Short definition
Field data — performance data from real website visitors, anonymously collected and aggregated over a 28-day period, as opposed to simulated lab tests.
Also known as: CrUX, real user data, RUM.
You can measure a website in two ways: in a lab or in the field. Both produce numbers, but they answer different questions. Field data is the information your visitors hand you for free — if you know where to look.
What field data is
Field data is performance data from real website visits. Google collects it anonymously from Chrome users who share it, aggregates it in 28-day windows, and publishes it as the Chrome UX Report (CrUX). You do not see individual visitors; you see percentiles. The well-known p75 of a metric is the value where three quarters of visits were faster.
That contrasts with lab data: a simulated test environment that measures the same thing every run. Lighthouse in PageSpeed Insights is such a lab. Great for diagnostics and comparison, less reliable as the single truth about what visitors experience. In practice, lab and field can diverge sharply; in a measurement of 19 website builders, lab LCP was up to 16.7x worse than the field value, mostly because external scripts load slowly only in the lab.
Where to find field data
PageSpeed Insights shows it at the top of every report, above the lab score. The Core Web Vitals report in Search Console shows it per page group for your own site. The public CrUX dashboard displays trends going back 25 weeks for any domain that meets the traffic threshold.
When to use which data
Use field data to know where you stand: do your real visitors clear the Core Web Vitals thresholds or not? Use lab data to find out why something is slow, because only the lab gives reproducible, diagnostic detail. The fixed order: measure the field first, then use the lab as a search engine for the cause.