Average the numbers to obtain the "mean." Do this by adding together all the numbers and dividing the sum by the count of numbers. For the set "-1,000, 1, 100," the average is -299.67.
Subtract the mean from each number. Multiply the result by itself (square it), and record the result. In this case, you should obtain the list: 490,462.1089, 90,402.4489, 159,736.1089.
Average the list of numbers you obtained in Step 2. The result will be the variance. For this example, it would be: 246,866.8889. As you can see, the variance will usually be a very large number.