gauravhesoyam wrote: ↑Tue Jul 30, 2013 10:55 am
Hi I'm having a hard time dealing with SD questions. Please help me out with the following question.
Sets A,B,C are shown below. If number 100 is included in each of these sets, which of the following represents the correct ordering of the sets in terms of the absolute increase in their standard deviation, from largest to smallest.
A{30,50,70,90,110} B{-20,-10,0,10,20} C{30,35,40,45,50}
A) A,C,B
B) A,B,C
C) C,A,B
D) B,A,C
E) B,C,A
----ASIDE-------------------------
For the purposes of the GMAT, it's sufficient to think of Standard Deviation as the
Average Distance from the Mean.
Here's what I mean:
Consider these two sets: Set A {7,9,10,14} and set B {1,8,13,18}
The mean of set A = 10 and the mean of set B = 10
How do the Standard Deviations compare? Well, since the numbers in set B deviate the more from the mean than do the numbers in set A, we can see that the standard deviation of set B must be greater than the standard deviation of set A.
Alternatively, let's examine the Average Distance from the Mean for each set.
Set A {7,9,10,14}
Mean =
10
7 is a distance of 3 from the mean of
10
9 is a distance of 1 from the mean of
10
10 is a distance of 0 from the mean of
10
14 is a distance of 4 from the mean of
10
So, the average distance from the mean = (3+1+0+4)/4 =
2
B {1,8,13,18}
Mean =
10
1 is a distance of 9 from the mean of
10
8 is a distance of 2 from the mean of
10
13 is a distance of 3 from the mean of
10
18 is a distance of 8 from the mean of
10
So, the average distance from the mean = (9+2+3+8)/4 =
5.5
IMPORTANT: I'm
not saying that the Standard Deviation of set A equals 2, and I'm
not saying that the Standard Deviation of set B equals 5.5 (They are reasonably close however).
What I am saying is that the average distance from the mean can help us see that the standard deviation of set B must be greater than the standard deviation of set A.
More importantly, the average distance from the mean is a useful way to think of standard deviation. This model is a convenient way to handle most standard deviation questions on the GMAT.
------NOW ONTO THE QUESTION!!!---------------
So, for this question, we have:
Mean of set A = 70
Mean of set B = 0
Mean of set C = 40
100 is furthest away from the mean of 0 in set B, so this will cause the GREATEST change in standard deviation.
100 is next furthest away from the mean of 40 in set C, so this will cause the 2nd greatest change in standard deviation.
100 is closest to the mean of 70 in set A, so this will cause the LEAST change in standard deviation.
Answer: E
Cheers,
Brent
