# Variance

March 29, 2018

Back to: Random Testing

The variance of a set of numbers, *x _{n}*,

*n*= 1, …,

*N*, with a known mean value of μ, is given by Equation 3:

(1)

Equation 3

If the mean value has been estimated from a set of numbers, then the variance is given by Equation 4.

(2)

Equation 4

The value in the denominator before the summation is the number of independent values used (or *degrees of freedom*) in the summation. Since x has been computed from the same set of numbers as used in Eq. 4, the value of the last term in the summation in Eq. 4 is predetermined, so the number of independent values in the summation is *N* – 1.

For a vibration signal with a mean value of zero, the variance is equal to the mean-square value. In general:

(3)

Equation 5

When summing independent random variables, the variances add (even with non-zero means).