Definition
A percentile is a value below which a given percentage of observations in a dataset falls. The nth percentile is the value that is greater than or equal to n% of the data. Percentiles help you understand where a specific value stands relative to the rest of the data.
How Percentiles Work
Percentiles divide your data into 100 equal parts, each representing 1% of the distribution.
On a standardized test, you score 720 out of 800.
Your score is at the 92nd percentile.
This means you scored higher than 92% of all test takers. It does not mean you got 92% of questions right - percentiles measure your rank relative to others, not your raw performance.
Why It Matters
Percentiles are used everywhere that ranking matters. Pediatricians track children's growth using percentile charts. Standardized test scores are reported as percentiles. Companies use the 95th and 99th percentile to measure server response times and identify performance issues.
Key percentiles have special names: the 25th percentile is Q1 (first quartile), the 50th is the median (Q2), and the 75th is Q3 (third quartile). Together, Q1, Q2, and Q3 form the basis of box plots, a popular way to visualize data spread.
Percentiles tell you how a value compares to the rest of the data. They are about ranking and relative position, not raw scores.