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OMARS
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Understanding CR assumption correctly

by OMARS Mon Nov 26, 2018 8:01 pm

Hi,

I'm particularly fuzzy about how to distinguish answers (true assumptions vs. strong strengtheners) on assumption questions. To make sure that I have my understanding of assumptions correctly, I tried to make up my own question below. Would you please let me know if my reasoning is correct?

I scored a 700 on the GMAT. Therefore, I will not apply to Stanford.

Which of the following is an assumption on which the argument depends?

A) Stanford does not accept applicants that score a 700 or below.
B) Stanford does not accept applicants that score a 730 or below.
C) Stanford does not accept applicants with a GMAT score of 730 or below and a GPA of 3.5 or less.

Here:

A) We definitely NEED this to be correct for the argument to work. Correct answer.
B) This is a strong strengthener, but we don't necessarily NEED the assumption to cover score ranges of 710-730. Incorrect answer.
C) Similar to above, but with an even clearer addition of something we don't NEED for the argument to work. Incorrect answer.

Is B definitely incorrect? If that's the case, wouldn't a negation of B destroy the argument?

Negation of B : Stanford accepts applicants that score a 730 or below.

Wouldn't that statement also cover scores of 700 and below?

Again, I'm trying to find the cut-off line that an assumption is the BARE minimum for an argument to work and a negation to destroy the argument.

Thanks!

Omar
Sage Pearce-Higgins
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Re: Understanding CR assumption correctly

by Sage Pearce-Higgins Tue Dec 04, 2018 1:41 pm

Thanks for your post, and apologies for the delay in replying. It's great to see you thinking closely about arguments and assumptions. There are a number of different issues that you touch on, so let me try to break it down.

First of all, what's an argument? You probably know that it's a claim supported by evidence. Of course, GMAT loves to use lots of different words such as conclusion, opinion, etc. for the claim, and premise, reasons, support, etc. for evidence. Importantly, the conclusion is something not certain, i.e. something we could disagree with. In the case of the argument you wrote 'I will not apply to Stanford' is not really a conclusion. I mean, it's just an expression of intent. What you wrote isn't really an argument, and some of your confusion may come from that.

Part of the problem is that we use 'therefore' in more than one way. For example, if I say 'It was raining, therefore Omar's shoes got wet.' I'm not making an argument, I'm just showing a cause. This is, I think, what your example is doing. An argument might be 'Look, Omar's shoes are wet [fact / premise], therefore it must be raining [claim / conclusion].' Don't confuse causes (which are about explanations) with arguments (which are about truth).

So, rewriting your argument would look something like: 'Omar scored 700 in his GMAT. Therefore, if he applies to Stanford he won't be accepted.'

In reference to your answer choices:

A) Stanford does not accept applicants that score a 700 or below.
Yes, this is an assumption that the argument makes. I describe an assumption as an unstated premise that the argument needs to work. This fulfills all three conditions. Of course the 'needs to work' bit is the toughest, and we have the negation test to help us for that. The negation is 'Stanford does accept applicants that score a 700 or below.' But, since the opposite of none is some (importantly, the opposite of none is not all) we might say 'Stanford does accept some applicants that score a 700 or below.' Then we know that Omar is in with a chance, we can't say that he definitely won't be accepted based on his GMAT score. The argument falls apart.

B) Stanford does not accept applicants that score a 730 or below.
We have to be super careful when taking the opposite of potential assumptions for the negation test. The opposite of this answer is not 'Stanford accepts all applicants that score a 730 or below'. You can think of an opposite of a statement as just adding 'It is not the case that...' to the beginning of the statement. This might help you see that the opposite of this answer is 'Stanford does accept some applicants that score a 730 or below.' Now, does this "destroy" the argument? Do we know that Omar has a chance of getting accepted? We're not actually sure. Maybe Stanford accepts people with 700, but maybe it only accepts people with 710. The argument is not actually broken, we're just in a situation of not enough information (similar to insufficiency on DS problems).

It's worth thinking about what 'destroys the argument' actually means. We might think that we need to make the conclusion false so that we show that Omar definitely will be accepted by Stanford. But that's too strong. To 'destroy' an argument means something more like 'show that the conclusion doesn't follow from the premises'. In this case, it's enough to show that, based on his GMAT score, Omar has a chance of getting accepted by Stanford.

To illustrate these ideas, I use a simple example of an argument: 'All people from England like tea. Therefore, Sage likes tea.' You can probably see that the missing piece of information is whether Sage is from England. The argument is assuming that Sage is from England. If we take the negation of the assumption, that Sage is not from England, then we can see that the argument falls apart. Sure, Sage might still like tea, we just have no reason to believe it. So far, so good.

Now, take the statement 'Sage is from London.' This statement definitely supports the conclusion (if it's true, then the argument is strengthened). However, we clearly don't need this to be true for the argument to work, and the negation test helps us. If we take the negation - 'Sage is not from London' - then it leaves open the possibility that Sage is from some other part of England. Perhaps the argument works, perhaps it doesn't. It hasn't been destroyed. As you say, an assumption is the bare minimum for the argument to work. Typically, strengtheners contain more information. That said, I've seen some OG strengthener problems in which the correct answer is an assumption.