To find a percentile rank all of the numbers in a given set from largest to smallest. Then, go along the list of numbers to the number that represents the percent you are looking for. For example:

In a set of 100 numbers, the number that is 10% of the way through the set is the tenth number. The value of this number is the tenth percentile.

Since the number 50% of the way through the set is the middle number and, since the median of a set is the value of the middle number, the 50th percentile of a set is equal to the median.

Since the 50th percentile represents a percentile divisible by ten it is what is called a decile. Since it is also divisible by 25 it is also a quartile.

Percentiles are a statistical tool used to examine the spread of a set.