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Intern  Joined: 12 Nov 2009
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A certain list of 100 data has an average (arithmetic mean) of 6 and  [#permalink]

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Question Stats: 77% (00:54) correct 23% (01:32) wrong based on 284 sessions

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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 and 0
B. 0 and 0
C. 0 and 6
D. 0 and 12
E. 6 and 6

Originally posted by Gmatter111 on 08 Dec 2009, 03:54.
Last edited by Bunuel on 10 Oct 2019, 04:45, edited 4 times in total.
Renamed the topic and edited the question.
Math Expert V
Joined: 02 Sep 2009
Posts: 60555
A certain list of 100 data has an average (arithmetic mean) of 6 and  [#permalink]

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itsreallytime wrote:
Hi Sriharimurthy,
thats a great explanation,but i am stuck with one point.
when u r calculating the new SD.how can u subtract 6 hrom both x and y??6 is the mean of the old set...for examle if u add 0 and 0 the new mean will be 600/102.ie,will be less than 6. the mean will change as well..
Kindly explain this point...will really appriciate..
Thanx

Yes, mean won't be the same for A, B, and C but as it will differ from 6 very little then the calculations above will still hold true, so it doesn't really matter.

As for the question, GMAT SD questions are fairly straightforward and don't require actual calculation of SD, they are about the general understanding of the concept.

So we have:
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 and 0
B. 0 and 0
C. 0 and 6
C. 0 and 12
D. 6 and 6

"Standard deviation shows how much variation there is from the mean. A low standard deviation indicates that the data points tend to be very close to the mean, whereas high standard deviation indicates that the data are spread out over a large range of values."

So when we add numbers, which are far from the mean we are stretching the set making SD bigger and when we add numbers which are close to the mean we are shrinking the set making SD smaller.

According to the above adding two numbers which are closest to the mean will shrink the set most, thus decreasing SD by the greatest amount.

Closest to the mean are 6 and 6 (actually these numbers equal to the mean) thus adding them will definitely shrink the set, thus decreasing SD.

For more on this issue please check Standard Deviation chapter of Math Book (link in my signature) and the following two topics for practice:
http://gmatclub.com/forum/ps-questions- ... 85897.html
http://gmatclub.com/forum/ds-questions- ... 85896.html

Hope it helps.
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Manager  Joined: 29 Oct 2009
Posts: 170
GMAT 1: 750 Q50 V42
A certain list of 100 data has an average (arithmetic mean) of 6 and  [#permalink]

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Gmatter111 wrote:
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 and 0
B. 0 and 0
C. 0 and 6
D. 0 and 12
E. 6 and 6

Logically, SD of a set will decrease if you add numbers which are equal to its mean. Thus the answer should be E.

I will however provide you with mathematical reasoning to justify the above statement.

S.D. for hundred numbers = d = $$\sqrt{\frac{S}{100}}$$ where 'S' is sum of the squares of the difference between each number and the mean.

Now let the two numbers added be 'x' and 'y'.

S.D after adding the two numbers will be = $$\sqrt{\frac{S}{102}+\frac{(x-6)^2 + (y-6)^2}{102}}$$

Now it is obvious that $$\frac{S}{102}$$ will be less than $$\frac{S}{100}$$. Also, the minimum value of $$\frac{(x-6)^2 + (y-6)^2}{102}$$ will be 0 when both 'x' and 'y' are equal to 6.

Thus if the two numbers added are equal to the mean, the SD of the set must decrease. (Unless of course SD of the set was 0 to start with (not in our case) and then in that case SD will remain constant).

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##### General Discussion
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Re: A certain list of 100 data has an average (arithmetic mean) of 6 and  [#permalink]

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3
I agree with sriharimurthy.
In most cases Standard Deviation question can be solved without using SD formula.
The main idea is to think of SD as of average deviation from mean: $$average (|x - x_{av}|)$$.
So, 6 and 6 add two 0s in average formula and must reduce SD.
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Re: A certain list of 100 data has an average (arithmetic mean) of 6 and  [#permalink]

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Hi Sriharimurthy,
thats a great explanation,but i am stuck with one point.
when u r calculating the new SD.how can u subtract 6 hrom both x and y??6 is the mean of the old set...for examle if u add 0 and 0 the new mean will be 600/102.ie,will be less than 6. the mean will change as well..
Kindly explain this point...will really appriciate..
Thanx
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Re: A certain list of 100 data has an average (arithmetic mean) of 6 and  [#permalink]

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Gmatter111 wrote:
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 and 0
B. 0 and 0
C. 0 and 6
D. 0 and 12
E. 6 and 6

Thanks!

We should keep in mind that the closer the data points are to the mean, the “lower” the standard deviation. Thus, since the mean is 6, when we add two additional values of 6 to the data set, the standard deviation will decrease.

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Re: A certain list of 100 data has an average (arithmetic mean) of 6 and  [#permalink]

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I have prepared a youtube video to explain this question. Hope you'll like it.

Check the explanation on this video
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Re: A certain list of 100 data has an average (arithmetic mean)  [#permalink]

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_________________ Re: A certain list of 100 data has an average (arithmetic mean)   [#permalink] 10 Oct 2019, 04:44
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