SD-Good One

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nitya34: Master | Next Rank: 500 Posts; Posts: 179; Joined: Tue Jan 08, 2008 7:23 pm; Thanked: 11 times; GMAT Score:590

by nitya34 » Mon Jul 13, 2009 6:18 am

Which data set has the greater standard deviation, data set K or data set L ?

(1) Every element in data set K is a multiple of 3.

(2) Every element in data set L is a multiple of 9.

OA is............ E...............Thanks

Last edited by nitya34 on Mon Jul 13, 2009 11:19 pm, edited 1 time in total.

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Source: Beat The GMAT — Data Sufficiency | Mon Jul 13, 2009 6:18 am

niraj_a: Legendary Member; Posts: 708; Joined: Sun Jun 01, 2008 4:59 amFollowed by: 1 member; Location: USA; Thanked: 13 times

by niraj_a » Mon Jul 13, 2009 6:39 am

not sure between C and E. i guess E.

since both sets contain numbers that are evenly spaced, the mean = median for both sets. but, if one set has more / less numbers than the other set, then the actual value of the mean, and by extension, the actual value of the SD could be different.

i sure hope i'm right.

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tohellandback: Legendary Member; Posts: 752; Joined: Sun May 17, 2009 11:04 pm; Location: Tokyo; Thanked: 81 times; GMAT Score:680

by tohellandback » Mon Jul 13, 2009 7:11 am

IMO E
consider sets {3,6,9} and {9}
now consider sets {3} and {9,18,27}

The powers of two are bloody impolite!!

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david2008: Senior | Next Rank: 100 Posts; Posts: 31; Joined: Sat May 09, 2009 7:45 pm; Location: Bay Area; Thanked: 4 times

by david2008 » Mon Jul 13, 2009 2:04 pm

Yup should be E

SD depends on number of items in the set as well as the mean and distribution of the numbers. Any 1 element set would have a 0 SD so by not knowing the number of numbers in each set, we can't conclude what the relative SD is,

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shibal: Master | Next Rank: 500 Posts; Posts: 345; Joined: Wed Mar 18, 2009 6:53 pm; Location: Sao Paulo-Brazil; Thanked: 12 times; GMAT Score:660

by shibal » Mon Jul 13, 2009 4:59 pm

E

K could be 3,3,3 or 3,6,9,12
L could be 9,9,9 or 9,18,27

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mehravikas: Legendary Member; Posts: 1161; Joined: Mon May 12, 2008 2:52 amFollowed by: 1 member; Location: Sydney; Thanked: 23 times

by mehravikas » Mon Jul 13, 2009 7:39 pm

Standard deviation changes when the numbers in the set are multiplied / divided by the same constant. However we dont know here how many elements are there in both the sets and the range of the set.

Therefore answer should be E.

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aj5105: Legendary Member; Posts: 1169; Joined: Sun Jul 06, 2008 2:34 amFollowed by: 1 member; Thanked: 25 times

by aj5105 » Mon Jul 13, 2009 9:03 pm

right said. (E). we need to know the number of elements in the set.

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