Q: Should I report on Session source/medium or First user source/medium?

Q: Should I report on Session source/medium or First user source/medium?

A: Pick based on whether you're measuring acquisition or engagement.

— Use First user source/medium to credit the channel that originally acquired the user — best for top-of-funnel and new-user campaigns.
— Use Session source/medium to credit the channel that started the session where action happened — best for last-touch and conversion analysis.
— Don't mix them in one report blindly: a user acquired by organic but converting via email will look totally different depending which you choose.

TL;DR: First user = who brought them in originally; Session = what drove this specific visit. Acquisition reports use first-user, conversion analysis usually wants session.

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