Report by days

[Tanya Tritelnitskaya]

Hello!

I know that CrUX aggregate data for 28 days. So just to clarify I'd like to ask if I noticed our site's score falling on May 26, does it mean that some issues had happened on April 26 or during these 28 days?

Also could you tell me please if I can generate report by day of the month.  

Thanks a lot in advance for your help!!

[Barry Pollard]

Since you specifically give a date of 26th of the month, I'm assuming you are talking about the CrUX API (which also powers PageSpeed Insights and other tools) or Google Search Console? If so, then it's a daily moving 28-day average based on the 75th percentile of data.

This is different to the monthly dataset (used by the CrUX Dashboard) which measures the whole of the given month (again at the 75th percentile).

This means if you notice your site dropping scores on 26th May, then enough page views have happened to mean that 75% of your visitors got that lower rating. Depending on the traffic profile, this could mean it happened up to 28 days before, at some point during the 28-days, or even quite recently (e.g. if you had a surge in slower traffic recently which made up 75% of your page views over the 28 days). Assuming even traffic every day, and that there was a radical change (both very big assumptions on my part), then it probably happened approximately 21 days before you noticed the drop (75% of the 28 days is 21 days).

I've more details on all this lag here: https://www.smashingmagazine.com/2021/04/complete-guide-measure-core-web-vitals/#the-chrome-user-experience-report-crux if that's helpful.

> Also could you tell me please if I can generate report by day of the month.  

The CrUX dashboard is only available monthly. If you query the daily API yourself, you can build up a daily dashboard (though it will still be subject to the 28-day aggregation, and hence lag), but this cannot be done retrospectively - you have to query it every day in advance and then have the results available to query.

A RUM solution that tracks Core Web Vitals often gives much more granular detail for all your pages, and also is not subject to the 28-day aggregation like CrUX, Though be aware that it can be more noisy because of that - one of the reasons CrUX aggregates over 28 days is to reduce the noise of daily fluctuations and visitor differences.

Thanks,

Barry

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