Sample Variance Versus Population Variance

We have used in class both formulas for variance:

(1)
\begin{align} \sigma^2 = \frac{ \sum (x - \bar{x})^2 }{n} \end{align}
(2)
\begin{align} s^2 = \frac{\sum(x-\bar{x})^2}{n-1} \end{align}

The first is called the population variance, and the second is called the sample variance.

Notice the difference between them is the n-1. This is called Bessel's correction. An explanation of why it is used is here:
http://en.wikipedia.org/wiki/Bessel%27s_correction

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