What is field data?

Field data is the sum of performance measurements experienced by real users on real devices and real networks.

It is the opposite of lab measurement: an auditing tool opens the page in its own environment assuming a fixed device and a fixed network; field data shows the distribution of real use.

The two diverge routinely, and usually in the same direction: lab good, field poor. The reason is that a substantial share of real users browse on low-powered devices and variable connections.

This is the data used in ranking. A verdict of “we are good” can therefore only be given from field data; a lab score is a diagnostic instrument, not a target.

Delayed — a twenty-eight-day window; the effect of today’s fix shows weeks later.
Requires a threshold — no data is collected for sites without enough traffic.
Aggregated — it may be reported for groups of similar pages rather than page by page.

The way past all three limits is to set up your own measurement: the browser reports all three metrics to the page, and those values can be sent to your own system. It requires no cookies and collects no personal data.

The place where the two measurements DIVERGE is itself a diagnostic: if lab is good and field is poor, the cause is usually the device and network distribution, and the treatment is reducing JavaScript. If both are poor, the diagnosis is made in the lab.

The practical rhythm is this: field data is read monthly and looked at for general health; lab measurement runs on every deployment and catches regressions. The two work at different rhythms because they answer different questions.

On this site field data is pulled into a cache file and shown on screen — but if there is no data the field is not rendered at all. The suite measures both directions: if a value exists the number on the strip must match the cache exactly, and if not the word “measured” must never appear.

“What is slow?” — lab. Repeatable, detailed.
“Did the change work?” — lab. Comparison under identical conditions.
“What are our users experiencing?” — field. The only honest answer.
“How does Google see us?” — field. The data used in ranking.
“How is our INP?” — field. It cannot be measured reliably in a lab.

The fifth line is critical: the interaction responsiveness metric looks by definition at real user interactions, and an auditing tool does not interact with the page.

And a note on scope: field data is collected only for pages that receive real visits. A page just published or never visited does not appear in that data at all, however technically flawless it may be.

If you want your own site measured and fixed at the source, our Core Web Vitals optimization starts from real-user data and delivers LCP, INP and CLS against a budget.

Performance

Focus trap A focus trap is when keyboard focus enters a region and cannot leave it again, and WCAG prohibits it explicitly at level A. Accessibility

Font subset A font subset is a font file regenerated with only the characters that are needed. Multilingual

G

GEO (Generative Engine Optimization) is the work of increasing the likelihood that a site is chosen as a source in the answers generated by AI systems. Answer engines

H

hreflang is the markup that declares a page’s different language and region versions to search engines. Multilingual

I

IndexNow is an open protocol that tells search engines immediately that a page has changed. Answer engines

INP (Interaction to Next Paint) is the time from a user interaction until the page responds visually, and Google counts anything under 200 milliseconds as good. Performance

J

JSON-LD (JavaScript Object Notation for Linked Data) is a structured data format that declares a page’s meaning in a machine-readable form without changing the page’s appearance at all. Answer engines

L

Language tag (BCP 47) A language tag is the standard coding form that tells machines which language a text is in. Multilingual

LCP (Largest Contentful Paint) is the paint time of the largest content element in a page’s visible area, and Google counts anything under 2.5 seconds as good. Performance

Live region A live region is an area of a page whose changes a screen reader has to announce to the user. Accessibility

llms.txt is an optional file placed in a website’s root directory that summarises, in Markdown, what the site is about and where its most important pages are for language models. Answer engines

Locale A locale is the setting that defines a user’s language and region together and determines the formatting rules. Multilingual

Localisation (Localization) is adapting content to the culture, habits and search language of a target market; it is a wider undertaking than translation. Multilingual

R

Reflow A reflow is the browser recomputing the positions and sizes of elements on a page after a change. Performance

Relative luminance is the value expressing a colour’s perceived lightness between 0 and 1, and it is the basis of the contrast ratio calculation. Accessibility

S

Screen reader A screen reader is assistive technology that turns on-screen content into speech or braille output and lets the user navigate by keyboard. Accessibility

Semantic HTML means choosing each element for its meaning rather than its appearance — a heading element for a heading, a button element for a button, a list element for a list. Accessibility

Structured data is an additional layer that labels a page’s content with a standard vocabulary machines can understand. Answer engines

T

Text expansion is the same content being of different lengths in different languages, and the layout having to flex accordingly. Multilingual

Translation memory A translation memory is a database that stores previously translated sentences and suggests them when the same or a similar sentence comes up again. Multilingual

TTFB (Time to First Byte) is the time between the browser sending a request and receiving the first byte from the server. Performance

W

WCAG (Web Content Accessibility Guidelines) is the international standard published by the W3C that defines the accessibility of web content through testable success criteria. Accessibility

X

x-default is a special hreflang value declaring the default page to show when no language version matches the user’s language. Multilingual

What is field data?

Field data is the sum of performance measurements experienced by real users on real devices and real networks.

It is the opposite of lab measurement: an auditing tool opens the page in its own environment assuming a fixed device and a fixed network; field data shows the distribution of real use.

The two diverge routinely, and usually in the same direction: lab good, field poor. The reason is that a substantial share of real users browse on low-powered devices and variable connections.

This is the data used in ranking. A verdict of “we are good” can therefore only be given from field data; a lab score is a diagnostic instrument, not a target.

Delayed — a twenty-eight-day window; the effect of today’s fix shows weeks later.
Requires a threshold — no data is collected for sites without enough traffic.
Aggregated — it may be reported for groups of similar pages rather than page by page.

“What is slow?” — lab. Repeatable, detailed.
“Did the change work?” — lab. Comparison under identical conditions.
“What are our users experiencing?” — field. The only honest answer.
“How does Google see us?” — field. The data used in ranking.
“How is our INP?” — field. It cannot be measured reliably in a lab.