CWV JavaScript library

[Matthew Delambo]

I work on a highly trafficked website with a custom analytics tracker. We're trying to compare the CrUX values for our site against our field data using the CWV JavaScript library. One problem that we are running into with the library is that we see very low reporting across all of our Chrome page views. For example, we only see CLS reporting on ~31% of page views  and FID at ~43%. This may be partly due to our analytics tracker. I brought this issue up with a maintainer of the library, but I closed it with a question that I hope this wider group can help answer:

If you are using the CWV JS library on the websites that you maintain, how often do metrics get reported on page views?

[Rick Viscomi]

+cc Philip Walton

[Matthew Delambo]

We made some adjustments to our analytics tracker, and we're seeing much better reporting for CLS! Thanks Philip! Here's our current breakdown for how often CWVs are reporting for a page on our site over the last day:

FID: 39%
CLS: 93%
LCP: 79%

If I may piggyback one more question for the CrUX team: Does CrUX see a similar breakdown for CWVs on page views? If not, do you think that the JS library is missing important data, and that we need to account for that in some way when we compare the two?

I'd imagine that since the CrUX reporting happens from within the browser internals that it would have near 100% reporting on page views. The reason for our concerns are that we'd like to make sure that our field data matches, at least directionally, the CrUX data.

[Rick Viscomi]

Hi Matthew,

We don't have any public data on the percent of page views that report on the CWV metrics. The closest we have is origin-level data in BigQuery showing that 67% of origins have enough FID data to include in the report, 97% LCP, and 97% CLS. These don't necessarily translate to page view-level reporting, but it's helpful to see that FID coverage is expected to be much lower due to the dependency on user input.

Rick

[François-Xavier Brottier]

Hi everyone,

this is an interesting topic but the reply that helped Matthew is now deleted.

Are there general guidelines to properly setup such a tracker and have an optimal reporting %?

Thanks

FX

[Matthew DeLambo]

Sorry for the confusion! Philip helped me in an issue that I opened in the the CWV library repository. I cross posted here with the hope that I would get more insight from CrUX users.