Lab-based performance tests often give different results from data that's collected from real users. This article explains the differences between the

Why does Lighthouse lab data not match field data?

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2021-08-16 17:00:08

Lab-based performance tests often give different results from data that's collected from real users. This article explains the differences between these two ways to measure site speed and how they lead to different test results.

We'll focus on Lighthouse and the Chrome User Experience report, but note that there are many other ways to collect lab or field data.

Lighthouse is the tool PageSpeed Insights uses to collect lab data. It can run tests on demand in a test environment using a fixed network and CPU speed.

Because of this, lab data is a great way to identify how changes on a website impact its performance. As the environment is fixed, results between different test runs are relatively consistent.

The lab environment is also able to capture detailed diagnostic data. Everything you see on PageSpeed Insights below the metric summary is comes from the Lighthouse test.

Field data is collected by measuring the experience of real users. Google collects data on Core Web Vitals through Chrome and publishes it in the Chrome User Experience Report (CrUX).