75th percentile
Google Chrome is collecting data of most users and then exposing the 75th percentile via different channels, such as Core Web Vitals, PageSpeed Insights, Google Search Console and CrUX APIs. But in this context, what does the 75th percentile mean?
75th percentile explained
The shortest and non-mathematical explanation probably is as following in the context of Real User Monitoring data:
Google looks at the best experience within the group of 25% worst experiences.
Erwin Hofman
In other words, Google basically wants you (in order to pass Core Web Vitals) to deliver an optimal experience for at least 75% of the pageviews on your site or shop. That means that numbers shown in different Google tools are telling you that 74% of pageviews had an experience that was better than that number. The remaining 25% had a worse experience.
Looking at the experiences from this group of users instead of average numbers is a healthy decision for your conversion and business. That's because you're then making sure that you aren't ignoring those that are more like to be browsing with more challenging conditions.
Why Google is using the 75th percentile
Which the above explanation in mind, looking at the 75th percentile makes sense. But in early PageSpeed Insights days, Google was using different percentiles for different metrics, for example the 75th and 95th percentile. That can end up being confusing. So they went with the 75th percentile for all metrics, and for the following reason:
There will be extremes in field data and removing the worst 25% experiences, it is intended to give a value that the majority of visitors can reasonably be expected to achieve.
web.dev
Using the 75th percentile instead of the 95th percentile makes sense, as 95th percentile might not be realistic. That would also include users under very challenging circumstances. The 50th percentile could be used, but you obviously don't just want to optimize for your average visitor. Using the 75th percentile then makes even more sense.
Limitations of the 75th percentile
The 75th percentile is a very good starting point. However, when you're serious about site speed, you would want to know experiences across different percentiles as well. That's because when doing optimizations, there is always a chance that any improvements is a coincidence, instead of being the result of your efforts.
When analyzing the result of pagespeed related deploys (or other deploys of which you aren't aware it could impact pagespeed), but sure to look at different percentile numbers to correlate improved or regressed pagespeed data with your deploys.
Percentiles in RUMvision
RUMvision dashboarding allows users to select the percentile you want to look at. Moreover:
- Domain percentile
you can configure a default percentile for your domain's profile via domain dashboard settings. All users will use this percentile by default to ensure that everyone is looking at the same data - Percentile chart
For reasons explained above, you want to precent blind spots by only looking at a single percentile. That's why RUMvision's technical panel is equipped with a percentile chart.
Below is a screenshot of this percentile chart:
