Add all the numbers from your list of percentage differences together. Note the total.
Divide your total by the number of percentage differences in your list. For example, if the total of your percentage differences is 55 and you have 5 numbers in your list, you would divide 55 by 5 (55/5). Write down the result of the calculation. This is the average of your percentage differences.
Subtract the average of your percentage differences from each percentage difference on your list. For example, if the average of your percentage differences is 11 and your percentage differences are 9 percent, 12 percent, 11 percent, 13 percent and 10 percnet, then subtract 11 from 9, 12, 11, 13 and 10. Write down each of the resulting numbers.
Find the square of each of your resulting numbers. If one of your resulting numbers is 2, for example (13-11 = 2), multiply it by itself (2 x 2) to find its square (4). Make a note of each of these squared numbers.
Add your squared numbers together. Write down this sum.
Divide the sum of your squared numbers by the number of percentage differences in your list minus 1. For example, if the total number of percentage differences in your original list is 5 and the sum of your squared numbers is 7, then you will divide 7 by 4 (since 5 - 1 = 4). Make a note of this number, your divided sum of squares.
Use the calculator to find the square root of your sum of squares. This number is the standard deviation of your percentage differences.