The Measurement Gap: Is Your GA4 Accurate? Why Marketing Mix Modelling (MMM) is the True Test of Media Activity

Let's start with a short potted history. The last 10 years in the marketing measurement industry have seen a lot of change. The Big 4 include

1. A rise of privacy-first web tracking & measurement
2. Walled gardens between tech firms (Google not talking to Meta, for example)
3. Transition from Universal Analytics to Google Analytics 4
4. An increasing desire for Brands to know “would that sale have happened anyway?”

With those key trends impacting the desire for measurement accuracy, let's see how GA4 and MMM compare.

What is driving my sales?

This is where we see a diversion. Use the chart below to compare how drivers change between a GA4 & MMM output.

The fundamental Divergence: Why do GA4 and MMM produce different results?

One thing we at Linea hear very often is: “GA4 is not answering our key media performance questions - especially for upper funnel channels”.

At its heart, they are answering different questions.

GA4 = of those things we can track. What element had the final contribution
MMM = of all things that occurred. Which factors contributed

See the table below comparing what is included in each approach

Group	Category	GA4	Linea MMM
Approach	Measurement type	Tracked	Statistical
	Customer journeys	✅	✖
	Longer-term impact	7–30 day impression window	Up to 12 months via adstocks
	Frequency	Live / daily	Monthly (can be faster)
Non-media factors	Price, product changes, promotions	✖	✅
	Market and economic factors	✖	✅
	Seasonality	✖ Trends visible with YoY data	✅
Channel coverage	Google Search	✅	✅
	Brand Search	? Often overvalued	✅
	Meta, TikTok, Display	✅ Requires dedicated setup	✅
	Mid / upper funnel digital (YouTube, CTV)	? Undervalues non-click channels	✅
	Offline channels	✖	✅

Determining Measurement Accuracy: When to use GA4 and When to use MMM

Which approach is the most accurate? This is where we come back to the key question of what specific goal you are trying to achieve. Both tools are highly accurate for their designated purpose, but they serve different decision-making needs.

Here at Linea, we recommend the following framework for effective budget allocation and cross-channel measurement:

• Use GA4 when...
  • You are looking to compare performance within a specific media channel (e.g., comparing two different Google Search campaigns).
  • You need to analyse digital customer journey steps at a user level.
• Use MMM when...
  • You need cross-media channel comparison (e.g., comparing the true return of TV vs. Paid Social).
  • You need to determine incremental ROI and calculate overall marketing budget allocation.
  • You need to understand the influence of non-media factors (like price changes, economic trends, or seasonality) impacting your media performance.

For modern marketers, it is vital to have full clarity on the tools you are using, where they should be used, and what questions they are best equipped to answer- see more here.

Only when you combine the tactical digital attribution from GA4 with the strategic, holistic media measurement of Linea’s MMM can you make cross-channel media decisions that will truly drive business growth.