Calculate the mean of the set. In the example, you add 1 + 2 + 3 + 4 to obtain the result of 10, and then you divide 10 by 4, which yields a mean of 2.5.
Subtract each variable from the mean and square each answer. In the example, (1-2.5)^2, (2-2.5)^2, (3-2.5)^2, (4-2.5)^2, which equals 2.25, 0.25, 0.25, and 2.25.
Add the numbers calculated in Step 2. In the example, 2.25 + 0.25 + 0.25 + 2.25 = 5.
Divide the number calculated in Step 3 by the number of variables. In the example, 5 is divided by 4, which yields a variance of 1.25.