Skewness

Skewness is a measure of how the size of length of one tail on one side of a distribution compares to the size or length of the tail on the other side of the distribution. When a distribution is symmetric (can be cut in half to produce mirror images), it has a skewness of 0. In these distributions, the mean, median, and mode are typically equal to each other.

When a distribution has a positive skewness, the right side tail is either fatter or longer than the left side tail. Distributions with positive skewness typically have a mean which is greater than the median which is greater than the mode. When a distribution has a left-side tail that is longer or fatter than the right-side tail, it has negative skewness. In this case, typically the mean is less than the median which is less than the mode.