Standard deviation is a positive square root of variance. Variance is the dispersion of observations of a data set from mean value. Standard deviation is equal to the square root of mean of square of deviation from the mean value or average. To calculate the standard deviation first find the average of data set. Secondly, calculate the deviations of all the observations from the average. Take sum of all the deviations and divide it by total number of observations to get a mean. The value obtained is variance. Take square root of this variance, you will get your required standard deviation.

Standard deviation= [ summation(observation - mean )

Standard deviation= [ summation(observation - mean )

^{2}/ number of observations ]^{1/2}