variance

Variance is a statistical term that measures the mean-square difference bwteen a value and its expected value (often its mean, μ). It is usually written σ², and it is the square of the standard deviatoon (σ), which is often a more meaningful measure, albeit less useful for calculations. For a simple series of data items (x_i) the variance is calculated by
      σ² = ( Σ_i (x_i &munus; μ) ² However, for more complicated models the expected value for each data item may be differenct.

Defined on page 139

Used on pages 139, 141