The 99th percentile is the highest percentile you can get. It means you are among the top scorers since you scored higher than 99% of the group who took the test. Only 1 in 100 the group scores in this range, so it places you at the very top of the pool.
Most of us want to be in the 1% percent that scored the highest. We want to be the top earners, performers, the fastest, the fittest, and generally the best in our crafts.
However, in application performance metrics, the users in the 99th percentile have the highest latency score (or response time, first contentfull paint, etc.…). Therefore, they had the worst experience in the measured period.
Why you should care?
You probably think, okay, it's only 1% who experience some shitty performance. Why should I care about 1%?
The 1% percent could happen where your system doesn't scale, for example, when working on too much data - which usually occurs for the top clients.
The 1% can be the edge cases when one user's action might affect other users' experience when the system runs on shared resources.
The 1% percent could lead your way through capacity planning. By analyzing this metric, you can determine the level of infrastructure, compute power, or database capacity needed to handle the occasional high-latency requests without sacrificing overall performance.
Outliers can skew the mean score. Removing them can give a more accurate representation of the average user experience.
Monitoring this metric is only the first step; the next step is finding the root cause of the score and fixing it for good. But it is only sometimes straightforward.
In my newsletter "Percentile 99th", I'll cover, every other week, the principles, how-to's, and deep dive analysis into different technologies, including real-life examples from my experience and top companies, to help you decrease the percentile 99th metric and help you become the top 1% percent of engineers who knows to improve the system to work like magic (and design new systems better), a real performance speedster.
See you there, at Percentile 99th
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