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How many questions can you get incorrect to get 51 in math?

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How many questions can you get incorrect to get 51 in math? [#permalink] New post 13 Jul 2008, 04:58
Hi,

I consistently get 50 in all the tests I have taken (GMAT prep, Kaplan etc).

Does anyone have any idea how many questions can one get wrong to get a 51 in math, not taking into account 9 experimental questions (i.e. the worst case scenario)?

I know, it also depends on when you get it wrong (pretty early or later- after 30+).

Please tell me how many questions you got wrong and still got a 51. We can consider the lowest of them as the safe limit. :)

Thanks,
Sid
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 13 Jul 2008, 06:27
While my Q scores are not encroaching 50, I wanted to make a quick note. When you get the questions wrong is important, but so is which questions you get wrong. If you're geting so many right, the CAT might give you a question that in reality that when solved quickly still takes 10 minutes. If you get this question wrong, it is not likely to count against you with much weight.


sid1009 wrote:
Hi,

I consistently get 50 in all the tests I have taken (GMAT prep, Kaplan etc).

Does anyone have any idea how many questions can one get wrong to get a 51 in math, not taking into account 9 experimental questions (i.e. the worst case scenario)?

I know, it also depends on when you get it wrong (pretty early or later- after 30+).

Please tell me how many questions you got wrong and still got a 51. We can consider the lowest of them as the safe limit. :)

Thanks,
Sid

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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 13 Jul 2008, 08:59
jallenmorris wrote:
While my Q scores are not encroaching 50, I wanted to make a quick note. When you get the questions wrong is important, but so is which questions you get wrong. If you're geting so many right, the CAT might give you a question that in reality that when solved quickly still takes 10 minutes. If you get this question wrong, it is not likely to count against you with much weight.


What reason do you have to think this is true? Unless something has profoundly changed, the GMAT uses a three parameter IRT model as the basis of its scoring algorithm, and the time taken to solve a question is irrelevant.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 13 Jul 2008, 09:21
Well, one of my friends told me that he got 50 in GMATPrep 1 and he had got 4 questions wrong in total. And no two of them one after the other.

Kaplan tests, as unrealistic as they are, give 50 even when you get all questions correct.

@Ian,

What is this IRT model that you are talking about? I mean, I know how the score is calculated but don't know about this IRT model. Please enlighten. :)

Thanks,
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 13 Jul 2008, 09:33
Expert's post
My experience with GMATPrep: 2 mistakes (one of them in 2nd question) - 50Q, 6 mistakes at the end of section - Q51.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 13 Jul 2008, 09:54
You missed the point of my post. It was not about the length of time taken to answer the question. The point is that it is a very difficult question and the CAT expects most people to get that question wrong. If you get it wrong, it will not count against you as much as a getting a question wrong that asks you 1+2= ?

Maybe I should have just left out any reference to time taken to answer the question as it has nothing to do with the point I was attempting to make.

IanStewart wrote:
jallenmorris wrote:
While my Q scores are not encroaching 50, I wanted to make a quick note. When you get the questions wrong is important, but so is which questions you get wrong. If you're geting so many right, the CAT might give you a question that in reality that when solved quickly still takes 10 minutes. If you get this question wrong, it is not likely to count against you with much weight.


What reason do you have to think this is true? Unless something has profoundly changed, the GMAT uses a three parameter IRT model as the basis of its scoring algorithm, and the time taken to solve a question is irrelevant.

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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 05:55
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My apologies, jallenmorris, I misunderstood your point, and I agree with the conclusion (that some questions affect your score more than others). I didn't entirely agree with the original premise: I don't find that 'difficult' GMAT questions are difficult because they are more time-consuming to solve. Instead they are normally more difficult conceptually. Every GMAT math question I've ever seen, hard or easy, can be solved in a minute if you look at it in the right way. But if we ignore the question of the time involved in answering, you're entirely right.

sid1009 wrote:
@Ian,

What is this IRT model that you are talking about? I mean, I know how the score is calculated but don't know about this IRT model. Please enlighten. :)


Item Response Theory (IRT) is the theoretical basis for computer-adaptive tests like the GMAT. The theory describes how you can estimate a test-taker's ability by knowing his or her responses to a set of calibrated questions. The math behind the theory gets complex very quickly (i.e. on a long test), which is why there are so many different (and, for the most part, unsubstantiated) opinions floating around about things like optimal timing strategy on the GMAT, the importance of the first few questions, etc. And, one consequence of the theory is that there is no precise answer to the question in the original post ('how many questions can you get wrong and still get a 51?'); it very much depends on the difficulty level of the questions you get wrong. As jallenmorris points out, if you happen to see a question that is off-the-charts difficult (i.e. considered too difficult even for the average 51-level test-taker), it won't hurt you much at all if you answer incorrectly. Without knowing the range of difficulty levels in the GMAT question bank, it's impossible to know how often, if at all, you might see such 'off-the-charts' questions, but from data I've seen about the GRE, my guess is that they exist in the database. That's speculation, however.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 06:02
Thanks Ian, for the detailed answer. Let's just hope not to get any questions wrong to be on the safer side! :)
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 06:04
walker wrote:
My experience with GMATPrep: 2 mistakes (one of them in 2nd question) - 50Q, 6 mistakes at the end of section - Q51.


