Skewness of Binomial Distribution

Theorem
Let $X$ be a discrete random variable with a Binomial distribution with parameter $n$ and $p$ for some $n \in \N$ and $0 \le p \le 1$.

Then the skewness $\gamma_1$ of $X$ is given by:


 * $\gamma_1 = \dfrac {1 - 2 p} {\sqrt {n p q} }$

where $q = 1 - p$.

Proof
From Skewness in terms of Non-Central Moments:


 * $\gamma_1 = \dfrac {\expect {X^3} - 3 \mu \sigma^2 - \mu^3} {\sigma^3}$

where $\mu$ is the mean of $X$, and $\sigma$ the standard deviation.

We have, by Expectation of Binomial Distribution:


 * $\mu = n p$

By Variance of Binomial Distribution, we also have:


 * $\var X = \sigma^2 = n p \paren {1 - p}$

so:


 * $\sigma = \sqrt {n p \paren {1 - p} }$

To calculate $\gamma_1$, we must calculate $\expect {X^3}$.

We find this using the moment generating function of $X$, $M_X$.

By Moment Generating Function of Binomial Distribution, this is given by:


 * $\map {M_X} t = \paren {1 - p + p e^t}^n$

From Moment in terms of Moment Generating Function:


 * $\expect {X^3} = \map {M_X'''} 0$

So:

Setting $t = 0$:

Plugging this result back into our equation above: