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Standard deviation

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by andrew.ng » Wed Oct 28, 2009 7:09 am
A list of 4 numbers a, b, c, d has standard deviation equal to that of which of the following list?

1) |a|,|b|,|c|,|d|
2) a+1, b+1, c+1, d+1
3) 5a, 5b, 5c, 5d
4) a^4,b^4,c^4,d^4
5) 2a+1, 2b+1, 2c+1, 2d+1
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Source: — Data Sufficiency |
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by mp2437 » Wed Oct 28, 2009 8:14 am
Choice B. Mean will change with a uniform shift in all numbers, but standard deviation doesn't.
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gmat prep q.

by khushboo143 » Mon Nov 02, 2009 2:53 pm
i came acroos one mor std dev question...couldn solve it..

A certain list of 100 data has an average (arithmethic mean) 6 and std dev d ,where d is positive. Which of the following data when added to the list must result in the list of 102 with std dev lass than d?
-6 &0
0&0
0&6
0&12
6&6

Correct ans is 6&6
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Re: gmat prep q.

by mehravikas » Mon Nov 02, 2009 7:32 pm
There are some rules of thumb for standard deviation. You don't need to calculate SD on gmat.

1. if you add data that is farther from the mean, the standard deviation will increase
2. if you add data that is closer to the mean, the standard deviation will decrease

khushboo143 wrote:i came acroos one mor std dev question...couldn solve it..

A certain list of 100 data has an average (arithmethic mean) 6 and std dev d ,where d is positive. Which of the following data when added to the list must result in the list of 102 with std dev lass than d?
-6 &0
0&0
0&6
0&12
6&6

Correct ans is 6&6
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