First user source dimension
The first user source dimension records the origin of a user's very first session — their acquisition source — and keeps it fixed for the user's lifetime. GA4 sets it from the referrer or campaign on the first visit. It is user-scoped, so it answers 'how did we acquire this person?' rather than 'where did this visit come from?', and confusing it with session source distorts attribution.
What this means
First user source is the origin of the first session GA4 attributes to a user. It is locked at acquisition and does not change — every later session keeps reporting the same first-user source for that user.
This user-scope makes it the right dimension for cohort analysis: grouping users by how they were first found, then watching what they do over time.
Why it differs from session source
Session source updates each visit; first-user source never does. A user acquired via organic search who later returns from a newsletter shows first-user source = google/organic and session source = newsletter/email. Both are correct at their own scope. Treating them as interchangeable double-counts or mis-credits acquisition.
- User-scoped: fixed at the first session
- Never changes on return visits
- Distinct from per-session session source
How it appears in analytics and logs
A first-user source value is the user's original acquisition origin. It will diverge from session source for any returning user — that divergence is the point, not an error.
Diagnostic use case
Use first-user source to analyse acquisition cohorts, and never compare it directly with session-scoped source, which changes on every return visit.
What WebmasterID can help detect
WebmasterID can hold a user's first-touch origin separately from per-session origin, so acquisition cohorts and return-visit behaviour stay cleanly distinguishable.
Common mistakes
- Comparing first-user source with session source as equals.
- Expecting first-user source to update on return visits.
- Using it for per-visit reporting instead of cohorts.
Privacy and accuracy notes
First-user source derives from the first visit's referrer and campaign tags, not from persistent cross-site identity. WebmasterID attributes first-touch first-party.
Related pages
- Session source dimension
The session source dimension attributes a whole session to a single origin — google, a newsletter, a domain — fixed when the session begins. GA4 derives it from the campaign tags or referrer present at session start. It differs from the event-scoped source (which can vary within a session) and from first-user source (the very first acquisition), so mixing the three is a frequent attribution error.
- New vs returning dimension
The new vs returning dimension classifies a visitor as new (no prior recorded visit) or returning. The classification depends on a persistent client identifier surviving between visits. When cookies or storage are cleared, browsers cap identifier lifetime, or a user switches devices, returning visitors are recounted as new — so this dimension systematically tilts toward 'new' and should be read with that bias in mind.
- First-click vs last-click referrer
A visitor often touches several sources before converting. First-click attribution credits the referrer that started the journey; last-click credits the one immediately before conversion. The same session can therefore be attributed to different referrers depending on the model — which is why the original inbound source is easy to lose without persisted campaign data. This page explains the distinction and the UTM approach.
- Attribution analytics
First-touch acquisition vs per-session origin.
Sources and verification notes
Last reviewed 2026-06-24. Facts are checked against primary/official sources where available; uncertain specifics are marked “Data not yet verified” rather than guessed.