Variance

Measures the variance of a series of values around their mean value.

$Var(X)=\frac{1}{N}\sum_i{(X_i-\bar{X})^2}$

where $\bar{X}=\frac{1}{N}*\sum_i{X_i}$

The variance can be computed incrementally:

$Var(X)=\bar{X^2}-(\bar{X}*\bar{X})$

Thus, we sum all $X_i$ and all $X_i^2$ , then build the difference of their mean values.