percentile

Percentile

The value below which a given percentage of observations fall — the 90th percentile (p90) means 90% of values are below that point.

When you'd see it: Performance engineering (p99 latency = the worst experience 1% of users had), compensation benchmarking (50th percentile salary = median), sales performance (a rep at the 80th percentile by quota attainment). Percentile distributions are the honest way to report skewed data.

Why it matters: Percentiles slice distributions without assuming any particular shape. The difference between p50 and p95 latency tells you whether your worst-case users are slightly worse or catastrophically worse than your typical users — information the average completely hides.

Common mistakes: Confusing 'the 90th percentile' with 'the top 10%.' Someone at the 90th percentile is above 90% of others — they are in the top 10%. The wording gets inverted in casual conversation and causes errors in compensation and performance conversations.

Related median, mean, outlier, normal distribution