In statistics, a percentile (or centile) is the value of a variable below which a certain percent of observations fall. For example, the 20th percentile is the value (or score) below which 20 percent of the observations may be found. The term percentile and the related term percentile rank are often used in the reporting of scores from norm-referenced tests.

The 25th percentile is also known as the first quartile (Q₁), the 50th percentile as the median or second quartile (Q₂), and the 75th percentile as the third quartile (Q₃).

There is no standard definition of percentile, however all definitions yield similar results when the number of observations is very large.

One definition of percentile, often given in texts, is that the P-th percentile (Failed to parse (Missing texvc executable; please see math/README to configure.): 0 \le P < 100 ) of N ordered values (arranged from least to greatest) is obtained by first calculating the (ordinal) rank

rounding the result to the nearest integer, and then taking the value that corresponds to that rank. (Note that the rounded value of n is just the least integer which exceeds Failed to parse (Missing texvc executable; please see math/README to configure.): \frac{P}{100} \times N .)