Percentile

A percentile is a value indicates the percentage of the data set values that are equal to or below it. The percentile is often used to estimate the extremes of a distribution. For example, if Jill is the 4th tallest in a group of 20 people, then 80% of the group is the same height or shorter than Jill. Thus Jill is said to be in the 80th percentile.

Percentiles are often used in classifying extremes of statistics in various climate situations. For example, a daily temperature is classified as extreme when that day's high (or low) is in the 90th percentile of the same day in previous years.

Values can also be separated into groups known as deciles and quartiles. Deciles split the data into tens while quartiles split it into quarters (fours). From the earlier example, Jill would be in the upper quartile as she is taller than over 75% of the other people, and would be in the 8th decile. Figure 1 visually represents percentiles are quartiles on a bell curve.

Figure 1. Interquartile range diagram showing minimum, 25th percentile, median, 75th percentile, and maximum.

This term is often confused with percent. While percent describes an individual value, percentile compares the values to each other.