Starting in May 2021, Google will begin using their Core Web Vitals metrics to inform search ranking. Will they will use field or lab data as the source of these metrics?

Our site scores well on field data but poorly on lab data according to Google PageSpeed Insights.

  • What is Field data and Lab data?
    – Rob
    Jan 20, 2021 at 11:13
  • @Rob Field data is data collected by Chrome from sites used by real users. Lab data is data collected when running the site on Google's own servers that exist specifically for the purpose of testing site perf. Jan 20, 2021 at 11:25
  • My site does well in lab data but "needs improvement" and "does not pass" when using field data. See this screenshot: i.stack.imgur.com/FeWSZ.png I am also looking for an answer to this question and have started a bounty in the hopes of getting one. Jan 27, 2021 at 10:50

1 Answer 1


It will be field data. In their announcement of the algorithm update, Google are clear about using "real-world, user-centered metrics". By definition, lab measurements are not real-world, and what I see in my lab might not be the same as what you see in yours.

Google themselves are clear on this in their documentation for speed tools: lab data "might not capture real-world bottlenecks" and "cannot correlate against real-world page KPIs".

The Web Vitals documentation is also clear on this point (emphasis mine):

"While all of the Core Web Vitals are, first and foremost, field metrics, many of them are also measurable in the lab."

The data used will almost certainly come from their CrUX (Chrome User Experience Report), since it "provides user experience metrics for how real-world Chrome users experience popular destinations on the web".

Edit: This is now confirmed by Google.

  • I'm a bit surprised by this for a couple reasons. 1. The field data seems to include a large number of users with very slow connections. I don't think there is any way to get a good score on some of the metrics with field data. 2. Google doesn't seem to have very good field data coverage. On my site with the domain name of my last name, I have nearly 1000 pages, but Google Search Console only reports that it has field data for 17 of them. Jan 27, 2021 at 20:24
  • (1) That's precisely the point. They want to influence real world user experience, and the real world does contain a lot of people with relatively slow connections. Even so, ≤2.5s for a "good" LCP rating doesn't seem prohibitively high. (2) Your experience may not be typical, lower traffic sites won't easily meet data quality thresholds, it's unlikely that Google are making their entire dataset public, and we're 4 months away from roll-out.
    – GDVS
    Jan 28, 2021 at 10:17

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