Debug Web Vitals in the field - Implementation Suggestions

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George Arnaoutoglou | Relevance

May 25, 2021, 9:58:29 AM5/25/21
to, George Lambrou | Relevance



We are trying to implement the “Measure and debug performance with Google Analytics 4 and BigQuery” guide and we chose to implement the measurement and collection from the GTM template by Simo Ahava, that it is the recommended approach linked by your guide to the GitHub repository of the Core Web Vitals library.


In the main article however, there are many references on the “Debug Web Vitals in the field” and the additional debug information that can follow the core web vitals event hits.


Can you please help us with suggestions on implementing this additional debug information solution with our current setup, that is through GTM in GA4 event hits.

As I understand, we need one more parameter to follow every hit, that of the debug_target.



Thank you in advance,


George Arnaoutoglou | Relevance

May 26, 2021, 4:05:02 AM5/26/21
to, George Lambrou | Relevance

Greetings again,


I’m also facing the same issues reported here:


After following the instructions here (, no templated report is created. Furthermore, when I try to use that data source as a source for a new report, I get the error “The data source no longer has access to the underlying data set”.


Please advise us on how to fix those issues in order to use your work.


Thank you,


Philip Walton

May 28, 2021, 4:00:11 PM5/28/21
to George Arnaoutoglou | Relevance,, George Lambrou | Relevance
Hi George, I understand that there currently isn't much guidance on how to capture the debug info with Simo's GTM template. I reached out to Simo to get his thoughts and this was his response:

"Currently the sandbox of GTM’s custom templates doesn’t allow processing HTML elements (or any complex, constructor-based objects), so this will have to wait for new template APIs to be introduced"

Based on that, it sounds like it's not possible to add this to his GTM template, so you'll likely have to manually write code to capture this info in GTM.

Unfortunately, I'm not the best person to give guidance on how to capture this information with GTM (nor is anyone else who checks this email list), so I'd recommend asking in a GTM forum or on Stack Overflow.

Sorry I don't have a better answer for you!

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George Arnaoutoglou | Relevance

Jun 1, 2021, 4:19:41 AM6/1/21
to Philip Walton,, George Lambrou | Relevance

Greetings Philip,


I hope you are well.


Thank you for your answer and apologies for the late follow up.


I was able to follow another guide, based on your CWB js library, using a custom HTML tag, with a bit more data collection (page_type is very useful) and an altered version of the table materialization and Data Studio template with charts for a more thorough dive in.


This guide is more plug n play from my experience following both yours and the above guide for a GTM approach.


I’ve linked it also in the issuetracker thread.


I understand regarding the GTM template limitations. I’ve got the same comment reply from Simo.


However, the connector with the report template is still broken. Also, from what I’ve read, there will be a newer version of the js library to more accurately measure CLS, right? So, our current numbers are higher than actual, correct (


Even though, we are able to have a functional setup from collection to reporting for Core Web Vitals and we’re exploring ways to use it for offers, implementations and prioritization, I would like to be able to copy your report as well. Some charts from there can be of additional value.

Will you be willing to share it?


Thank you very much,


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