How to calculate variance and standard deviation? ^{1}
Definition:
To illustrate the variability of a group of scores, in statistics, we use "variance" or "standard deviation". We define the deviation of a single score as its distance from the mean: Variance is symbolized by ^{2}. Standard Deviation is . N is the number of scores.
However, when the data of a sample are used to estimate the variance of the population from which the sample was drawn, the population variance estimate is computed instead:
Summary of the calculation procedures:

subtract the mean from each score

square each result

sum all the square

divide the sum of square by N. Now you get variance
If divide the sum of square by N1, you will get the population variance estimate

Standard deviation is just the positive square root of the variance
