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need the standard deviation for dummies book

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allenkt Senior | Next Rank: 100 Posts Default Avatar
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need the standard deviation for dummies book

Post Sun May 06, 2007 1:29 pm
I have a very basic understanding of standard deviation but apparently I need more. I've taken both of the GMATPrep tests and both times a standard deviation question appeared near the beginning. And both times I was stumped.

Pretty much all I know about standard deviation is that it is a way to express how far away from the mean a value is. But I don't understand how to manipulate it or how other numbers would affect it. The first question on the quantitative section was something like I had a certain set of numbers and the mean was 33. Then what numbers would need to be added to change the standard deviation. Obviously that's not exactly it but that is the kind of information I'm clueless about.

Can someone point me to a resource that will help me gain a basic understanding of this concept?



Last edited by allenkt on Mon May 07, 2007 6:41 am; edited 1 time in total

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allenkt Senior | Next Rank: 100 Posts Default Avatar
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Post Mon May 07, 2007 6:40 am
To answer my own question, I did find some helpful information in the math review of OG 11. Here is what I learned, hopefully this will help others who are struggling as I was:

To find the standard deviation:

1) find the mean of the numbers
2) find the difference between each number and the mean
3) square each number from step 2
4) find the average of all the numbers in step 3
5) take the nonnegative square root of that average

The number from step 5 = standard deviation.

I doubt I would have to actually calculate the sd in a problem, so the key point to remember is that the standard deviation depends most on values that are farthest from the mean. Adding numbers to a set that are close to the mean will not change the standard deviation very much. Adding numbers that are far from the mean will change the standard deviation more.

Hope that helps.

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Stacey Koprince GMAT Instructor
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Post Thu May 17, 2007 7:19 pm
Yeah, I've never seen them ask us to calculate SD in a problem - it would take too long.

Also, FYI, we're unlikely to see more than one SD question (if any) on the test - I definitely applaud you for figuring this out since you saw one on each test - but for others, studying SD may not be the best use of time. This falls into the "very infrequent" category.

Also FYI - it actually doesn't matter too much even if the question is towards the beginning. One question won't make or break a score.

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