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If the answer to the stem question is "No"

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Source: — Data Sufficiency |
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by kvcpk » Thu Aug 12, 2010 10:39 am
findmimi wrote:is the statement considered sufficient to answer the question?

See below example:
In the xy-plane, region R consists of all the points (x.y) such that 2x+3y=6. is the point (r,s) in region R?
(1) r=3 and s=2
(2) ???????

Thanks in advance.
2x+3y=6 is a straight line. Question is asking if (3,2) is on the line.
2(3)+3(2) = 12 i snot equal to 6.
Hence (r,s) does not belong to region R.
Answer is a definite NO.

Hence Sufficient.

Hope this helps!!
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by Brian@VeritasPrep » Thu Aug 12, 2010 11:16 am
Hey, findmimi:

Great question - honestly, I think that this is one of the five easiest mistakes to make on the math section, so I'll add to kvcpk's post just to make the point clear.

On Data Sufficiency questions, your only job is to determine when you have "enough information" (or "sufficient data") to answer the question.

In order to answer the question, you need to be able to provide one and exactly one answer. They can ask you the question in one of two ways:

1) Yes/No (e.g. "Is x > 5?")

Here, you need to be able to answer "definitely yes" or "definitely no". If you can get exactly one answer, the information is sufficient.

If you can get both answers (sometimes yes, sometimes no), then the information is not sufficient.

Strategy Tip: As soon as you find that you "can" get "yes", then try to get "no". Often there may be only one number (maybe 0) or type of number (e.g. nonintegers) that would give you that opposite answer, but that's all you need to prove that the information is not sufficient.


2) Find the value (e.g. "What is the value of x?"

Here, you need to be able to give exactly one value. If there's a range of values or if the answer could even have two values (x^2 = 16 means that x is 4 or -4, so that's not sufficient), the information is not sufficient.

Again, strategically, do what you can to find a second possible value. If you can, then it's not sufficient.



Now, back to that first point, "No" means "Sufficient". I'd advise this - write the word "SUFFICIENT" at the top of your noteboard on test day, and make sure that you tap it with your pen before you submit your answers to any Data Sufficiency questions to remind yourself that your job is not to determine whether the answer is yes, but rather to determine whether the information is sufficient. Our knee-jerk reaction is to equate "no" with "wrong", and that's why it's such an easy mistake to make on this test!


While we're talking about easy-to-make mistakes, I should point out another one that my friend David wrote about on BTG this morning. Answering "No" vs. "Sufficient" is an example of his point - make sure to answer the right question: https://www.beatthegmat.com/just-somethi ... 63685.html Another handy reminder to yourself - you can write a question mark (?) at the top of your noteboard to remind yourself to double-check the question being asked to make sure that your answer directly answers it.
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by Ian Stewart » Thu Aug 12, 2010 1:00 pm
I agree with all of what Brian says, but I'd just add: on real GMAT yes/no Data Sufficiency questions, it is very rare that you have sufficient information to give a 'no' answer to the question. If you look at the 60 or so yes/no questions in the DS section of the Official Guide, there is only 1 question where you can be certain, using the statements, that the answer is 'no'. On all of the other questions, if you have sufficient information, the answer you get is 'yes'. So while in a question like this one:

Is x positive?
1) x = -3
2) x < -2

each statement is certainly sufficient - we can be certain the answer is 'no' in each case - it is very unlikely you'd ever see such a question on test day. You should certainly understand this technicality about Data Sufficiency questions, but in my experience, many prep companies greatly overstate the importance of this 'trap'. I see tons of prep company questions where you can use the statements to get a 'no' answer, but I almost never see official questions where that's the case.
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by Brian@VeritasPrep » Thu Aug 12, 2010 2:32 pm
Hey guys,

Don't let me turn this into an argument, but a quick note on Ian's post - GMAC itself will tell you that the Official Guide series is much more a "collection of GMAT questions" than a "proportional representation of the frequency of topics/concepts/question types on the exam". It's not designed to be the latter, so I've tended to be leery about using it that way - in fact, to be included in the Official Guide questions must have been retired for at least five years, so it's a look backward but not necessarily a look at today's test.

