Google provides a number of tools to help site owners monitor their Core Web Vitals scores. These tools fall into two main categories:
The problem is that sometimes the data reported by lab tools can be quite a bit different from the data reported by field tools! Your lab data might indicate that your site performs great, but your field data suggests it needs improvement. Alternatively, your field data may say all your pages are good, but your lab data may report a very low score.
The following real example of a PageSpeed Insights report from web.dev shows that in some cases lab and field data can be different across all of the Core Web Vitals metrics:
Difference between tools is an understandable source of confusion for developers. This post explains the main reasons these differences could exist—with specific examples covering each of the Core Web Vitals metrics—and what to do when you find differences on your pages.
To understand why lab and field tools might report different values—even for the exact same web page—you need to understand the difference between lab and field data.