Attribution Scope

GA4 has three distinct attribution scopes. Using the wrong scope gives misleading results. Choose based on the question you are answering.

The three scopes

Dimensions by scope

Session scope — use for traffic analysis:

• sessionDefaultChannelGroup
• sessionSource
• sessionMedium
• sessionCampaignName
• sessionSourceMedium

First-user scope — use for acquisition analysis:

• firstUserDefaultChannelGroup
• firstUserSource
• firstUserMedium
• firstUserCampaignName

Event/attribution scope — use only in advertising/conversion contexts:

• defaultChannelGroup (no prefix)
• source (no prefix)
• medium (no prefix)

How to choose

"Where is my traffic coming from?" → session scope

dimensions: ["sessionDefaultChannelGroup", "sessionSource", "sessionMedium"]
metrics: ["sessions", "totalUsers", "engagedSessions", "keyEvents"]

"Which channels are bringing in new users?" → first-user scope

dimensions: ["firstUserDefaultChannelGroup", "firstUserSource"]
metrics: ["newUsers", "totalUsers"]

"How should I attribute conversion credit across channels?" → event scope (Use only with advertising campaign analysis; this is AI-weighted attribution.)

Common mistake

A user acquired via Instagram who later converts via Google Organic will show:

• sessionDefaultChannelGroup = "Organic Search" (where this session came from)
• firstUserDefaultChannelGroup = "Organic Social" (where they were first acquired)

These are different questions. Mixing scopes in one query produces meaningless results. Never combine sessionSource with firstUserMedium in the same report.

Nested filters

When filtering, use the same scope as your dimensions:

// Filtering traffic acquisition report — use session-scoped dimension
{"filter": {"fieldName": "sessionDefaultChannelGroup", "stringFilter": {"value": "Organic Search", "matchType": "EXACT"}}}

// NOT source (attribution scope) — wrong scope for this report
{"filter": {"fieldName": "source", "stringFilter": {"value": "google", "matchType": "EXACT"}}}