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Which of the following has the same standard deviation as {s, r, t}?

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Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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Which of the following has the same standard deviation as {s, r, t}?

I. {r - 2, s - 2, t - 2}
II. {0, s - t, s - r}
III. {|r|, |s|, |t|}

(A) I only
(B) II only
(C) III only
(D) I and II only
(E) I and III only
[Reveal] Spoiler: OA

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Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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New post 07 Apr 2018, 14:06
the answer should be E

I. The spread is consistent with each value so it shouldn't affect the SD
II. We don't know the spread of each value. It could be big or small
III. The absolute value sign measures the distance from 0. So it should be the same.
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Re: Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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New post 07 Apr 2018, 14:43
For Same STD Deviation, both the sets need to have values dispersed in a similar way...
eg Option 1) all values are same as Original delta of 2
Option 2) first value is 0, second is s-t, and then s-r all 3 are differently spread.
Option 3) absolute values of s,r,t are going to have same spread as original set.

So Answer E with option 1 and option 2.


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Re: Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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New post 08 Apr 2018, 01:25
hdavies wrote:
the answer should be E

I. The spread is consistent with each value so it shouldn't affect the SD
II. We don't know the spread of each value. It could be big or small
III. The absolute value sign measures the distance from 0. So it should be the same.



Absolute can change the value of average from the original so E cannot be the option

IMO A
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Re: Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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New post 08 Apr 2018, 06:04
Dear Bunuel, what’s the answer? Thank you.


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Re: Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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Re: Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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New post 08 Apr 2018, 08:11
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I think the answer is D.

I. {r - 2, s - 2, t - 2}
If every element in a set is increased/decreased by a constant, c, the standard deviation does not change. Thus, this set has the same standard deviation as {s, r, t}.

II. {0, s - t, s - r}
We can subtract constant, s, from each of the 3 elements in this set and not change the standard deviation.
The set becomes: {-s, -t, -r}
This is a mirror image of {s,t,r}. The relative positions of {-s,-t,-r} on the number line are the same as the relative positions of {s, r, t}. Thus, standard deviation is the same.

III. {|r|, |s|, |t|}
If {r,s,t} = {-1,1,1}, we have some positive standard deviation.
Then {|r|, |s|, |t|} = {1,1,1}, which has standard deviation = 0.
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Re: Which of the following has the same standard deviation as {s, r, t}? [#permalink]

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New post 08 Apr 2018, 09:54
The answer should be (D) i.e. Both I AND II

I.
The SD for a set of number remains same if added or subtracted by a constant. So, (r-s, s-2, and t-2 ) has same SD as (r, s, and t).
This is because the spread between the nos. is not changing. Only the set of nos. is shifting 2 units away.

II
(-s, -r, and -t) are just the mirror image of (s, r and t) on the number line. Thus the set (-s, -r and -t) has the same spread as the set (s, r and t).
Thus the SD for (-s, -r and -t) is same as that of (s, r and t).
Also if a set is added by a constant number its SD remains the same.
So, (-s+s, -r+s and -t+s) has same SD that of (-s, -r and -t)
Thus, (0, s-t, s-r) has same SD as that of (s, r, t)
Re: Which of the following has the same standard deviation as {s, r, t}?   [#permalink] 08 Apr 2018, 09:54
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Which of the following has the same standard deviation as {s, r, t}?

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