Calculate the standard deviation of the data. For this calculation compute the difference between each one of the observations and the mean, (this number may be positive or negative), then multiply each of these differences by itself, then add all of these products, and finally divide the sum total by the number of observations. You'll get the "VARIANCE". Now take the square root of this number and you get what is called the "STANDARD DEVIATION" that measures the dispersion of the data from their mean.