Let be observations of the variable . The skewness of is defined as:
…where is ’s standard deviation.
Notice that the second and third moment of , which we’ll denote by and respectively, are involved in this definition:
When skewness is negative, mass tends to be concentrated to the right of the mean (with a tail on the left); when it’s positive, it tends to be concentrated to its left (with a tail on the right):
Negative and positive skew are sometimes referred to as left-skewed and right-skewed, respectively.