First user medium dimension
The first user medium dimension records the medium of a user's very first session — organic, cpc, referral, email, (none) for direct — and keeps it fixed for the user's lifetime. GA4 derives it from utm_medium or the inferred channel on the first visit. It is user-scoped, so it answers 'through what kind of channel did we first acquire this person?' rather than how a later visit arrived, and confusing it with session medium distorts cohort attribution.
What this means
First user medium is the medium GA4 attributes to a user's first session, locked at acquisition and never updated. Medium is the channel-type half of the source/medium pair: google is a source, organic or cpc is the medium.
Because it is user-scoped and fixed, it is the right dimension for cohort analysis grouped by how a person was first reached — for example comparing retention of email-acquired versus organic-acquired users.
Scope and derivation
GA4 sets first-user medium from utm_medium when present, otherwise from the medium it infers from the referrer (organic for search engines, referral for other sites, (none) for direct). Session medium, by contrast, is re-derived each visit. Treating the two as interchangeable double-counts or mis-credits acquisition across scopes.
- User-scoped: fixed at the first session
- Derived from utm_medium or inferred channel type
- Distinct from per-session session medium
How it appears in analytics and logs
A first-user medium value is the channel type of the user's first session. It diverges from session medium for any returning user, and that divergence is expected rather than an error.
Diagnostic use case
Use first-user medium to build acquisition cohorts by channel type and to compare lifetime behaviour of organically- versus paid-acquired users.
What WebmasterID can help detect
WebmasterID can retain a user's first-touch medium separately from per-session medium, so channel-type acquisition cohorts stay distinct from return-visit reporting.
Common mistakes
- Comparing first-user medium with session medium as equals.
- Expecting first-user medium to change on return visits.
- Reading (none) as missing data rather than direct traffic.
Privacy and accuracy notes
First-user medium derives from the first visit's utm_medium and referrer, not from persistent cross-site identity. WebmasterID attributes first-touch first-party without fingerprinting.
Related pages
- 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.
- Medium dimension: the channel-type half of origin
Medium is the dimension that records the general category of how a visit arrived: organic, cpc, referral, email, affiliate, and so on. It is the channel-type half of source/medium. In GA4 and earlier tools it is set by the utm_medium parameter or inferred from the referrer, and it feeds channel grouping. The distinction between an empty medium, 'none', and '(not set)' trips up many reports.
- Session campaign dimension
The session campaign dimension attributes an entire session to a single campaign, fixed when the session begins. GA4 derives it from utm_campaign or the auto-tagged Google Ads campaign present at session start. It is session-scoped, so it answers 'which campaign drove this visit?' — different from event-scoped campaign (which can change within a session) and from first-user campaign (the lifetime acquisition campaign). Mixing the three is a common attribution error.
- Attribution analytics
First-touch channel type vs per-session medium.
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.