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Data Sufficiency Structure

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Data Sufficiency Structure

by fskilnik@GMATH » Fri Jul 27, 2012 3:53 am
Dear Rebecca,

I´ve been teaching students in Brazil for the quantitative section of the GMAT for more than a decade and although the (D) alternative in data sufficiency does NOT necessarily mean EXPLICITLY that both statements answers in the same way to the question asked it is usually the case in official problems. After many years thinking about this small (but interesting) detail and having read good arguments for the fact that the answers (in this (D) right choice case) "must" be the same (for instance: https://www.beatthegmat.com/mba/2012/01 ... ufficiency) I found a good "mathematical" reason to also believe on that, too. (My explanation follows the article mentioned.)

Well, my question is: could you please tell me/us the "official" position in this matter? In other words, may we (students/teachers) be 100% sure all GMAT data sufficiency problems are created in the following sense: there is one single scenario presented pre-statements and both statements must refer to the same scenario and, therefore, all info given in the statements are "coherent" between them without any possible situation in which one statement could give us a conclusion that is not compatible to any possible conclusion taken from the other statement?

I hope I could make my question clear.

Thanks a lot,
Fabio.
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by OfficialGMAT » Fri Jul 27, 2012 8:01 am
Hi Fabio,

Thank you for asking this Data Sufficiency question. I have forwarded your question to one of our psychometricians in order to get the best answer possible. When I receive a response I will post it on this thread.

Thanks!
Leah
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by fskilnik@GMATH » Fri Jul 27, 2012 9:16 am
Hi Rebecca!

Thank you for your quick reply, your friendly tone and your carefull support.

The service you provide here is really amazing.

I´m looking forward to your response (but no hurry, of course)!

Regards,
Fabio.
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by OfficialGMAT » Mon Aug 06, 2012 12:54 pm
Hello, Fabio! There are not any "official" rules with regard to your scenario. DS has been on the GMAT exam since 1991 and it is certainly possible that someone very familiar with DS would be able to find a counter example in one of the OGs. That said, it is highly unlikely that you would come across a question with independently sufficient statements that contradict each other.

I hope that answers your question.
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by fskilnik@GMATH » Tue Aug 07, 2012 4:05 am
Dear Rebecca,

Thank you for your very careful and honest answer. That´s a bit surprising, because most "experts" would be ready to *guarantee* his/her students that this "contradiction" would be impossible in official gmat problems!

It´s nice to see that GMAT psychometricians are "conservative" on this matter. That´s an elegant position, by the way, because in my humble opinion the DS rules interpretation really leaves the answer to my question (strictly speaking) "open".

Regards and thanks a lot,
Fábio.
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Data Sufficiency Structure

by fskilnik@GMATH » Tue Sep 11, 2018 5:39 pm
Dear Leah (and other Official GMAC Representatives),

I am still teaching students for the quantitative section of the GMAT... and now I have another "conceptual question" related to Data Sufficiency.

I hope I will have (again) the pleasure to count on your authoritative definitive answer, for the benefit of myself, my students, other "GMAT Experts", their students, and the full BeatTheGMAT community!

The problem that "motivates" my question is the one below, presented in the Official Guide.
(I don´t know the year of the publication because it was posted by the BTG moderator. Link at the end of this post.)

------------------------------------------------------------------------------------------------------------
The perimeter of a rectangular garden is 360 ft. What is the length of the garden?

1) the length of the garden is twice the width
2) the difference between the length and width of the garden is 60 ft

Official Answer: D
------------------------------------------------------------------------------------------------------------


The issue is related to statement (2) ALONE, therefore let us focus on it, exclusively.

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ARGUMENT 1:

Statement (2) is sufficient, because we may ALWAYS consider this statement as "equivalent" (meaning: exactly the same information contained) to the one below:

2b) the difference IN THAT ORDER between the length and width of the garden is 60 ft

Hence we MUST have L-W = 60, and using L+W = 180 (from the question stem pre-statements), we find a unique numerical value for L (the length of the garden).
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ARGUMENT 2:

Statement (2) is sufficient, because we must ALWAYS consider the length of the rectangle as the greater dimension of the rectangle. (When we have a square, we understand "greater" as any of the two equal dimensions, of course.)

Hence we MUST have L-W = 60 (because W-L = 60 would imply W>L and this is ALWAYS impossible), and using L+W = 180 (from the question stem pre-statements) we find a unique numerical value for L (the length of the garden).
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To be absolutely sure that you (psychometricians inlcuded) are able to give us a definitive answer, I would like to propose a problem in which those two arguments DIVERGE, so that the official answer to this problem will close the case without any doubt...

Question:
The perimeter of a rectangular garden is 360 ft. What is the length of the garden?
1) the length of the garden is twice the width
2) the length and the width of the garden differ by 60 ft

If ARGUMENT 1 is correct, the answer is (A) , because statement (2) would translate to |L-W| = 60 and, consequently, two different possibilities are viable:
> Length = 120 and Width = 60 , answering 120 (feet)
> Length = 60 and Width = 120 , answering 60 (feet)

If ARGUMENT 2 is correct, the answer is (D), because statement (2) would translate to L-W = 60 (because W-L = 60 would imply W>L , impossible) and, consequently,
> Length = 120 and Width = 60 , answering 120 (feet)

The link for the original question and for many different points-of-view is the following:
https://www.beatthegmat.com/the-perimet ... 04164.html


Thank you very much for your support!

Regards,
Fabio Skilnik (GMATH).
Fabio Skilnik :: GMATH method creator ( Math for the GMAT)
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Portuguese-speakers :: https://www.gmath.com.br
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