utm_medium vs utm_source

This is the single most common UTM mix-up, and it quietly corrupts more reports than any other tagging mistake. The distinction is simple once stated plainly: utm_source says who sent the click; utm_medium says how it arrived.

The rule of thumb

Ask two questions about any link. Where did the click physically happen? That is the source: facebook, google, newsletter, partner-blog. What kind of channel carried it? That is the medium: paid-social, cpc, email, referral. One link, two answers, two parameters.

Worked examples

Where teams go wrong

• Putting the platform in medium. utm_medium=facebook wastes the parameter — facebook is a source. The medium should be paid-social or social.
• Putting the channel in source. utm_source=email tells you nothing about which email. Source should be newsletter or welcome-series; medium is email.
• Repeating the same value in both. utm_source=social&utm_medium=social doubles the noise and halves the information.
• Case drift. Email vs email splits one channel into two rows in every report.

A memory aid

Source is a proper noun — a name. Medium is a common noun — a category. If the value sounds like somewhere (google, newsletter, partner-blog), it belongs in source. If it sounds like something (email, cpc, referral, display), it belongs in medium. When in doubt, generate a consistent set with the utm_medium generator and pin it in the team wiki.