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
| Scope | Dimension prefix | Question it answers | When to use |
|---|---|---|---|
| Session | session |
Where did THIS session come from? | Traffic Acquisition — volume, engagement, conversions by source |
| First-user | firstUser |
Which channel originally acquired this user? | User Acquisition — who brought in new users |
| Event/attribution | (no prefix) | Which channel gets credit for this conversion? | Advertising reports — AI-weighted conversion credit allocation |
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"}}}
Load it in a session with search_skills("attribution-scope"), or read the MCP
resource skill://attribution-scope. Markdown twin:
/skills/attribution-scope/index.md · Source:
skills/attribution-scope/SKILL.md.