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# Gmat Club Math Book Doubt

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Manager
Joined: 10 Jan 2013
Posts: 156
Location: India
Concentration: General Management, Strategy
GMAT 1: 600 Q43 V30
GPA: 3.95
Gmat Club Math Book Doubt  [#permalink]

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18 Aug 2018, 23:08
1
Hello Everyone,

I have a doubt in the attached snippet from the GMAT Club math book.

I would be grateful if someone can help me decode this in plain English (especially points 1,2,3,4)

Bunuel chetan2u
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SVP
Joined: 15 Jul 2015
Posts: 1901
Location: India
GMAT 1: 780 Q50 V51
GRE 1: Q170 V169
Gmat Club Math Book Doubt  [#permalink]

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19 Aug 2018, 20:28
2
saurabh9gupta wrote:
Hello Everyone,

I have a doubt in the attached snippet from the GMAT Club math book.

I would be grateful if someone can help me decode this in plain English (especially points 1,2,3,4)

Bunuel chetan2u
The first bit is just saying that if you add a number (positive or negative) to every element in the set, the standard deviation remains the same. For example:

1. For the set $${1, 2, 3, 4, 5, 6, 7, 8, 9, 10}$$ we have $$σ=2.87$$ (approximate value) and mean $$μ=5.5$$
2. For the set $${11, 12, 13, 14, 15, 16, 17, 18, 19, 20}$$ (adding 10 to each element) we have standard deviation $$σ=2.87$$ (approximate value) and mean $$μ=15.5$$

What matters for the standard deviation is how far the elements are from the mean. The actual values of the elements do not matter here.

The second portion seems to be talking about the inclusion of one more element in the set, but as far as I know, it is not very easy to predict the impact of such an inclusion on the standard deviation, so I'll leave that to someone else. What we usually discuss is the inclusion of a pair of values that are symmetric about the mean.

Let's say the original set is $${1, 3, 5}$$. Here $$σ=1.633$$ and $$μ=3$$. If we now put a pair of elements $$x$$ and $$y$$ into the set, the new set will be $${1, 3, 5, x, y}$$. If $$x$$ and $$y$$ are very far from the mean, the standard deviation will go up, and if $$x$$ and $$y$$ are very close to the mean, the standard deviation will go down.

1. $$x=(3-4)=-1$$ and $$y=(3+4)=7$$: The new set is $${-1, 1, 3, 5, 7}$$ and its standard deviation is $$2.828$$, which is greater than the original standard deviation.

2. $$x=(3-1.633)=1.367$$ and $$y=3+1.633=4.633$$: The new set is $${1, 1.367, 3, 4.633, 5}$$ and its standard deviation is $$1.633$$, which is the same as the original standard deviation.

3. $$x=(3-1)=2$$ and $$y=3+1=4$$: The new set is $${1, 2, 3, 4, 5}$$ and its standard deviation is $$1.414$$, which is less than the original standard deviation.

4. $$x=(3-0)=3$$ and $$y=3+0=3$$: The new set is $${1, 3, 3, 3, 5}$$ and its standard deviation is $$1.265$$, which is the lowest the standard deviation can go with a pair of new elements added to the set (the difference between these values and the mean is $$0$$).
_________________
Gmat Club Math Book Doubt &nbs [#permalink] 19 Aug 2018, 20:28
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# Gmat Club Math Book Doubt

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