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Webpagetest.org offers "own" Lighthouse test. It can run as a part of the general WPT test or as a standalone test from the URL /lighthouse.

My question: is the WPT's Lighthouse test a LAB or a FIELD ?

I've got this question because of the following experience:

I've fixed a CLS issue on a website. I realized the CLS issue was gone when I tested this website with Lighthouse out of Chrome DevTools and PageSpeed Insights.

In the PageSpeed Insights results, it was displayed very clear:

  • while the field data displayed the CLS issue (correct, because this is the data from the past 28 days, and I fixed the issue just today),
  • the lab data displayed no CLS issue (correct too - the lab data comes in real-time from a specific environment).

And then, I tested the website with WPT's Lighthouse the CLS issue was displayed again. This is the cause, why I think WPT's Lighthouse data is the field and decided to ask.

PS: Firstly I thought to publish this question at https://webmasters.stackexchange.com/, but the most important tag [webpagetest] doesn't exist there.

Evgeniy
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  • It runs 3 different tests and combines them. So it certainly isn't field data, it is lab data. Whether the test is performed in the same way at the exact same screen size is not clear though (same formula but differences in throttling etc. could cause differences). Do you have a URL for us to test against that has differences between lighthouse and WPT? – GrahamTheDev Dec 01 '21 at 15:47

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It was my fail on terminology.

Lighthouse does always lab tests, independently of where it starts. On PageSpeed Insights there are just two different results sources:

  1. Field data, from CrUX database,
  2. Lab data, from Lighthouse test
Evgeniy
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