Comparison
Field datavsLab data
Field data
the dashboard shows the metric at the 75th percentile across everyone who actually loaded the page last month.
Measurements collected from real visits on real devices and networks. It is the only source that tells you what your users experience, and it is what Core Web Vitals are assessed against. It is also slow to react and hard to attribute — a regression appears days later as a shifted distribution, with no profile attached to explain it, which is why lab and field data are used together rather than as alternatives.
Full entry →Lab data
you ran the audit on your laptop, got a score of 98, and shipped a page that is slow for everyone else.
Measurements from a controlled synthetic run: one device, one network profile, no cache, no extensions, no real users. It is reproducible and therefore good for comparing two builds and for catching regressions in CI. It is a poor description of reality, because it has no distribution — no slow devices, no third-party variance, no returning visitors.
Full entry →