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I know this isn't very common on the gmat but can this approach be used to calculate S.D?

Example 1,3,5

Mean 9/3 = 3

subtract difference from each term so -2,0,2

square each term 4,0,4

divide the above sum by number of terms 8/3

S.d = square root of the sum = square root of 8/3?

is this approach correct?

\(SD = \sqrt{Sum(Ai - Aavg)^2/n}\) Ai represents the terms from 1 to n. Aavg is the average of all the terms. n is the number of terms.

Basically, you are using this formula.

The process is tedious so it is not asked on GMAT. All you need to know is how to compare SDs of two sets and how a change in a set affects the SD. Check out this post: http://www.veritasprep.com/blog/2012/06 ... deviation/ _________________

GMAT does ask you to compute the standard deviation of a finite set, however they will provide you with an expression that will tell you how to compute the standard deviation. You will just need to interpret the different terms in the expression for the standard deviation. For example, they will give you an explicit formula for the standard deviation and in the description they may say things like m is the mean of the numbers in the set and n is the number of terms in the list, etc. So technically you don't have to know the formula for the standard deviation, but they do expect you to interpret the expression for standard deviation. I would classify this type of a question as medium, partly because a lot of students will be thrown off by the tedious looking expression for standard deviation.

Now, I also do want to point out that you may also be asked to recognize that the standard deviation is related to the sum of the square of the deviations from the mean in a data sufficiency context, however this question would likely fall in the Q51 range.

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