Google Analytics 4 Integration

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TL;DR

Integrate Optimizely Web Experimentation with Google Analytics 4 (GA4) to analyze client-side experiment performance, create audiences from experiment variations, and leverage GA4 reporting capabilities.

Overview

The GA4 + Web Experimentation integration enables you to:

  • Track visitor experiment assignments automatically in GA4

  • Export variations as audiences to GA4 for remarketing and analysis

  • Build custom reports comparing variation performance

  • Segment all GA4 data by experiment and variation

How It Works

The sequence diagram below shows the flow of experiment data from the Optimizely snippet to Google Analytics 4:

sequenceDiagram
    participant Browser as User Browser
    participant Snippet as Optimizely Snippet
    participant GA4 as GA4 gtag.js
    participant Server as GA4 Server

    Browser->>Snippet: Page loads
    Snippet->>Snippet: Activate experiment
    Snippet->>GA4: Set experiment_id dimension
    Note over GA4: exp.EXPERIMENT_ID<br/>VARIANT_ID
    Browser->>GA4: Track page view / events
    GA4->>Server: Send hits with experiment data
    Server->>Server: Segment by experiment
    Note over Server: Reports show<br/>experiment breakdown

The sequence diagram below shows the flow of experiment data from the Optimizely snippet to Google Analytics 4:

sequenceDiagram
    participant Browser as User Browser
    participant Snippet as Optimizely Snippet
    participant GA4 as GA4 gtag.js
    participant Server as GA4 Server

    Browser->>Snippet: Page loads
    Snippet->>Snippet: Activate experiment
    Snippet->>GA4: Set experiment_id dimension
    Note over GA4: exp.EXPERIMENT_ID<br/>VARIANT_ID
    Browser->>GA4: Track page view / events
    GA4->>Server: Send hits with experiment data
    Server->>Server: Segment by experiment
    Note over Server: Reports show<br/>experiment breakdown

Prerequisites

  • An active Optimizely Web Experimentation account

  • A GA4 property with the Google tag configured

  • Optimizely snippet installed on your website

  • Project Owner role in Optimizely

  • Editor role in GA4 property

Method 1: Built-In Integration (Recommended)

The simplest way to integrate - enable in Optimizely settings.

Step 1: Enable GA4 Integration

  1. Log into Optimizely Web Experimentation

  2. Navigate to Settings → Integrations

  3. Find Google Analytics 4 - Report Generation

  4. Toggle the integration ON

  5. Click Accept

Step 2: Add to Experiments

  1. Open an experiment in Optimizely

  2. Go to the Integrations tab

  3. Check the Tracked box next to Google Analytics 4

  4. Click Save

Step 3: Verify Events in GA4

  1. Navigate to your website and enter an experiment

  2. In GA4, go to Reports → Realtime

  3. Within 30 seconds, you should see experience_impression events

Report Generation Feature

Report Generation automatically creates GA4 audiences for each variation, eliminating the need to manually create custom dimensions and segments.

Configure Report Generation

  1. In your experiment, click the Integrations tab

  2. Click Configure Report Generation

  3. Select your GA4 property from the dropdown

  4. Choose which variations to export as audiences

  5. Click Send Variations to GA4

Optimizely will automatically:

  • Create GA4 audiences for each variation

  • Populate audiences with users who saw each variation

  • Update audiences daily as new users enter the experiment

Method 2: Google Tag Manager Integration

For advanced users who need custom logic or conditional tracking.

Create GTM Variables

Create two data layer variables in GTM:

  1. exp_variant_string → Data Layer Variable Name: exp_variant_string

  2. Holdback → Data Layer Variable Name: Holdback

Create GA4 Event Tag

  • Tag Type: Google Analytics: GA4 Event

  • Event Name:experience_impression

  • Event Parameters:

    • exp_variant_string → {{exp_variant_string}}

    • Holdback → {{Holdback}}

  • Trigger: Custom Event → experience_impression

Event Parameters Explained

The experience_impression event includes:

exp_variant_string

Format:[Experiment ID]-[Variation ID]

Example:28475839201-28491038274

Holdback

Values:true or false

  • false: User is IN the experiment

  • true: User is in the holdback group

Common Use Cases

Funnel Analysis by Variation

Compare checkout funnel completion rates between variations:

  1. GA4 → Explore → Funnel exploration

  2. Define steps: experience_impression → begin_checkout → purchase

  3. Breakdown by custom dimension or use segments

Revenue Attribution

Calculate incremental revenue from winning variation:

  1. Use Report Generation to create audiences

  2. GA4 → Monetization → Overview

  3. Apply audience filters to compare revenue

Troubleshooting

Events Not Appearing

Check:

  • Integration enabled in Optimizely Settings

  • Experiment marked as Tracked

  • Experiment is Running (not Paused)

  • GA4 tag loaded on page

  • Visitor qualifies for experiment

GTM Integration Issues

Verify:

  • Variables have correct names (case-sensitive)

  • Trigger listens for experience_impression

  • GTM container is published

  • Event fires in GTM Preview mode

Next Steps

  • Verify events in GA4 Realtime reports

  • Wait 24-48 hours for custom dimensions

  • Create user segments in GA4 Explorations

  • Build custom reports comparing variations

  • Export to BigQuery for advanced analysis

For the complete guide with 20+ code examples, GTM setup details, troubleshooting, and advanced use cases, see web-ga4-article.md