Understand how attribution in GA4 works vs. Universal Analytics and what the differences mean for conversion tracking.
Attribution determines the role of each touchpoint in driving conversions and assigns credit for sales to interactions in conversion paths.
As Google’s deprecation of Universal Analytics (UA) nears, it’s crucial to understand attribution in Google Analytics 4 (GA4) – including what is new, what is missing, and what the differences mean for search marketers.
(If you are new to attribution, read the Google Analytics help article on attribution first.)
How Google Analytics 4 attribution works
Universal Analytics reports attributed the entire credit for the conversion to the last click. A direct visit is not considered a click, but for the avoidance of doubt, this attribution model was also called the last non-direct click model. Other attribution models were only available in the Model Comparison Tool in the Multi-Channel Funnels (MCF) reports section.
GA4 offers a wider availability of different attribution models, but it depends on the scope of the report – whether it is the user acquisition source, session source or event source.
In Universal Analytics, the source dimensions had session scope solely. The MCF reports made it possible to analyze the sources of all sessions on the conversion path. The three scopes of source dimension in GA4 (user, session, event) are the most important and fundamental changes in the attribution area.
This guide will use the term “source” in a broader meaning as any dimension that indicates the origin of a visit, e.g., channel grouping, source, medium, ad content, campaign, ad group, keyword, search term, etc.
Session source
Session-scope attribution – unsurprisingly – determines the source of the session. It is used, among others, in the Traffic acquisition reports in the Reports section. It works similarly to Universal Analytics in always using the last non-direct click model.
The session source is the source that started the session (e.g., social media referral or organic search result). However, if a direct visit started a session, the session source will be attributed to the source of the previous session (if there was any).
Quick reminder: A direct visit means that Analytics does not know where the user came from because the click does not pass the referrer, gclid, or UTM parameter.
Therefore, exactly as it was in Universal Analytics, the session source will be direct only if Analytics cannot see any other source of visit for the given user within the lookback window. The default lookback window in GA4 is 90 days, while in Universal Analytics, it was six months by default. We will return to the lookback window matter later in this article.
By the way, what is a session?
A Google Analytics session is not the same as a browser session.
In GA4, a session begins when a user visits the website or app and ends after the user’s inactivity for a specified time (30 minutes by default – see this Analytics help article).
Closing the browser window does not end the session. If the browser window is closed, another visit to the website within the time limit would still belong to the same session – unless the browser deletes cookies and browser data after closing the browser window, for example in incognito mode.
In Universal Analytics, when a user re-visits the website from a new source during an existing session, the existing session is terminated, and a new session starts with that new source.
In GA4, it is no longer the case. If a visit from a new source occurs during a session, a new session will not start, and the source of the current session will remain unchanged.
It does not mean that the visit from the new source is ignored. GA4 records the source of this visit, and the event-scope attribution reports (more on that later in this article) will take into account all sources of all sessions. (See this Analytics help article.)
A new visit during an existing session may happen, for example, if a user returns from a payment gateway or a webmail site after password recovery or registration confirmation. In GA4, these visits will not artificially inflate the number of sessions, as in Universal Analytics.
Nevertheless, sources of these visits are so-called unwanted referrals and should be excluded. Visits from excluded referrals are reported as direct visits.
In GA4, these visits are de facto ignored because the session source and the session count remain unchanged. The non-direct attribution modeling in GA4 will assign no credit to this (direct) source (as described later in this article).
In Universal Analytics, the session (regardless of duration) ends at midnight, which is no longer the case in GA4.
First user source
First user source (source of the first visit) is new to GA4. It shows where the user came from to the website or app for the first time.
It is a part of Google’s new approach to measurement in online marketing, which no longer focuses only on the classic ROAS (revenues vs. costs), but also analyzes the CAC vs. LTV (customer acquisition cost vs. lifetime value).
This approach reflects the app logic: we have to acquire the app user first, and after the app is installed, further marketing efforts engage and monetize the user. However, for the web traffic, it also makes more sense.
The new customer acquisition goal in Google Ads, available in Performance Max campaigns, also represents a similar approach. In this case, the focus is on the first-time buyer, not the first visit.
In GA4, the first user visit is recorded by the first_visit event for the website or the first_open event for the app. The naming is self-explanatory.
Therefore, the source of the first visit is a user attribute and indicates where this user’s first visit to the website or application came from.
The first visit source is attributed using the last non-direct click model. Of course, this attribution applies only to interactions before the first website visit or the first open of the app (interactions following the first visit or first open are not taken into account).
Once assigned, the source of the first visit remains unchanged – of course, as long as Google Analytics can technically link the user’s activity on the website and in the app with the same user.
The first user source will be reset if the tracking of the user is lost, for example, if the user does not visit the website for a period longer than the Analytics cookie expiration date.
We will return to the Analytics cookie expiration period and other data collection limitations in GA4 later in this article.
Event scope attribution
In GA4, events replaced sessions as the fundament of data collection and reporting. GA4 makes it possible to report attribution using a selected attribution model for any event (not only for conversions).
The model is set in the Attribution Settings of the GA4 property. There are several pre-defined models to choose from (see the screen below).
A common belief is that Google Analytics 4 no longer uses the last-click attribution model. But is that the case?
In practice, it applies only to customized reports that use event-scope dimensions and metrics, for example, Medium – Conversions.
The default traffic and user acquisition reports use session source and first user source, respectively, and these dimensions use the last click model. It is indicated in the dimension name (e.g., Session – Campaign or First User – Medium).
Remember: source, session source and first user source are three different dimensions where different attribution models apply.
Scope | Attribution Model | Where available |
Session | Last click | E.g., traffic acquisition reports |
User (first user source) | Last click | E.g., user acquisition report |
Event | Model set in the GA4 property settings (data-driven by default) | E.g., in the Explore section |
Attribution settings
The attribution model set in the property settings applies to all reports in the property.
There are several attribution models, known from Universal Analytics (described in the earlier mentioned Analytics help article), to choose from. However:
- All the models do not assign value to direct visits unless there is no other choice because there is no other interaction on the path. In other words, they all use the non-direct principle, which was not the case in the Universal Analytics pre-defined attribution models, except for the last non-direct click model.
- The Ads-preferred models assign the entire conversion value to Google Ads interactions if they occur in the funnel. At the moment, there is only one Ads-preferred model available – the last click model, which is the equivalent of the “last Google Ads click” known from Universal Analytics. In the absence of Google Ads interactions on the funnel, this model works like a regular last-click model.
- In addition to clicks, models take into account “engaged views” of YouTube ads, that is, watching the ad for 30 seconds (or until the end if the ad is shorter) and other clicks associated with that ad (see this Google Analytics help article for more details).
Again, a change of the attribution model settings works retroactively (i.e., it applies to the historical data before the change). Saved explorations will be recalculated when viewing them.
Lookback window
Google Analytics property settings determine the length of the lookback window. The lookback window determines how far back in time a touchpoint is eligible for attribution credit. The default lookback window is 90 days, but you can change it to 60 or 30 days.