The standard deviation of a data set is a measure that tells something about how much the observations of the data set deviate from the mean value of the data set. In that respect it is similar to mean deviation but it is calculated differently.
Calculating Standard Deviation
Standard deviation is calculated based on the variance of the data set. The standard deviation is calculated as the square root of the variance of the data set. More formally:
standardDeviation(data) = squareRoot( variance(data) )