Web development and performance
What is Field data?
Field data is performance measured on real visitors using their own devices and connections, as opposed to lab data from a simulated test. Google’s Chrome User Experience Report aggregates it, and it is what Core Web Vitals assessment uses.
Also called: RUM · real user monitoring · CrUX
Why it differs from lab results
- Real devices span a decade of hardware, not one simulated mid-range phone
- Real connections vary constantly; simulated throttling does not
- Real users interact, which is the only way INP can be measured at all
- Repeat visitors have warm caches, which no cold lab test reflects
The 28-day window
Assessment uses a rolling 28-day aggregate, so a fix deployed today appears gradually and is only fully reflected after about a month. Lab measurements move immediately, which is precisely why they are useful for verifying a fix landed.
Getting it for a small site
Sites with insufficient traffic may have no CrUX data at all. In that case a lightweight web-vitals script reporting to your own endpoint gives you real numbers, and it is a few kilobytes rather than an analytics platform.
Where this is covered in depth
A definition can only go so far. Core Web Vitals, and the order that actually reduces them covers this properly — 3 minutes, free, no email required.
Who wrote this
Anas Bin Masud builds e-commerce sites and does technical SEO for businesses in the UK, Canada and Pakistan. These definitions come from client work rather than from a content brief — where an entry describes a mistake, it is usually one found on a real site. More about how I work.