Attribution
Source / medium
Also called source-medium pair.
Source / medium is a combined key made from where a visit came from and the kind of channel it used, written like google / organic or newsletter / email. It is the raw label behind channel reports.
How it is measured
The tool takes the source and medium from campaign tags when present, otherwise from the referrer. Search engines map to organic, ad tags map to cpc, and unknown sites map to referral.
Export the full list for a month and look at the long tail. Variants such as Email, email, and e-mail make separate rows and need to be unified in your tagging standard.
Worked example
A language-school site exports source / medium and finds 74 rows. The top two are google / organic with 6,100 sessions and (direct) / (none) with 2,300. Beneath those are 'Newsletter / email', 'newsletter / email', and 'newsletter / Email'.
The three spellings add up to 1,450 sessions, ranking just below direct when combined. After standardizing to lowercase, the school sees its email program as its third-largest source.
How it differs
A source / medium pair is the raw label. A channel grouping is the rule set that rolls pairs into a few buckets. Fix the pairs and the channels fix themselves.
Common errors
Allowing free-form tags. Leaving capitalization unchecked. Not reviewing the long tail. Using medium for the platform name. Changing tag names mid-year.
In practice
Write a short tagging guide that lists allowed values. Clean existing rows when you report. Check the list once a quarter.