In statistics there are many useful indices that we can use to describe data. Some common terms are median, mode and mean.

Arithmetic mean is a simple but very important index to describe the center point of data. It is the sum of values in the data divided by the number of values. It is very sensitive to outliers. As for example, in a class of eight kids and one teacher, the mean is ( 3 + 4 + 4 + 4 + 4 + 4 + 4 + 4 + 50 ) ÷ 9 = 9, which cannot represent the center point of age in this class.