Why Do Core Web Vitals Scores Always Change?

John Mueller answered a question in the SEO Office-hours session about why the core web vitals scores keep changing even though the web pages themselves have not changed.

There are two kinds of core web vitals scores:

1. Field Data
   These are the core web vitals scores that are recorded from actual visitors visiting a website in the real world.
   The purpose of Field Data is to show the real-world user experience of actual site visitors to a given site.
2. Lab Data
   These are core web vitals scores that are generated by a simulated visit. A bot from Google Lighthouse that is using a user agent identified as Chrome/94.0.4590.2 will visit the page that is being tested and an algorithm will simulate that the visit was on a mobile phone and on a throttled internet connection. The page loading information is collected and an artificial core web vitals score generated.The purpose of Lab Data is diagnostic. Lab Data helps a site publisher or an SEO troubleshoot web page performance. Lab Data provides information that will help a publisher or an SEO understand what is slowing down the web page and provide hints for fixing the problems.

Screenshot of John Mueller Discussing Core Web Vitals Scores

Lab Data is generated using Google’s Lighthouse tool.

In fact, the bot that visits a site when generating the core web vitals scores for the lab data uses the word Lighthouse in the user agent.

According to the official Lighthouse web page:

“Lighthouse is an open-source, automated tool for improving the quality of web pages.

…It has audits for performance, accessibility, progressive web apps, SEO and more.

You can run Lighthouse in Chrome DevTools, from the command line, or as a Node module. You give Lighthouse a URL to audit, it runs a series of audits against the page, and then it generates a report on how well the page did.

From there, use the failing audits as indicators on how to improve the page. Each audit has a reference doc explaining why the audit is important, as well as how to fix it.”

Core Web Vitals Scores Keep Changing

The question that John Mueller answered didn’t specify if it was about field data or lab data.

The person asking the question simply asked when does the core web vitals scores stop changing, without specifying field data or lab data.

Mueller answered the question from the point of view of Field Data, which is the core web vitals scores based on real-world visitors.

This is the question that was asked:

“When does the core web vitals give steady and correct information?

It keeps changing without changing any data on the website.”

Core Web Vitals Scores Are Influenced by Random Factors

The answer focused on the randomness of the data that is used to generate the core web vitals field data.

John Mueller answered:

“So, I think this is probably a side effect of how the core web vitals and the page experience update is processed.

And that’s something where I would try to look up those details to understand a little bit more about how the field data, …the data that users actually see, kind of plays a role into this.

And that is something where if users from a wide variety of backgrounds and different locations and different device types access your pages, you will probably see some fluctuations over time there as well.”

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Internet Speeds Vary

A fast server and a fast website is only one part of obtaining a high core web vitals score and it’s the only part that’s under the control of the SEO or site publisher.

As John Mueller said, there are a wide variety of other factors that influence the core web vitals scores and many of those factors cannot always be controlled.

Even if a website is hosted on a fast dedicated server, network congestion, an outdated cell phone used by a site visitor and a poor mobile data connection can all significantly impact the core web vitals performance of an otherwise fast website, thereby contributing to a less than good core web vitals score on the field data.

And some site visitors might be on a newer model cell phone that has a powerful processor that is operating on a newer 5G network. That site visitor will generate high scores on the core web vitals field data.

Things like network latency (how slow the entire Internet is at any given moment) can also affect both the real-world field data and the simulated lab data.

So even though the lab data is not a real visitor, an actual Chrome browser-based bot is traveling through the Internet to visit the web page being tested. And that will also affect the simulated lab data scores.

Google’s Web.dev website published an explainer about why the field data is always changing and not always the same from month to month.

Web.dev explains it like this:

“The most important thing to understand about field data is that it is not just one number, it’s a distribution of numbers. That is, for some people who visit your site, it may load very quickly, while for others it may load very slowly.

The field data for your site is the complete set of all performance data collected from your users.

As an example, CrUX reports show a distribution of performance metrics from real Chrome users over a 28-day period. If you look at almost any CrUX report you can see that some users who visit a site might have a very good experience while others might have a very poor experience.”

Whether you’re testing a site for lab data or reviewing the real-world field data, the core web vitals scores will tend to fluctuate and likely not provide a stable score month after month.

Citation

When Do Core Web Vitals Scores Settle Down?

Watch John Mueller answer the question at the 19:05 minute mark: