Variance

For a series of (x_i) elements, the variance is calculated as

$$ {\text var}\ =\ \frac{\sum (x_i - x_{avg})^2}{N} $$

with x_avg the average value of all the x_i, and N the total number of elements in the series.

Compare with CoVariance, that relates two different series of elements.

See http://en.wikipedia.org/wiki/Variance