1. Types of CPAs based on the attribution model being used
2. Prerequisites for calculating CPA in Google Analytics
3. How Google Analytics calculates attribution model specific CPAs?
4. How to find attribution model specific CPAs in Google Analytics
5. Types of CPAs based on the attribution window being used

Here is how most optimizers define CPA (cost per acquisition):

“It is the average cost of generating a conversion via a particular marketing campaign/channel.”

However, CPA is actually the average cost of generating a conversion via a particular marketing campaign/channel under a particular attribution model and attribution window. 

Depending upon the type of attribution model, attribution window and the marketing platform being used, there can be dozens of different types of CPAs. For example:

• Last-non-direct click CPA (7 days)
• Last-non-direct click CPA (30 days)
• First Interaction CPA (15 days)
• Linear CPA (60 days)

So when you say CPA what do you mean exactly? 

It means nothing.

The only thing that a stand-alone CPA metric communicates to your decision-makers is that you are reporting the performance of marketing campaigns like attribution models and attribution windows do not exist.

Types of CPAs based on the attribution model being used

Depending upon the attribution model being used, we can have nine different types of CPAs in Google Analytics multi-channel funnel reports:

1. Last-interaction CPA – It is the average cost of generating a conversion via a particular marketing campaign/channel under the last-touch attribution model.
2. Last non-direct click CPA – It is the average cost of generating a conversion via a particular marketing campaign/channel under the last non-direct click attribution model.
3. Last Google Ads click CPA – It is the average cost of generating a conversion via a particular marketing campaign/channel under the last Google Ads click attribution model.
4. First-interaction CPA – It is the CPA reported under the first-touch attribution model.
5. Linear CPA – It is the CPA reported under the linear attribution model.
6. Time-decay CPA – It is the CPA reported under the time-decay attribution model.
7. Position-based CPA – The CPA is reported under the position-based attribution model.
8. Data-driven CPA – The CPA is reported under the data-driven attribution model.
9. CPA based on a custom attribution model – It is the CPA reported under a custom attribution model.

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Prerequisites for calculating CPA in Google Analytics

Google Analytics can calculate and report the CPA of a marketing channel/campaign only when its advertising spend data (aka cost data) is available in the reporting view. 

If you have connected your Google Ads account to your Google Analytics account then GA can automatically pull the cost data from your Google Ads account and calculate the CPA of Google Ads channel and campaigns.

But for other marketing campaigns/channels, you would have to manually import their cost data to your GA property. You can import the cost data via the data import feature. 

For example, if you want to calculate the CPA of your Bing ads channel and campaigns then you would first have to manually import the Bing Ads cost data via the data import feature to your GA property. 

Once the Bing Ads cost data is available in your GA reporting view, Google Analytics will automatically calculate and report the CPA of the Bing Ads channel and campaigns.

How Google Analytics calculates attribution model specific CPAs?

Google Analytics calculates attribution model-specific CPA via the following formula:

Advertising spend / (The total number of conversions attributed to a marketing channel under a particular attribution model and attribution window) 

Let us suppose that under the last non-direct click attribution model and the default 30 days attribution window, the paid search marketing channel generated 100 ecommerce conversions and 50 goal conversions against a total ad spend of $1,500. 

So the total number of conversions attributed to the paid search marketing channel under the last non-direct click attribution model and the default 30 days attribution window would be 100 + 50 = 150 conversions.

Now GA will calculate and report the last non-direct click CPA for paid search marketing channel as $1,500 / 150 = $10

Now let us suppose that under the last non-direct click attribution model and 60 days attribution window, the paid search marketing channel generated 200 ecommerce conversions and 100 goal conversions against a total ad spend of $1,500. 

So the total number of conversions attributed to the paid search marketing channel under the last non-direct click attribution model and 60 days attribution window would be 200 + 100 = 300 conversions.

Now GA will calculate and report the last non-direct click CPA for paid search marketing channel as $1,500 / 300 = $5

Note: Longer your attribution window, the more conversions could be claimed by a marketing campaign/channel under a particular attribution model.

Similarly, let us suppose that under the linear attribution model and the 7 days attribution window the paid search marketing channel generated 10 ecommerce conversions and 5 goal conversions against a total ad spend of $1,500. 

So the total number of conversions attributed to the paid search marketing channel under the linear attribution model and 7 days attribution window would be 10 + 5 = 15 conversions.

Now GA will calculate and report the linear CPA for paid search marketing channel as $1,500 / 15 = $100

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How to find attribution model-specific CPAs in Google Analytics

To see all of the different types of attribution model-specific CPAs, follow the steps below: 

1. Login to your GA account and then navigate to your main reporting view. 
2. Navigate to the ‘Conversions’ > ‘Multi-channel funnels’ > Model Comparison Tool report. 
3. Select two or more attribution models from the drop-down menus. 
4. Look at the data table. It should show you the attribution model specific CPAs for each MCF Channel grouping (provided your reporting view is collecting the cost data).

From the screenshot above, we can conclude that paid search marketing channel has the Last Interaction CPA of $40.41 and the Last Non-Direct Click CPA of $32.49.

Note: You can also use the ROI Analysis report (available only to GA360 customers) to view the attribution model-specific CPAs.

Types of CPAs based on the attribution window being used

The attribution model-specific CPAs can also be further classified based on the attribution window (lookback window) being used. For example:

• Last-interaction CPA (7 days) – It is the average cost of generating a conversion via a particular marketing campaign/channel under the last-touch attribution model and 7 days lookback window.
• Last-interaction CPA (15 days) – It is the average cost of generating a conversion via a particular marketing campaign/channel under the last-touch attribution model and 15 days lookback window.
• First-interaction CPA (60 days) – It is the average cost of generating a conversion via a particular marketing campaign/channel under the first-touch attribution model and 60 days lookback window.

Note: Google Analytics does not report the attribution window along with the attribution model-specific CPAs. However, you should report it explicitly especially if you are not using the default 30 days attribution window.

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