Let $X \in \text{Random}(\mathbb{R})$ be a D3161: Random real number such that

(i)	\begin{equation} \mathbb{E} |X|^2 < \infty \end{equation}

The standard deviation of $X$ is the D4767: Unsigned real number \begin{equation} \text{Std} X : = \left( \mathbb{E}|X - \mathbb{E} X|^2 \right)^{1/2} \end{equation}