Essential Ideas Section 10.4: Measures of Dispersion Range The range of a set of data is the difference between the largest and the smallest numbers in the set. Standard Deviation The standard deviation of a population, denoted by , is the square root of the variance. To find it, carry out these steps: Determine the mean of the set of numbers. Subtract the mean from each number in the set. Square each of these differences. Find the sum of the squares of these differences. Divide this sum by the number of pieces of data. The result is the variance of a population: the mean of the squares of the deviations from the mean. Take the square root of the variance.