Page "Kurtosis" Paragraph 9

which is also known as excess kurtosis.

The " minus 3 " at the end of this formula is often explained as a correction to make the kurtosis of the normal distribution equal to zero.

Another reason can be seen by looking at the formula for the kurtosis of the sum of random variables.

Suppose that Y is the sum of n identically distributed independent random variables all with the same distribution as X.

Then