Field data (real-user data)
What is field data (real-user data)?
Field data is speed measured on real visits, as opposed to lab data, which comes from one simulated test. In PageSpeed Insights and Search Console, Google takes it from Chrome users over the last 28 days and reports the 75th percentile: the value that three visits in four come in at or under. Core Web Vitals are assessed on field data, and a site with little traffic may have none.
Example
A product page in a homeware shop gets 1,000 mobile visits in 28 days. 700 come from recent phones on good connections, with an LCP of 1.8 seconds; the other 300 come from older phones, with an LCP of 4 seconds. Line the visits up from fastest to slowest and the 750th lands among the slow ones, so the 75th percentile is 4 seconds and the page fails, even though seven visits in ten were fast. To pass, at least 750 visits must come in under 2.5 seconds, which means the mobile optimisation has to reach the weaker phones too.
A lab test of the same page might score 85, because it simulates one device on one connection. The two numbers do not contradict each other; they measure different things.
Why it matters for a business
Field data shows what your customers actually went through, not what happened in one test, so it is the fair way to judge whether speed work paid off. It has two limits. First, it lags: after a fix on 1 March, the 28-day window is not made up entirely of new visits until about 29 March, so the report improves gradually. Second, it comes only from Chrome on Android and on desktop. Visits from an iPhone are not counted at all, and small sites often lack enough visits for a report; for them, lab tests and the site’s own measurements have to fill the gap.
What to ask
- Is the before-and-after result field data or a lab test? The two should never be mixed in one comparison.
- Is there field data for this specific page, or only for the whole site?
- Which period does it cover, and how much of it comes after the fix?
- What does Search Console show for mobile and for desktop, separately?
In our website speed optimisation work we record both kinds of data before we start, so the result afterwards is compared like with like.