\begin{definition}[Definition:Sample Variance of Stochastic Process]
Let $S$ be a stochastic process giving rise to a time series $T$.
The '''sample variance''' of $S$ over a set of $N$ successive values $\set {z_1, z_2, \dotsb, z_N}$ is defined as:
:$\ds \hat \sigma_z^2 := \frac 1 N \sum_{t \mathop = 1}^N \paren {z_t - \overline z}^2$
where $\overline z$ denotes the sample mean of $S$ over $\set {z_1, z_2, \dotsb, z_N}$.
\end{definition}