Why do results for “web core vitality” always change?

John Mueller answered a question in an SEO Hours session about why the results of core web vitals keep changing even though the web pages themselves haven’t.

There are two main types of web biometrics results:

  1. wrong data
    These are the basic results of web vitals that are logged from actual visitors who visit a website in the real world.
    The purpose of Field Data is to show the realistic user experience of the actual site visitors of a particular site.
  2. lab data
    These are the results of basic Web Vital Indicators generated through a simulated visit. A Google Lighthouse bot using a user agent identified as Chrome/94.0.4590.2 will visit the page being tested and an algorithm will simulate that the visit was on a mobile phone and on a throttled internet connection. Page load information is collected and a web biosynthetic result is generated, and the lab data is intended for diagnostics. Lab Data helps the site publisher or SEO troubleshoot web page performance. Lab Data provides information that will help the publisher or SEO understand what is slowing down a web page and provide tips for fixing problems.

Screenshot of John Mueller discussing the results of “Vital Web Fundamentals”

Lab data is generated using Google’s Lighthouse tool.

In fact, a bot visiting a site when generating basic web vitality results for lab data uses the word Lighthouse in the user agent.

According to the official Lighthouse webpage:

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

… has performance and accessibility audits, progressive web applications, SEO, and more.

You can run Lighthouse in Chrome DevTools, from the command line, or as a node module. You give Lighthouse an audit URL, and it performs a series of checks on the page, then generates a report on how well the page is performing.

From there, use failed audits as indicators on how to improve the page. Each audit contains a reference document that explains why the audit is important, as well as how to fix it.”

The results for “basic web vitality” are constantly changing

The question answered by John Mueller did not specify whether it was related to field data or lab data.

The person asking the question is simply asked when the results of basic web vitals stop changing, without specifying field data or lab data.

Mueller answered the question from the viewpoint of Field Data, which is the primary findings of web vitals based on real-world visitors.

This is the question asked:

When do basic web biomarkers give consistent, valid information?

It is constantly changing without changing any data on the site.”

The results of ‘Basic Web Vitality’ are affected by random factors

The answer focused on the randomness of the data used to generate field data for web vital indicators.

John Mueller replied:

“So, I think this is probably a side effect of how we handle basic web vitality and refresh the page experience.

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

And that’s something where if users from a variety of different backgrounds, locations, and different types of devices access your pages, you’re likely to see some fluctuation over time there as well.”

Internet speeds vary

A fast server and fast website is only one part of having a high BBI score and the only part that is under the control of SEO or site publisher.

As John Mueller said, there are a variety of other factors that affect basic web vitality results and many of these factors cannot always be controlled.

Even if the website is hosted on a fast dedicated server, network congestion, old cell phone used by the website visitor and poor mobile data connection can all greatly affect the performance of the basic web vitality of a fast website and thus contribute to the reduction of good basic web vitals on field data.

Some site visitors may be using a newer model cell phone that has a powerful processor running on a newer 5G network. This site visitor will score highly on the basic field data of web vitals.

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

So even though the lab data is not a real visitor, an actual Chrome based bot travels over the internet to visit the webpage being tested. This will also affect the results of the simulated lab data.

Google’s Web.dev posted an explanation of why field data always changes and not always the same from month to month.

Web.dev explains it as follows:

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

The wrong data Your site is the complete set of all performance data collected from your users.

For example, CrUX reports show a breakdown 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 may have a very good experience while others may have a very poor experience.”

Whether you’re testing a site for lab data or reviewing real-world field data, results for core web vitals tend to fluctuate and likely won’t provide a consistent score month after month.

the quote

When are the results for “basic web vitality” settled?

Watch John Mueller answer the question at 19:05:

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