One of the most frequently asked questions I receive while supporting GA4 is about the "Source/Medium" dimensions. Many users are unsure which "Source/Medium" to select when creating the specific report they need. In this column, I will explain the various "Source" and "Medium" dimensions related to traffic acquisition and how to choose the right one for your reports.
Source/Medium for Users, Sessions, and Events (Attribution)
When creating traffic acquisition reports on Exploration Report, searching for "Source/Medium" will display several related dimensions, such as "Source/Medium," "First User Source/Medium," and "Session Source/Medium." Each of these dimensions provides different insights depending on whether you're analysing users, sessions, or events.
These differences arise because GA4 aggregates data based on different scopes: user, session, and event. Therefore, "Source/Medium" dimensions are prefixed with "First User," "Session," or are left unprefixed, depending on the scope they are related to.
To illustrate these differences, we'll explain the relevant "Source/Medium" for each scope in red text, using the example of User A’s visits from (1) to (4) and User B’s visits from (5) to (6).
In standard reports, you can view this under the 'Acquisition > Traffic Acquisition' report.
Source/Medium (Event Scope)
The "Source/Medium" without a prefix aggregates the source information of events where Key Events occurred, which can be found under "Advertising > Attribution." Metrics such as "Key Events" and "Purchase Revenue" are most suitable for this scope.
The relevant data is based on the source information of the event that triggered a “Key Event” within the session marked in the purple box (4). The attribution model you select will adjust the weighting based on past source information.
You can view this in each report under 'Advertising > Attribution.' report.
What is Source Precedence in GA4
So far, we have covered the various "Source/Medium" dimensions for traffic acquisition based on different scopes. When users visit a website, they may come from various media, such as ads, organic search, or social networks. However, in GA4, same as in Universal Analytics (UA), there is special handling for visits with no referrer.
For the visits with "no referrer", such as the traffic coming from "favourites," "direct input," or links without UTM parameters from email clients. GA4 applies a rule known as "Source Precedence". This rule states that if there is historical source data available, the no-referrer visit data will not overwrite it.
For more details, please refer to the "Processing" section of this article:
[GA4] Campaigns and traffic sources
https://support.google.com/analytics/answer/11242841
To illustrate, consider User B's second visit (6) in the diagram, which is a no-referrer visit classified as "direct/none" coming from "favourites." Because there was a previous visit recorded as "yahoo/cpc," the "Source Precedence" rule will be applied.
As a result, during the reporting period, the traffic attribution will show:
"yahoo/cpc: 2 sessions", but not "yahoo/cpc: 1 session, direct/none: 1 session."
Conclusion
In this column, we’ve explored the different “Source/Medium” dimensions and the important points to keep in mind when using them in reports. Remember, the “Source/Medium” in the event scope has the same name as in Universal Analytics (UA), which can easily cause confusion when selecting dimensions in Exploration Report. Always choose the dimension that best fits the goal of your report to ensure accurate data analysis.
