ds:/SD set T

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advita: Senior | Next Rank: 100 Posts; Posts: 62; Joined: Mon Sep 13, 2010 8:06 pm; Thanked: 2 times

by advita » Mon Jan 24, 2011 7:58 am

If set T consists of odd integers divisible by 5, is the standard deviation of T positive?

1. All members of T are positive
2. T consists of only one member.

Q:1. 0 is neither positive nor negative. is that true.??

i doubt OA-..... B

Q2. if i calculate SD to be zero, then can i say SD for the set is positive.... i dont think so.

pl explain....thanks.

source: gmat club.

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Source: Beat The GMAT — Data Sufficiency | Mon Jan 24, 2011 7:58 am

maihuna: Legendary Member; Posts: 1578; Joined: Sun Dec 28, 2008 1:49 amFollowed by: 9 members; Thanked: 82 times; GMAT Score:720

by maihuna » Mon Jan 24, 2011 8:06 am

I think SD is always >=0 as it is sum of square of diff with mean divided by mean and taking squrt. Yes 0 is neither positive nor negative it is 0. So OA B is reasonable as you know for 1 elem SD=0.

For more than 1 elelm, if all same(2, 2, 2) SD = 0, for 1, 2, 3 SD >0

Charged up again to beat the beast

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advita: Senior | Next Rank: 100 Posts; Posts: 62; Joined: Mon Sep 13, 2010 8:06 pm; Thanked: 2 times

by advita » Mon Jan 24, 2011 8:16 am

i agree what you say.

but my question is that: from B. we can conclude that SD=0 , which is neither positive nor negative.

so, are we answer the question: is SD +ve.

ok...i got it .. we cant say sd is positive....thats why OA is B.

thanks..buddy.

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Stuart@KaplanGMAT: GMAT Instructor; Posts: 3225; Joined: Tue Jan 08, 2008 2:40 pmFollowed by: 614 members; Location: Toronto; Thanked: 1710 times; GMAT Score:800

by Stuart@KaplanGMAT » Mon Jan 24, 2011 10:45 am

advita wrote:i agree what you say.

but my question is that: from B. we can conclude that SD=0 , which is neither positive nor negative.

so, are we answer the question: is SD +ve.

ok...i got it .. we cant say sd is positive....thats why OA is B.

thanks..buddy.

Hi,

for "yes/no" questions, a statement is sufficient when it gives a definite answer to the question; a definite "no" is just as good as a definite "yes".

Here, the question is "is the SD of the set positive?". From (2), we can conclude that the SD is definitely NOT positive, so (2) is sufficient.

It's when a statement gives you a "sometimes yes, sometimes no" answer that it's insufficient.

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

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