Lab Data vs. Field Data
Core Web Vitals are measured in two ways.
In the lab (Lab Data)
Lab Data is based on a simulated page load in a consistent, controlled environment. Specifically, Lab Data is collected by emulating a Moto G4 phone on a fast 3G connection.
Lab Data does not capture data from user interactions, so First Input Delay (FID) is not available in synthetic tools. In its place, lab tools report on Total Blocking Time (TBT), a proxy metric.
Do not rely on the scores shown in Lab Data because they may not reflect the users’ experience in the real world.
In the field (Field Data)
Field Data, also known as Real User Metrics (RUM), is based on real users actually loading and interacting with the page. Field Data comes from the last 28-days of data in Chrome User Experience Report (CrUX), which collects anonymized data from actual user visits and interactions.
Google uses the 75th percentile of all page views to determine the value of each CWV. For example, a page will have a Good LCP score if at least 75% of page views meet the Good threshold.
Considering the above, an important fact to consider moving forward: Google uses 28 days’ worth of data and the 75th percentiles. We will get into more details later, but know that this computation method delays CWV reporting in all Field Data tools, which can be frustrating.
Finally, it’s important to note that Google uses Field Data to measure Core Web Vitals for search rankings.
When there is no Field Data
Field Data will only be available if your page has generated 28 days of data. If there is no Field Data, some measurement tools will display origin data; this is the Field Data for the whole domain.