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A certain list of 100 data has an average (arithmetic mean)

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A certain list of 100 data has an average (arithmetic mean) [#permalink]

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New post 22 Mar 2008, 17:17
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A certain list of 100 data has an average (arithmetic mean) of 6 and a standard deviation of d, where d is positive. Which of the following pairs of data, when added to the list must result in a list of 102 data with standard deviation less than d?

(A) -6 & 0
(B) 0 & 0
(C) 0 & 6
(D) 0 & 12
(E) 6 & 6

Pls explain. Thanks.
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Re: PS:Standard Deviation [#permalink]

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New post 22 Mar 2008, 19:02
E - Same question was posted a day ago. SD depends on the deviation from mean, so for values in E the deviations are zero which is the minimum we can get.
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Re: PS:Standard Deviation [#permalink]

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New post 22 Mar 2008, 19:59
sreehari wrote:
E - Same question was posted a day ago. SD depends on the deviation from mean, so for values in E the deviations are zero which is the minimum we can get.


Sorry guys, can you give more discription? Why "values in E the deviations are zero" make the new SD less than the old?
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Re: PS:Standard Deviation [#permalink]

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New post 22 Mar 2008, 20:07
add 6 to the population of mean 6 does not change it mean
so the 2 new squares of difference are 0 -> reduce ur SD (divide by N+2)
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Re: PS:Standard Deviation [#permalink]

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New post 22 Mar 2008, 20:19
AlbertNTN wrote:
add 6 to the population of mean 6 does not change it mean
so the 2 new squares of difference are 0 -> reduce ur SD (divide by N+2)


Thank you,

I plan to make a stupid calculation.
:oops:
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Re: PS:Standard Deviation   [#permalink] 22 Mar 2008, 20:19
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A certain list of 100 data has an average (arithmetic mean)

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