Understanding GA4 Attribution in BigQuery

Google Analytics 4 (GA4) BigQuery exports include a wide range of traffic source attribution fields. This variety can feel overwhelming when you’re first trying to understand and calculate attribution. You might envision it in a specific way. In this guide, I’ve outlined the key concepts and steps I’ve gathered so far in navigating GA4 attribution. I hope these insights will support analysts and marketers in making more informed decisions. They can also help in building a clearer attribution framework.

First-Click Attribution (User Scoped)

The traffic_source record in GA4 BigQuery captures the first-touch attribution—i.e., the campaign, medium, and source that initially brought the user in. This data does not update with later user interactions and is not available in intraday tables.

• traffic_source.name: Initial campaign name
• traffic_source.medium: Initial traffic medium (e.g., email, organic)
• traffic_source.source: Initial traffic source (e.g., Google, Facebook)

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This is useful for understanding user acquisition origin, not ongoing session behavior.

Last-Click Attribution (Session Scoped)

The session_traffic_source_last_click record in GA4 BigQuery captures the last-click attributed traffic source for a session. This includes detailed campaign data from manual campaigns and various Google marketing platforms (e.g., Google Ads, SA360, DV360, CM360).

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🔹 Manual Campaigns (UTM-based):

Includes fields such as campaign ID/name, medium, source, term, content, platform, creative format, and marketing tactic — based on the last clicked campaign before the session started.

🔹 Google Ads Campaigns:

Tracks Google Ads-specific details like customer ID, account name, campaign and ad group IDs/names.

🔹 Cross-Channel Campaigns:

Captures last-click campaign data across multiple platforms including campaign name/ID, medium, source, and source platform.

🔹 SA360 (Search Ads 360):

Provides campaign, ad group, creative, and engine-level details — including manager and engine account names and types.

🔹 DV360 (Display & Video 360):

Extensive metadata including advertiser, campaign, creative, exchange, insertion order, line item, partner, source, and medium.

🔹 CM360 (Campaign Manager 360):

Includes campaign, creative, placement, site details, and creative attributes like type, version, and cost structure.

This record is essential for performing session-level last-click attribution analysis across both manual and paid campaign contexts. It enables marketers and analysts to understand what campaign interaction immediately preceded a user session. This supports accurate performance tracking and optimization.

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Last-Click Attribution (Event-Scoped)

The collected_traffic_source record in GA4 BigQuery exports contains traffic source data collected with each event. It includes both manual UTM parameters and automatic identifiers:

• Auto Tracking IDs:
  • gclid (Google Ads),
  • dclid (Display & Video 360 / Campaign Manager),
  • srsltid (Google Merchant Center).
• Manual UTM Parameters:
  • utm_id, utm_campaign, utm_source, utm_medium, utm_term, utm_content,
    utm_creative_format, utm_marketing_tactic, utm_source_platform.

https://support.google.com/analytics/answer/7029846?sjid=2838849962983543989-NA

Choosing the Right Fields for Channel Grouping

Most accurate field: session_traffic_source_last_click.cross_channel_campaign (available after Oct 9, 2024) is closest to what the GA4 UI shows, including values like direct and (not set).

Attribution Model in GA4

• GA4 uses a last non-direct click attribution model:
  • Assigns 100% credit to the last non-direct source before conversion.
  • Direct traffic is ignored unless it’s the only source in the path.
• Example: If the final conversion is preceded by Paid Search, Organic, and Direct, credit goes to Paid Search (the last non-direct touchpoint).

Lookback Window

• Default is 30 days, adjustable to 60 or 90 days.
• Impacts how far back an interaction is considered for attribution.
• Ensure your BQ queries align with the configured lookback window in GA4 Admin settings.

Reporting Identity & Session Calculation

• GA4 allows different reporting identity settings: Device-based, Blended, or Observed.
• Session ID generation in BQ should reflect the selected identity:
  • Blended: concat(coalesce(user_id, user_pseudo_id), ga_session_id)
  • Device-based: concat(user_pseudo_id, ga_session_id)
• Consent Mode can affect attribution in the UI due to AI-driven adjustments not visible in BQ.

Known Issues

• Incorrect source/medium when gclid/dclid is present. Bug causes mis-attribution, especially for Google Ads traffic.
• gclid is not present on iOS due to Apple’s AppTrackingTransparency (ATT)
• Attribution can take 1–2 days to stabilize due to data delay
• GA4 in BQ groups null, (not set), and direct under “direct”, leading to overcounting.