Those formulas are just going to be confusing; the notation is completely different from anything you'll ever see on the GMAT. Technically, it's version (b) that you would use on the GMAT; you can ignore the others. But, as Testluv says, you never need to compute standard deviation on the GMAT. You *do* need to understand what standard deviation *measures*, so you need to be able to judge whether the set {0, 5, 5, 10} has a larger standard deviation than the set {99, 100, 100, 101} (it does, because elements in the first set tend to be further from the mean).
When you calculate standard deviation, the steps are as follows (and be aware that if you find yourself computing a standard deviation on the GMAT, you've missed something - it is *never* required, so what follows is for interest only). Let's take one of those rare sets that will actually give us an integer answer for standard deviation - {2, 3, 4, 5, 6, 7, 8}.
* You first find the mean, which is 5.
* You then find how far each element is from the mean and list those distances: 3, 2, 1, 0, 1, 2, 3.
* Now we square each of these numbers and average the result: (3^2 + 2^2 + 1^2 + 0^2 + 1^2 + 2^2 + 3^2)/7 = 28/7 = 4.
* Finally we take the square root of this average: sqrt(4) = 2 = standard deviation
Notice in the third step, we divide by 7, the number of elements in the set. In your formula sheet, this is n; we do not subtract 1 from n.
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