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Assumptions and Strengthen the conclusion..

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Assumptions and Strengthen the conclusion..

by iongmat » Mon Jun 09, 2014 9:14 pm
Hello. I am going through the critical reasoning bible and had a question. The book says that for arguments that depict causal reasoning, the assumptions questions tend to work in exactly the same manner as strengthen the conclusion questions.

Let us take an argument where with causal reasoning:

Smoking causes lung cancer.

One of the answer choices that would strengthen this conclusion would be:

80 % of all lung cancers are attributed to cigrate smoking.

Hence this option will definitely qualify to be the correct answer in response to strengthen this conclusion question; however I do not think that this option is in any way an "assumption" of the argument.

So I am not very sure what the book means when it says that for arguments that depict causal reasoning, the assumptions questions tend to work in exactly the same manner as strengthen the conclusion questions.

Can someone please clarify.
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by theCodeToGMAT » Tue Jun 10, 2014 2:26 am
Assumptions are the bridge between Facts & Conclusion..

Lets assume that there were some facts... Say "Lung cancer is a dangerous disease"

Conclusion is "Smoking causes lung cancer"

Now, "80 % of all lung cancers are attributed to cigarette smoking" acts as assumption .

Here we can see that the conclusion is justifiable based on the assumption..

If we negate the assumption.. "80% cases are NOT attributed to cigarette smoking", then we cannot say confidently that "Smoking cause lung cancer"
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by David@VeritasPrep » Thu Jun 12, 2014 7:45 am
If we negate the assumption.. "80% cases are NOT attributed to cigarette smoking", then we cannot say confidently that "Smoking cause lung cancer"
Rahul - I hate to by picky but this is not the correct way to negate an assumption that features a quantifier.

For example, the following statement is wrong because of a woman named Margaret Thatcher: "All British Prime Ministers have been men." Or to put it into a percentage "100% of British Prime Ministers have been men."

How do we capture the fact that this is not true - how do we account for Mrs. Thatcher?

The way that you have negated it above I would say, "All British Prime Ministers have NOT been men." You can see that this is false (nearly all of the prime ministers have been men). So the incorrect technique led us to say something we did not mean to say.

What we want is something that acknowledges the prime minister who was not male, but that is not unnecessarily strong. The statement we want is "NOT all British Prime Ministers have been men."

So you see when we negate an assumption we first negate the quantifier (such as all, none, some, 80%, etc). If there is no quantifier we negate the main verb.

So the proper negation of "80 % of all lung cancers are attributed to cigarette smoking" is "NOT 80% of all lung cancers are attributed to cigarette smoking."

My favorite way to negate an assumption to see if it is necessary is to simply ask, "what if answer choice this is not true?" "Does this harm the logic and ruin the conclusion?"

So I would say, "what if some percentage other than 80% of all lung cancers are attributed to cigarette smoking?" What does this do to the conclusion.

Having taught the LSAT for many years I have lots of experience with this sort of thing, and this is how we instruct LSAT students to do negation...

First negate any quantifier
if no quantifier then negate the main verb.

Remember that statements that are definite need only become indefinite. If the answer choices says "He will be home at 10PM" you need only change this to "He MIGHT NOT be home at 10PM."

I hope that helps!
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by David@VeritasPrep » Thu Jun 12, 2014 8:03 am
Hence this option will definitely qualify to be the correct answer in response to strengthen this conclusion question; however I do not think that this option is in any way an "assumption" of the argument.
iongmat - you are correct. "80 % of all lung cancers are attributed to cigarette smoking" is a very good strengthen answer, but it is a terrible assumption answer. It is absolutely one of the worst assumption answers you could come up with and I am shocked if the book you are using says that it is a valid assumption answer.

Basically, a good assumption answer is a bad strengthen answer and vice versa. An great inference answer (one that is easy to prove) also makes a very good assumption answer."

For example, with inference "some" or "may happen" are easier to prove than "all" and "must happen." The same words help to make a good assumption choice.

For example, you offer the conclusion, "Smoking causes lung cancer." The proper assumption is not the one offered but is "at least some cases of lung cancer are attributed to smoking."

Do you see how this is required? What if we negate this answer. Negate the quantifier, and the opposite of "some" is "none." So the negated answer becomes "NO cases of lung cancer are attributed to smoking." As you can see that destroys the conclusion!

A weak answer as written is a great assumption because it becomes strong when you negate it.

Now what about "80 % of all lung cancers are attributed to cigarette smoking" - why is this a great strengthen, but a terrible assumption answer? As a strengthen you want to add something strong. 80% is a strong number. It really shows that smoking causes lung cancer. But is this necessary?

One thing you want to watch out for on assumptions is an answer that is too specific. Try this example: conclusion, "you are rich" strengthen answer: "you have 27 million dollars." This is a very good strengthen answer as 27 million likely makes you "rich." But it is a terrible assumption. You see all I have to do to negate this is to give you a different amount of money. Something other than exactly 27 million. You could have 25 million (still rich!) or even 50 million (it is not 27 million right?)

So you see an answer that is too strong or too specific is not a good assumption.

The book, if it said this is quite wrong.

Hope it helps!
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