Given a variable with values , the quantity

 

 

is defined as the moment of . The first moment of , , is the arithmetic mean, .

 

 

The moment about the mean of , , is defined as

 

 

If , then . If , then , the variance.

 

 

The moment about any center, , of is defined as

 

 

where are the deviations of from . If , Eq. (3) reduces to Eq. (1), for which Eq. (1) is often referred to as the moment about zero.