Last updated: 03/04/2015
The variance is the average difference of each value in the sample from the mean. As, by definition, the sum of the difference of each value from the mean would equal 0, the difference of each value from the mean is squared. (1st image below)
The standard deviation is simply the square root of the variance. (2nd image below)
It is very important to understand the difference between the standard deviation and the standard error, see SE vs. SD calculation.
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