Source / Medium

Source / medium is a pair that describes an entry more precisely than a channel: the source says “from where” (e.g. google) and the medium says “by what mechanism” (e.g. organic or cpc).

A channel is fine for budget decisions but coarse; source / medium takes the same entry one level finer so two campaigns on one network can be told apart. The source is usually a domain or service name and the medium is the type of relationship — organic for a natural search entry, referral for a link from another site, cpc or paid for an ad click, email for a newsletter. It is this pair that later maps to a channel, so a mis-filled UTM (medium social instead of cpc) propagates upward from here and skews the whole channel report. The golden rule is to standardize source / medium once for the whole team so one campaign does not get two names across two reports.