Lab Data vs. Field Data
Covered here
- What lab data and field data are
- Where each appears in PageSpeed Insights and Search Console
- Why the two often disagree
- How to use each in an audit
Short answer
Field data comes from real Chrome users through the Chrome UX Report and shows how pages actually perform over 28 days. Lab data comes from a single simulated test, such as Lighthouse, under fixed conditions. Use field data to judge whether pages pass Core Web Vitals, and lab data to diagnose causes and test fixes.
A client says, “Our PageSpeed score is 38! We are failing!” But Search Console shows every URL passing Core Web Vitals. Another client scores 92 in Lighthouse, yet real users fail LCP. Both are confused because they are mixing two kinds of data.
Field data: what real users experience
- Collected from real Chrome visits through CrUX
- Rolling 28-day window, judged at the 75th percentile
- Shown in Search Console’s Core Web Vitals report and the top of PageSpeed Insights (“Discover what your real users are experiencing”)
- This is what Google uses for page experience
Lab data: a controlled test
- One simulated visit, usually a mid-range phone on a throttled network
- From Lighthouse, PageSpeed Insights (lower section), Chrome DevTools, or WebPageTest
- Instant results, repeatable, with detailed diagnostics
- Not used for rankings
How to use both
- Judge with field data: which templates fail which metric on mobile?
- Diagnose with lab data: run Lighthouse on sample URLs from failing groups and read the opportunities and diagnostics.
- Verify with both: lab results confirm a fix immediately, field data confirms it about four weeks later.
Practice: lab or field?
Sort each item into the right group.
-
Search Console Core Web Vitals report
Answer: Field data.
-
Lighthouse score of 54
Answer: Lab data.
-
CrUX 75th percentile LCP of 3.1 s
Answer: Field data.
-
Chrome DevTools Performance panel trace
Answer: Lab data.
-
WebPageTest run from Singapore
Answer: Lab data.
Common mistakes
- Reporting the Lighthouse score as the main KPI.
- Expecting field data to change the day after a fix.
- Ignoring missing field data. Low-traffic pages need lab testing and origin data.
Your action
Next, you will identify slow pages and templates across the whole site.
Key terms
- Field data
- Performance measured from real user visits, also called real user monitoring (RUM).
- CrUX (Chrome UX Report)
- Google's public dataset of real Chrome user experience, used in Search Console and PageSpeed Insights.
- Lab data
- Performance measured in a controlled test with a set device, network, and location.
- Lighthouse performance score
- A 0 to 100 lab score. It is not used by Google for rankings.
Questions beginners ask
Does the Lighthouse score affect rankings?
No. Google uses field Core Web Vitals. The Lighthouse score is a diagnostic guide.
Why does PageSpeed Insights show no field data?
The URL or site does not have enough Chrome traffic in CrUX. Use origin-level data, your own RUM, or lab tests instead.
Why does lab LCP differ from field LCP?
Real users have different devices, networks, caches, and locations. Lab tests use one fixed setup.
How long until field data reflects a fix?
CrUX uses a rolling 28-day window, so expect about four weeks for full impact.
Related lessons
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