase, login, lead_submit, etc.

🔹 Step 7: (Optional) Sync User ID with CRM via HTTP Request

To connect GA4 activity to your CRM, you can send the user_id with event context:

Example HTTP POST Tag in ssGTM:

POST /user-event-sync
{
  "user_id": "USER_12345",
  "event": "purchase",
  "transaction_id": "ORD789",
  "timestamp": "2024-05-30T08:01:00Z"
}

✅ Use it to push activity into Salesforce, HubSpot, or your own backend.

🔹 Step 8: GA4 Reporting with User ID

Go to GA4 Admin > Reporting Identity
Set Blended or Observed (GA will combine User ID + Device ID)

Once set:

• Go to User Explorer Report
• You’ll see User ID-based journeys
• Filter by segments like “logged-in users” or “multi-session buyers”

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🔹 Step 9: Ensure Consent Compliance

If you use Consent Mode, make sure user_id is only sent after consent.

gtag('consent', 'update', {
ad_storage: 'granted',
analytics_storage: 'granted'
});

Only push user_id to dataLayer after consent granted.

🔹 Step 10: Debug and QA

• Use GA4 DebugView to verify user_id
• Use Chrome DevTools > Network > collect? to inspect payload
• Use Server GTM Preview Mode to inspect forwarded user_id
• Enable Realtime Reports > filter by device & user ID

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🔒 Tips for Security and Privacy

• Never expose PII (email, name, phone) in GTM or GA4
• Use hashed values if needed (SHA256)
• Sanitize and validate user ID before sending to GA
• Apply Consent Mode & Privacy Policy disclosures

✅ Summary Table

Step	Action
1	Add user_id to dataLayer
2	Create GTM variable
3	Add to GA4 Config tag
4	Route via Server GTM
5	Extract in ssGTM
6	Send as user property to GA4
7	(Optional) Sync with CRM
8	Enable GA4 Reporting Identity
9	Respect consent mode
10	Debug thoroughly