What is Field data?
Field data is produced in the browsers of real people, on their devices and their networks. For websites it is publicly available: Chrome collects it in the Chrome User Experience Report, which is where PageSpeed Insights and the Search Console report take their values for Largest Contentful Paint, Interaction to Next Paint and Cumulative Layout Shift. The lab test alongside it measures one standardised session. It is useful for finding causes, but it is no evidence of how fast your site actually is for your audience.
Two quirks are worth knowing. First, every value is a rolling mean across 28 days, so today's figure contains the four weeks before it. After a rebuild, old and new site are mixed for four weeks and no clean statement is possible before that. Second, an address needs a minimum number of visitors or it does not appear in the dataset at all. For many small sites the number therefore does not exist, which leaves the lab test, honestly labelled as such.
The values count for search, but they are not a switch. Google writes in its own documentation that Core Web Vitals are used by its ranking systems, that there is no single signal for it, and that relevant content still wins with a weaker page experience. The practical benefit therefore sits less in ranking than in the substance: a page that responds late loses people before they have read anything.
Why does Field data matter?
Field data is the one speed figure nobody can argue with, because it comes from visitors' devices rather than from the agency's test environment. Its memory is limited, though: the history reaches back 40 weeks, and for addresses below the visitor threshold it does not exist at all. Before a rebuild, the value belongs in your own records.
Field data in practice
- 01PageSpeed Insights shows field data for the last 28 days at the top and the lab test below it; for a small site the upper panel is often empty.
- 02The Core Web Vitals report in Search Console groups URLs by field data, not by a test run.
- 03After a relaunch the value stays a mixture of old and new site for four weeks, because the mean runs across 28 days.
