Session impact on attribution GA4

Channel attribution has become a hot topic with clients migrating to GA4. While we acknowledge that GA4 is a different model to universal, it’s understandable for senior teams to look for parity with legacy reporting to gain confidence in their data during the cutover period.

GA4 is an event-based data model, however, the session remains a required construct to measure and understand marketing attribution regardless of the model configuration in GA4.

GA4 Channel Scope

GA4 channel dimension is defined across 3 scopes; User, Session and event. In order to measure the familiar channel, campaign, medium and source parameters it is crucial we first understand how the scopes should be interpreted.

User Scoped Channel

Perhaps the easiest to understand, the channel dimension at a user’s scope is essentially first channel when the ‘new_user’ event parameter occurs. This reporting is found in Acquisition> User Acquisition in the GA reports menu. The data can be used to understand top of funnel activity that is bringing new users to you website or app.

Session Scoped Channel

As the name suggest session scoped channel dimensions are using the GA4 session to assign credit to the channel for the specific event you want to attribute credit. Simple, right? Well first we need to understand that a session in GA4 is not the same as a session in Universal Analytics.

GA4 sessions don’t terminate with a new UTM source

With the GA4 attribution model set to use last-click, the session channel reporting found under the Acquisition > Traffic acquisition : Session default channel grouping will provide different results to last-click channel data you can report on from Universal analytics.

GA4 sessions don’t terminate with a new UTM source, this means that if multiple channels are used within the 30 minutes activity window attribution output will be different.  In my experience this will differ from brand to brand, but if like most you have a strong mobile user segment and a paid media marketing mix then this will impact your data. 

Event Scoped Channel

The event scoped dimension isn’t readily available within the reports menu, but the data does exist in GA4 via the explore customized reporting. This means that depending where you report your channel revenue will have a real impact on how you view performance. Neither view is incorrect, but the devil is in the detail.

From our early analysis with the last-click attribution model enabled the event scope channel dimension is proving to be the closest match to universal last-click non direct model and so might be worth considering for your reporting or validation purposes as you move to GA4.


Google could do a better job at releasing documentation on this subject. Ultimately, the data model is so different teams will have to suck it up at some point and make the adjustment as the GA4 session rules impact channels regardless of the attribution model used.

If you’d like to discuss any of this further, do get in touch.