kurtosis

The term kurtosis refers to the degree to which a probability distribution has tails, regions of small probability extending high and/or low from the highest-probability distribution. A widely-used measure (due to Karl Pearson) is the fourth Pearson moment of the distribution. A formula:

           Σ (Yi-m)4
kurtosis = —————————
           Σ (Yi-m)2

• Y_i - probability density.
• m - mean of all the Y_i.

That of the normal distribution is always 3, so sometimes 3 is subtracted from the above formula, which is then more precisely termed the excess kurtosis.