CrUX bug: same wrong data since multiple 28-day-timeframes

[E. O.]

I realize a strange behavior of the CrUX data seeming to me like a bug:

Since Mai 2023 I run a site. Since start, through multiple timeframes, where CrUX updates its site data, I realize the following CrUX data behavior:

For my site PageSpeed Insights tool delivers only origin data. So, for the startpage of my site I get since the site is online same CrUX data, and same Lighthouse data, as on the screenshot:

What I mean, it should be a bug: it is the red CLS metric. 

In my understanding, the live test results are those by Lighthouse. Lighthouse CLS is deeply green - and this is and was always so. If a metric is constant in the timeframe of 28 days, CrUX takes it into its database. But here CrUX doesn't do it: the Lighthouse CLS is always green - but the CrUX CLS is always red.

How can it be so?

[Barry Pollard]

This suggests one of two things:

1. A different page is causing the CLS than the oost being tested. Note that PSI is only able to provide Origin-level CLS and not specific data for this URL:

or

2. Your shifts are caused post-load, rather than from the load that Lighthouse can check. This could be due to scrolling causing CLS (e.g. lazy loaded images without dimensions) or CLS that happens due to an interaction but after the 500ms grace period. CrUX measures CLS across the whole page life cycle from real users.

Both of these issues are discussed in the Optimize CLS guide: https://web.dev/articles/optimize-cls#understanding_where_your_shifts_are_coming_from

Thanks,

Barry

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