GA4 Traffic Allocation and Conversion Attribution (Part I: GA4 UI)

The concept of "attribution" often sparks confusion within the web analytics and advertising industry, mainly because it carries different meanings across platforms. This article aims to clarify how attribution works specifically within Google Analytics 4 (GA4) while also exploring how GA4 allocates visit and user acquisition.

Understanding GA4 Conversion Attribution Models

Attribution in GA4 is fundamentally about determining which marketing channel should receive credit for a conversion, a conversion in GA4 is a GA4 event that is set as a ‘key event’. You can set up key events under ‘Admin -> Data display -> Key events’: 

GA4 has two attribution model options: Last (Non-Direct) Click attribution and Data Driven Attribution model (DDA). By default, GA4 is set to use DDA model.

If you choose to use ‘Data Driven Attribution’ option, GA4 uses Shapley Game theory as the fundamental algorithm to score conversion rate for each channel that has ever been involved in a conversion journey. The below graph demonstrates how this works: 

If you choose to use ‘Last click’ model in the UI Admin, GA4 attributes the conversion to the last non-direct channel that brought the user to the site, ignoring any direct traffic that might have immediately preceded the conversion.

For example, if a user initially arrives at your site through a paid search campaign, revisits via an newsletter link, GA4 attributes the conversion to the email campaign. However, if the user had come directly to the site after the email touchpoint, and then convert during this 'direct' visit, GA4 would skip over that direct visit and still attribute the conversion to the email campaign.

You can set up the attribution model by ging to ‘Admin -> Data display -> Attribution settings’: 

How to report conversion in GA4 UI

Once you have set up key events and have chosen the preferred attribution model, you can start building reports in GA4 to analyse channel conversion performance. In GA4 UI, custom dimensions under ‘Attribution’ section should be used to report conversion. Note this is the ONLY time you would use the dimensions under Attribution section, you should never use these dimensions if you are reporting other none-conversion events. This applies to both GA4 ‘Reports’ and ‘Explore’. 

GA4 Dimension Panel in ‘Reports’

GA4 Dimension Panel in 'Explore'

The Attribution traffic source dimensions are also used in the Advertising section in GA4: 

GA4 Session Acquisition Allocation

The word ‘attribution’ is often misused when referring to session acquisition reporting, as these are distinct concepts. In GA4, 'attribution' specifically refers to conversion reporting, whereas 'acquisition' pertains to session allocation.

Understand how GA4 credits session acquisition

GA4 adheres to a straightforward rule when crediting a session (or a visit) to a channel: the first channel that brought the user to the site within a session is the channel attributed to that session. Similar to last-click attribution model, GA4 ignores the latest ‘direct’ traffic source and assign the session to the last non-direct channel. 



This allocation does not change even if the user interacts with other channels during the session, as long as the session remains active (i.e., no more than 30 minutes of inactivity). For instance, if a user visits your site via a paid search ad and then clicks on an affiliate link within the same session, the session source remains attributed to the paid search ad. In comparison, this in-session click will be taken into considerations in the DDA or last-click conversion attribution model. 

How to report session allocation in GA4 UI

In GA4 UI, session level traffic dimension names are prefixed with ‘session’. If you are analysing channel data at session level, make sure you do not use dimensions without this prefix. This again applies to both GA4 ‘Reports’ and ‘Explore’. 

Session Scoped Traffic Dimensions vs. Attribution Dimensions

User Acquisition Allocation

User acquisition reports or user scoped traffic source dimensions represent the channel that initially acquired the user. This is more straightforward than conversion attribution and traffic allocation models: GA4 attributes the credit to the channel that brought the user to the site for the very first time. GA4 uses the _ga cookie ID to identify a new user and fires a first_visit event to recognise a user’s first visit.



In GA4 UI, user level traffic dimension names are prefixed with ‘First user’.  The value of these dimensions persist throughout the user’s lifetime on the site (i.e. the ga cookie value’s lifetime in GA4). Here are the user source dimensions in GA4 UI’s Explore:

Different Types of Traffic Source Dimensions in GA4 UI

When reporting on traffic sources in GA4, it's easy to make the mistake of using incorrect traffic source dimensions across different scopes, which can lead to misleading channel data. As discussed earlier, there are three groups of traffic source dimensions in GA4, both in the Reports and Explore sections. It’s crucial to select the right group when building your reports to ensure accuracy.

These are the types of dimensions:

• Source/Medium/Campaign (no prefix): Use these dimensions for conversion or advertising analysis.
• Session Source/Session Medium/Session Campaign (Prefixed with ‘Session’): These are for session acquisition analysis, helping you understand how users arrived at your site during each visit.
• First User Source/First User Medium/First User Campaign (Prefixed with ‘User’): Employ these dimensions for user acquisition analysis, revealing how users first found your site.

Understanding these differences and choosing the correct dimension is essential for accurate reporting.

An Example of How Conversion is Attributed and How A User and Session Acquisition are Credited

To apply what we’ve discussed, we will demonstrate an example of a user who visited the site multiple times through different channels. showing how the GA4 UI calculates conversions and credits user and session acquisitions.



In this example, the user has a GA cookie ID of '1234567.1234567' and engages in three sessions. The first two sessions originate from 'Paid Search,' followed by a third session via an email. During the first session, the user clicks on an affiliate link.

Based on the logic discussed in this article, the following tables illustrate how conversions, session source/medium, and user source/medium are calculated: 

1.     The user is attributed to 'google/cpc' as their First User Source/Medium because this is how they arrived at the site during Session 1, their first visit.

2.     The Session Source/Medium for each session is determined by the UTM parameters or ‘gclid’ value captured from the initial hit that brought the user to the site for that session

3.     For conversion attribution, the Last Click Model assigns credit to the last non-direct channel before the conversion, ignoring any direct channels that may have immediately preceded it. In contrast, Data-Driven Attribution (DDA) uses Shapley values to distribute credit across all channels, taking into account the entire customer journey."

Understanding how GA4 attributes conversions, user acquisition, and session acquisition is crucial for accurate reporting. We hope this article has clarified the differences between these attribution and traffic source dimensions. If you have any questions, don’t hesitate to reach out to us — we’re here to help.