Cross-domain tracking in GA

A quick step back. How does GA track visitors?

As part of my 3rd week at CXL Institute, I would like to share a very important topic with you. Chris Mercer has done a great job in explaining the concept of cross-domain tracking in Google Analytics which is an essential part of GA implementation for some websites. Though topics like Custom Reports, Funnel Tracking, Alers, Measurement Protocol, and Segments have been covered during the 3rd week, I will focus on cross-domain tracking to explain the concept and clarify the dark side of it.

Introduction:

By default, Google Analytics tracks users with first-party cookies. You land on a website and GA (on behalf of your website) stores a _ga cookie in visitor’s browser (with some random identifier). When you, as a visitor, navigate from page A to page B (of your website), Google Analytics checks the value of that cookie.

Since the cookie is still present (and the same), GA understands that you are the same person who just was on page A, therefore, the 2nd-page view is also attributed to the same session and user.

And that’s great! With web analytics tools (not just GA), you can track how visitors are using your website, what are they doing, etc. Of course, cookies, as a technology, are far from being perfect and have a lot of drawbacks but let’s keep this topic for another blog post.

So where is the problem?

Where am I heading with all of this? First-party cookies (created on behalf of your website) cannot be shared between two separate domains (due to cross-domain policy). So if you have a website where the journey of a visitor starts on domainA.com and eventually, ends on the domainB.com, Google Analytics on the domainB.com will not able to understand that this is the same visitor as on the domainA.com (even if the GA property on both domains is the same).

When the visitor navigates from domain A to domain B, Google Analytics on domain B cannot reach the _ga cookie of domain A, therefore, it creates a new one.

The result? Google Analytics (and other cookie-based web analytics tracking tools) will see you as two separate visitors, one on the domainA.com and one on the domainB.com.

And that’s a big problem.

• First of all, you will see too many users in your GA reports. Every person who visits both domainA.com and domainB.com will turn into two users.
• You’ll lose the original traffic source. If a visitor lands on the domainA.com from Google Ads, then navigates to domainB.com and converts, that sale will not be attributed to Google Ads. Because whenever a visitor jumps from one domain to another, GA will start a new session and its traffic source will be domainA.com. This means that such conversion will be attributed to your own website and you won’t be able to properly measure the effectiveness of your marketing efforts. This situation is called “self-referral”.

What is cross-domain tracking?

It is a workaround that allows webmasters/marketers/analysts to send user’s/visitor’s identifier (a.k.a. client ID) from domain A to domain B and preserve the session information. That way, the visitor is being tracked as the same person across multiple websites/domains.

In order to make this work, you will need to have access to both websites/domains and make some configurations. This entire guide explains different approaches you need to keep in mind (because every situation needs a different treatment).

I will mention this multiple times throughout this article but cross-domain tracking is needed only if domains are totally different. If you are tracking the visitor across the subdomain of the same domain, you don’t need cross-domain tracking. I will explain what to do about this in a moment.

1.4. Examples of GA cross-domain tracking

If you’re very new to Google Analytics cross-domain tracking and don’t understand its main purpose, don’t worry! Here’s are several examples where you must implement it:

• A visitor lands on your eCommerce store > initiates a purchase > is redirected to the payment processor’s website (which is hosted on another domain) > makes a purchase and returns. Cross-domain tracking must be implemented in the jump between your store and the payment processor’s website. Otherwise, all your purchases will be attributed to that payment processor, not the original traffic source (like Google organic, paid ads, or something else).
• You run a travel blog and have an embedded form where visitors can check hotel prices and even book rooms. That form belongs to another domain (embedded via iFrame). So if you want to track how your blog visitors interact with that booking form (and you want Google Analytics to properly track it), you need to enable the cross-domain tracking (within that iFrame). Otherwise, all the bookings within that form will be attributed to the parent website (your travel blog) and GA will see users of the booking form as totally different users.

1.5. Google Analytics Cross-domain tracking to the rescue!

Not everything is hopeless here. Otherwise, this guide would not exist, right?

Even though you cannot access domainA.com’s first-party cookies on domainB.com, there is a workaround that can help you make sure that _ga cookie’s value on both domains is the same. Because that is your main goal. Google Analytics can track the same visitor on different domains without any problems (but some additional configurations will be needed) as long as _ga cookie’s value persists across those domains.

How does Google Analytics cross-domain tracking work?

Here is it in a nutshell.

When a visitor navigates from domainA.com to domainB.com, we’ll pass a special linker parameter to domainB.com’s URL. Thanks to that linker parameter, the GA on domainB.com will be able to update the _ga cookie and will understand that it’s actually just one person navigating between two websites.

Your main goal: the same value of ga cookie across different domains

I cannot stress this enough. Cross-domain tracking will work only if you make sure that the _ga cookie (that Google Analytics uses to identify visitors) has the same value on different domains.

Of course, there are some other additional configurations needed (I’ll explain them a bit later) but this is the key ingredient to success (and in many cases, the most difficult part to do and the most tricky part where GA users struggle).

You DON’T need Google Analytics cross-domain tracking on subdomains of the same domain

This is a common misconception of how cross-domain tracking works. Its purpose is to help track visitors that navigate between totally different domains, like domainA.com and domainB.com.

If you just have a website www.website.com and your visitors can also navigate between blog.website.com, support.website.com, you DON’t need to configure cross-domain tracking.

Just go to your Google Tag Manager container, open Google Analytics Settings Variable that is used by your GA tags and set Cookie Domain to auto. By default, all GA Settings Variables have it enabled.

That’s it. There is no need to implement GA cross-domain tracking for subdomains. Better spend your time somewhere else.

Also, make sure that you are using the same GA property across all subdomains. You can learn more about the subdomain tracking here.

But if, on the other hand, you indeed need to track visitors/users across different domains, then you will find the rest of this guide very useful (hopefully).

So that was the end of the first part of this guide. So far, so good. Nothing super difficult. But that might change in the 2nd part (I’ll do my best to keep it simple).