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## Standard Deviation

tagged by: Brent@GMATPrepNow

This topic has 2 expert replies and 2 member replies
abhirup1711 Senior | Next Rank: 100 Posts
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#### Standard Deviation

Tue Jun 25, 2013 1:45 am
Elapsed Time: 00:00
• Lap #[LAPCOUNT] ([LAPTIME])
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

Can I ask an expert to explain this?

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faraz_jeddah Master | Next Rank: 500 Posts
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Tue Jun 25, 2013 3:06 am
I'm not an expert.

I took the values as 1,2,-3,4

And only B or 2) does the SD remain unchanged.

What is OA and source?

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Brent@GMATPrepNow GMAT Instructor
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Tue Jun 25, 2013 5:54 am
abhirup1711 wrote:
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
It's important to note that standard deviation is a measurement of the variation (or dispersion) of a set of numbers.
So, adding the same value to every number in a set will have no effect on the variation of those numbers.

Example
The set {1,2,3,4} has a certain standard deviation. Adding 13 to every number in the set gives us {14,15,16,17}, and we can see that the standard deviation of this new set will be the same as the standard deviation of the old set (since the dispersion for each set is identical).

Here's another question that tests this concept:
- http://www.beatthegmat.com/statistics-ps-t145051.html

Cheers,
Brent

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faraz_jeddah Master | Next Rank: 500 Posts
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Tue Jun 25, 2013 9:08 am
wohoo so I was correct

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ceilidh.erickson GMAT Instructor
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Tue Jun 25, 2013 10:51 am
To explain why the other answer choices are wrong...

As Brent said, standard deviation (SD) is the variance (or more simply put, the spread) of a set of numbers. Adding the same value to each will effectively shift all of the values up or down the number line the same amount, but will not affect the spread.

1) |a|,|b|,|c|,|d|
If the set were all positive numbers, such as [1, 2, 3, 4], or all negative numbers, [-1, -2, -3, -4], then taking the absolute value would not affect the SD. If just one of those values was negative, though, as in Faraz's example: [-3, 1, 2, 4], then the SD of the absolute values would be different; [1, 2, 3, 4] - the spread is smaller.

2) a+1, b+1, c+1, d+1
Same SD, as discussed above.

3) 5a, 5b, 5c, 5d
Here, by multiplying each value by 5, we increase the range by a factor of 5 as well: [1, 2, 3, 4] --> [5, 10, 15, 20]. We can see that the set is more spread out, so the SD will be greater.

4) a^4,b^4,c^4,d^4
Taking terms to the 4th power will certainly affect the SD (unless every term is the same). If the terms are positive integers, they become more spread out: [1, 2, 3, 4] --> [1, 16, 81, 256]. If they're fractions, they become less spread out: [1, 1/2, 1/3, 1/4] --> [1, 1/16, 1/81, 1/256]. Any negative terms would become positive, etc.

5) 2a+1, 2b+1, 2c+1, 2d+1
Same issue as in 3). By multiplying each value by 2, the range is doubled. The +1 has no effect on SD, though.

For more on SD, see: http://www.beatthegmat.com/call-for-help-standard-deviation-ds-t153493.html#545420

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