Performance / Quiz
Reading the Tail: p95, p99, and Lies
Why you cannot average percentiles, what coordinated omission hides, and how fan-out amplifies the tail users feel.
- Difficulty
- Medium
- Format
- Quiz
- Points
- 150
- Estimate
- 9 min
// MISSION BRIEF
Your Mission
Averages comfort; percentiles convict. This quiz goes past "look at p99" into the traps: averaging percentiles, coordinated omission, tail amplification under fan-out, and reading a distribution instead of a single hero number.
// FIRST CONTACT
Battle teaser
Why can you NOT average two servers' p95 values to get the fleet p95?
- APercentiles are not additive or averageable; you must aggregate the raw distributions (or use a mergeable sketch like t-digest/HDR histogram)
- BBecause p95 is always the same on every server
- CBecause averaging requires the mean, not p95
- DYou can, the average of percentiles is the fleet percentile
Answers, scoring, hints, and the full battle stay sealed.
// SKILL TAGS
performancepercentilestail-latencymetrics