Tracking | Google Analytics - Why are results different in GA4 and Universal?

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At this time many Google Analytics (GA) users are seeing the above warning which states that Universal Analytics (UA) is being deprecated in favor of GA4. 

It seems logical that once both versions of GA are in place, GA4 reports should show the same numbers as UA reports.

In reality, these two versions process data differently which means that some discrepancy can be expected. This article will shed light on some of those discrepancies and why they exist. 

UA vs GA4 Differences

Models

Universal Analytics Measurement Model is based on sessions and pageviews whereas GA4's model based on Events with (optional) Parameters. Universal Analytics sessions may contain multiple Hits of the type Pageview and on each page there may be multiple Events as a result of user interactions. In GA4, both Pageviews and user interaction Events are just considered Events. 

Category, Action, and Label - In UA, we are prompted to supply values for the fields in an Event Tag. GA4 does not recommend these but instead, provide a unique Event Name. If you still need to capture a variable value that is currently in a UA Label, for example, add that variable as an event parameter. 

In the example below, the value for a UA Event Label is converted to a GA4 Event Parameter

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Session Count

In Universal Analytics, sessions are defined as ended after 30 minutes have elapsed with no interactions from the user. By contrast, GA4 sessions begin with an automatically collected event called "session_start". Sessions in GA4 are defined by the time span between the first and last automatically collected event. These differences between UA and GA4 can result in differences in session count.

Active Users

GA4 automatically detects user activity whereas UA relies on the firing of an event. Therefore, if a user interacts with a page and no manually configured UA Tag fires, GA4 will recognize them as an active user but UA will not, thus leading to a discrepancy in reporting. 

Late Hits

Under certain circumstances, analytics hits are delayed not sent immediately. The rule for Universal Analytics is to process these hits/events so long as they arrive withing 4 hours of the close of the day. The rule of GA4 is different -- those events will be processed so long as they arrive within 72 hours. As a result:

  • GA4 may process more total hits
  • Discrepancies may exist when comparing data over the past 72 hours as late hits may still process

Conclusion

The differences between these two GA versions include modeling and processing rules. Overall trends should be the same but numbers will not match up exactly. 

From an implementation perspective, tracking teams must ensure that tags fire based on the same user interactions. Triggers have not changed and can be used to fire both UA and GA4 Tags simultaneously. Ideally tracking teams will implement both versions and allow them run for several months before Universal Analytics stops processing data on July 1, 2023. 

 

 

 

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