The Official Guide is obviously a terrific resource for practicing official questions, but be cautious in trying to use it to rule out concepts or question devices that you can ignore because "the OG doesn't have very many questions on that". If the GMAT has used a line of questioning even once and found it to be a good differentiator between ability levels, it certainly can and will use it again. The device of making it easy for a distracted or rushed examinee to answer the wrong question and therefore answer the overall item incorrectly has been quite a successful one for the GMAT in the past, so you can be certain that they'll continue to use it in the future.
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by Ian Stewart » Thu Aug 12, 2010 3:31 pm
Brian@VeritasPrep wrote: Don't let me turn this into an argument, but a quick note on Ian's post - GMAC itself will tell you that the Official Guide series is much more a "collection of GMAT questions" than a "proportional representation of the frequency of topics/concepts/question types on the exam". It's not designed to be the latter, so I've tended to be leery about using it that way - in fact, to be included in the Official Guide questions must have been retired for at least five years, so it's a look backward but not necessarily a look at today's test.

The Official Guide is obviously a terrific resource for practicing official questions, but be cautious in trying to use it to rule out concepts or question devices that you can ignore because "the OG doesn't have very many questions on that". If the GMAT has used a line of questioning even once and found it to be a good differentiator between ability levels, it certainly can and will use it again. The device of making it easy for a distracted or rushed examinee to answer the wrong question and therefore answer the overall item incorrectly has been quite a successful one for the GMAT in the past, so you can be certain that they'll continue to use it in the future.
I'm sure this is a discussion that's only of great interest to us GMAT specialists, but I was only discussing OG questions because it's a convenient example that others can easily verify. What I've said I've found to be equally true of GMATPrep questions or GMATFocus questions (well, I know of two GMATFocus questions where the statements give a 'no' answer, but that's from quite a large pool of questions) and of any real GMAT I've taken. Of course there are some questions where you can arrive at a 'no' answer from the statements, but I've never found these to be common no matter what official questions I look at.

I'm also curious what has led you to think that "to be included in the Official Guide questions must have been retired for at least five years". Many of the new questions in OG12 were drawn from GMATFocus, and my impression, though I may be mistaken, was that GMATFocus questions were drawn from those questions that GMAC was forced to retire because they had been publicized on illegal forums. Those questions would be less than five years old.
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by findmimi » Thu Aug 12, 2010 5:53 pm
Thanks so much to all!
Just so I am 100% clear, here is another one:
If xyz>0, is xy2z3<0?
(1) y>0
(2) ????
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by Jai_itguys » Fri Aug 13, 2010 12:27 am
If xyz>0, is xy2z3<0?
(1) y>0
(2) ????

xy2z3=(xyz)(yz2)=(xyz)(z2)(y) since xyz>0 (given) and z2 will always be positive(or zero) the sign of xy2z3 will always be the sign of y

1) if y>0 then xy2z3 is >0 hence you get a definiter answr "No" -SUFFICIENT
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by Brian@VeritasPrep » Fri Aug 13, 2010 3:16 pm
Yeah, Ian - we may be getting a little too specific, but for what it's worth, the 5-year rule is something that the reps from GMAC mentioned at a recent GMAT summit (they do those every two years or so for the top prep companies to discuss trends and developments).

Actually, it was at one of those summits this fall that I mentioned a device that I admired in one of their questions (I forget which now...maybe the logical correctness facet of Sentence Correction or something like that) to one of the GMAC rep, and asked if they'd be using it more often as they attempt to build difficulty. His answer was to the extent of "well, if we're not, we should" and we talked about how they do over time realize that they've hit on particular devices that allow them to increase the difficulty level of test items, and then plan to use them more often as needed. So whenever I see even a couple of iterations of something that I know gives students fits, I try to point it out as I know that the test has a tendency to evolve the same way.

The best part of those meetings is that you develop a pretty healthy respect for the test and those who administer it. For any students still reading, rest assured that GMAC puts a ton of research and development into creating a test that is fair and valid.
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