!@*! 6 mistakes and you can still get effectively a perfect score?
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 06:12
Ian,

Thanks for that great explanation regarding IRT. That's exactly why places like ScoreTop.com are such a threat to GMAC and our legitimate test scores. With such a large sample (hundreds of thousands) the results of the GMAT exam are going to be very reflective of a bell curve, and those that score 700+ are going to be few. Since the test is adaptive, and the harder questions are given to those that demostrate superior knowledge of Quant early in the GMAT, those hard questions are likely to come out because the theory Ian states wants to use other, more difficult questions, to judge the ability of that person to a hard set of questions. I highly doubt GMAC creates as many 700+ level questions and they do 500-650 level questions. Thus, as you get into the harder questions, there is a smaller pool for GMAC to select questions from.

to the OP - good luck on your quest to "not get any wrong" Don't lose sight of what the GMAT is...it's a tool used to help us get into a great b-school. It's nothing more than that. It's not a quest, it's not something we must conquer. It's a hurdle we have to jump to get on with the rest of the process. Perfection (even an 800) doesn't guarantee you admission to any program you choose, if anything, the adcoms might look closer into the other parts of that person's application to ensure that person will fit in well and work well within a team environment. I've meet many extremely intelligent people that I would not let watch my goldfish while I was out of town. Awesome brain power....little real world application of that ability.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 06:55
Maple wrote:
walker wrote:
My experience with GMATPrep: 2 mistakes (one of them in 2nd question) - 50Q, 6 mistakes at the end of section - Q51.


!@*! 6 mistakes and you can still get effectively a perfect score?


I've noticed that too. i did gmat prep twice. the first time I got one question wrong early (though i think GMATprep has it wrong) and I got 51, the second time I got 3 questions wrong but closer to the end and also got 51. Here is what I've learned: stupid early mistakes are deadly.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:07
i got a 50 yesterday on Gmatprep1 with 12 wrong...and i had 3 wrong in a row..

so i still have not been able to get an understanding of how this scoring algorithm works
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:18
maratikus wrote:
the first time I got one question wrong early (though i think GMATprep has it wrong)


Do you remember the question? There's definitely one (hard-level) question which has the wrong answer recorded in the GMATPrep software, and I'm curious if there's more than one.

(and I'm not talking about the two questions that are well-known for having typographical errors in the question itself- the question with 2(x+y)^2/2(x-y)^2, and the DS question with remainders and powers of 3).
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:34
IanStewart wrote:
maratikus wrote:
the first time I got one question wrong early (though i think GMATprep has it wrong)


Do you remember the question? There's definitely one (hard-level) question which has the wrong answer recorded in the GMATPrep software, and I'm curious if there's more than one.

(and I'm not talking about the two questions that are well-known for having typographical errors in the question itself- the question with 2(x+y)^2/2(x-y)^2, and the DS question with remainders and powers of 3).


That question was the following. First, they defined a function of integers:
f(n) = 3*n, if n is odd
f(n) = n/2, if n is even

Then they asked about f(9)*f(6)=(3*9)*(6/2)=27*3=81 but their answer was 27.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:36
I have done the GMAT twice in the past year, and I did not see any "off the charts" difficult questions. After all, a 51 is for the top 1 or 2% of all test takers, and I'm sure that much fewer than 1% of all test takers can get through the 37 questions without a single mistake. No question that I saw on the GMAT was more difficult that what you could find in GMATPrep.

I agree with Ian that each question can be done in 1 minute if you know how to best approach it from the start (something that is hard to do when you are under pressure!)
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:37
maratikus wrote:
IanStewart wrote:
maratikus wrote:
the first time I got one question wrong early (though i think GMATprep has it wrong)


Do you remember the question? There's definitely one (hard-level) question which has the wrong answer recorded in the GMATPrep software, and I'm curious if there's more than one.

(and I'm not talking about the two questions that are well-known for having typographical errors in the question itself- the question with 2(x+y)^2/2(x-y)^2, and the DS question with remainders and powers of 3).


That question was the following. First, they defined a function of integers:
f(n) = 3*n, if n is odd
f(n) = n/2, if n is even

Then they asked about f(9)*f(6)=(3*9)*(6/2)=27*3=81 but their answer was 27.


Don't overlook the fact that the answer choices are f(27), f(54), f(81)..., not 27, 54, 81 (At least they are in my software)
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:39
i had the question and the ans is 27..

goes like m and [m] is 3m..if m=odd and [m]=m/2

then asks [9]*[6]=27*3

which becomes [27]

maratikus wrote:
IanStewart wrote:
maratikus wrote:
the first time I got one question wrong early (though i think GMATprep has it wrong)


Do you remember the question? There's definitely one (hard-level) question which has the wrong answer recorded in the GMATPrep software, and I'm curious if there's more than one.

(and I'm not talking about the two questions that are well-known for having typographical errors in the question itself- the question with 2(x+y)^2/2(x-y)^2, and the DS question with remainders and powers of 3).


That question was the following. First, they defined a function of integers:
f(n) = 3*n, if n is odd
f(n) = n/2, if n is even

Then they asked about f(9)*f(6)=(3*9)*(6/2)=27*3=81 but their answer was 27.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:42
that helps, gentleman. I guess I overlooked f(27). I feel better now ...
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:43
51 is hardly a "perfect" score. I'm pretty certain that the great majority of people who score 51 get at least 1 question wrong.

My 2 cents: I just think it's a misnomer to call a 51 a "perfect" score.
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Re: How many questions can you get incorrect to get 51 in math? [#permalink] New post 14 Jul 2008, 07:49
maratikus wrote:

That question was the following. First, they defined a function of integers:
f(n) = 3*n, if n is odd
f(n) = n/2, if n is even

Then they asked about f(9)*f(6)=(3*9)*(6/2)=27*3=81 but their answer was 27.


That is what I call a "gotcha" question. It is a simple math question, except they catch people by a syntax. I wonder what percentage of people got it right the first time.
Re: How many questions can you get incorrect to get 51 in math?   [#permalink] 14 Jul 2008, 07:49
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