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core-web-vitals

How are the Chrome User Experience Report origin values calculated

How are the origin values calculated in the Chrome User Experience Report?

Solution

  • By scores I presume you mean the values surfaced in the likes of PageSpeed Insights (PSI) and Google Search Console?

    These are calculated at the 75th percentile over the previous 28 days for the dimension reported against. You can see this if you use the "Expanded View" section of PSI:

    PSI Expanded View

    That works best with an example. Let's say we had the following page views over that 28-day period, ordered by LCP time:

    page view # visitor # page LCP time
    1 1 page1.html 2.0 seconds
    2 2 page1.html 2.2 seconds
    3 3 page1.html 2.4 seconds
    4 4 page1.html 3.5 seconds
    5 1 page2.html 3.6 seconds
    6 1 page2.html 4.0 seconds
    7 2 page2.html 4.2 seconds
    8 3 page2.html 4.3 seconds
    9 1 page3.html 6.0 seconds
    10 1 page3.html 7.0 seconds

    The 75th percentile is the smallest value that 75% of the page views achieved.

    CrUX also has minimum eligibility thresholds which Chrome does not disclose, but let's say it's 3 visitors for this example.

    In this case:

    • The page1.html p75 reported LCP would be 2.4 seconds based on the 4 page views for that page.
    • The page2.html p75 reported LCP would be 4.2 seconds based on the 4 page views for that page.
    • The page3.html p75 LCP would not be reported as below the eligibility threshold of a minimum of 3 visitors (but it would be 7.0 seconds based on the 2 pages views if it had been reported).
    • The overall origin p75 value would be 4.3 seconds based on the 10 page views for that origin.

    It's a moving 28-day window so let's say the first page view was 28-days ago and the rest were within the last week. The next day, if there were no new pages views, then we'd ignore the first value and then we'd have:

    • The page1.html p75 reported LCP would now be 3.5 seconds based on the 3 page views for that page.
    • The page2.html p75 reported LCP would still be 4.2 seconds based on the 4 page views for that page.
    • The page3.html p75 LCP would still not be reported as below the eligibility threshold of a minimum of 3 visitors (but it would be 7.0 seconds based on the 2 pages views if it had been reported).
    • The overall origin p75 value would still be 4.3 seconds based on the 9 page views for that origin.

    There are also some other nuances but that broadly covers how it works.

    Other resources: