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West Side School, LSAT CR

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West Side School, LSAT CR

by madhukumar_v » Mon Apr 19, 2010 2:36 pm
Although the charter of Westside School states that the student body must include some students with special educational needs, no students with learning disabilities have yet enrolled in the school. Therefore, the school is currently in violation of its charter.

The conclusion of the argument follows logically if which one of the following is assumed?

A:All students with learning disabilities have special educational needs
B:The school currently has no students with learning disabilities.
C: The school should enroll students with special educational needs.
D: The schools charter cannot be modified in order to avoid its being violated
E: The only students with special educational needs are students with learning disabilities.

OA: E

Can some one explain why A is not an option, I got confused between A and E.
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by stephen@knewton » Mon Apr 19, 2010 4:45 pm
As both a GMAT and LSAT teacher, I love seeing questions like this on BTG! :)

The LSAT has a much greater focus on formal logic than the GMAT does, but GMAT students can definitely benefit from learning to think that way. So let's use formal logic to unpack this question. You'll be surprised how powerful it is.

First we'll use some shorthand to make things easier:

"Special educational needs" = SN
"Learning disability" = LD

So the stimulus basically says the following:

1) Evidence: school must have some students with SN [which means at least one].
2) Evidence: no students with LD enrolled.
3) Conclusion: school is in violation.

But we can make that even more concise:

E: Must have some SN.
E: ~LD enrolled.
C: Therefore in violation.

Before we look at the answer choices, we should pre-phrase our missing assumption. The author seems to assume that if we don't have any LD students, we don't have any SN students ... in other words, you can't have SN without LD! But this connection is never established by the argument (one thing the LSAT and GMAT share is a love of precise wording). Our assumption pre-phrase might therefore go something like:

"Any student with SN has LD" ... if that were true, this argument would make sense.

Now let's translate answer choices (A) and (E). The LSAT demands that test-takers be able to translate sentences into "If/Then" statements. This skill is definitely helpful in GMAT CR, but it's fundamental to the LSAT. Think about the language carefully, and you'll realize that (A) and (E) translate to:

A) If LD, then SN; this is also written LD --> SN, from which you can deduce ~SN --> ~LD (the contrapositive).
E) If SN, then LD; this is also written SN --> LD, from which you can deduce ~LD --> ~SN ... our author's assumption!!

So the correct answer is (E). It established the link that the author needs from special needs to learning disabilities.

And lastly, if you are still uncertain, try the negation test. The negation test works this way: if you are assessing whether something is a required assumption, write down the OPPOSITE of it. If that new statement invalidates the argument, you've got an assumption on your hands!

Let's try it:

(A) "Some students with learning disabilities do not have special educational needs"
(E) "There are students with special needs who do not have learning disabilities."

Which one invalidates the original argument? It's E! You've got your assumption.

Hope this helps!

Cheers, Steve P.
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by lkm » Mon Apr 19, 2010 6:56 pm
Hi

Stephen has already explained it in detail.
However, here is my explanation in shorter form.

Statement A: It states that all student with disabilities have special educational needs. However, it leaves an open space here for the other students who might have special educational needs BUT they don't have any any kind of disabilities.

So school could have enrolled them.

Since we don't know whether school has enrolled students who required special educational needed students but don't have disabilities. So, it leads us to assume the things. Therefore, school may or may not have violated the law.

So, with statement A, we cannot conclude the argument.

Statement E: States that only students who need the special educational needs have disabilities.

So here author closes the room of argument. Since there is no other kind of student who needs the special education and it is already stated in premises that school hasn't enrolled disabled students. Hence, school has clearly violated the charter.

So statement E leads to the conclusion of the argument.
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by madhukumar_v » Mon Apr 19, 2010 7:37 pm
Thanks Stephen and lkm. Both your explanations are excellent.
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by Testluv » Mon Apr 19, 2010 8:53 pm
And lastly, if you are still uncertain, try the negation test. The negation test works this way: if you are assessing whether something is a required assumption, write down the OPPOSITE of it. If that new statement invalidates the argument, you've got an assumption on your hands!
Note that this question asks for a sufficient and not a necessary assumption.

A necessary assumption is one which the argument cannot do without; it is a premise upon which the argument relies. That's why we can use the negation test (or "denial test") to prove whether or not an answer choice is a necessary assumption.

A sufficient assumption, on the other hand, is a premise which, if true, would guarantee or validate the argument. But an arguer doesn't (necessarily) rely on a sufficent assumption. Thus, we should be VERY careful about using the denial test to prove sufficient assumptions. In fact, I always tell my students to never use the denial test in sufficient assumption questions: It will only work if the sufficient assumption also happens to be necessary. However, in a sufficient assumption question, it is possible that one of the incorrect answers is a necessary (but insufficient) assumption. In that case, using the denial test could cause you to select an incorrect answer (epecially if the correct answer--the sufficent assumption--is unnecessary).

The kinds of questions for which it is absolutely safe to use the denial test are:

*necessary assumption
*strengthen/weaken
*inference (we can ask whether the denial of answer choice would falsify any of the facts in the stimulus; if it does, then it is something that must be true, and therefore the corect answer to an inference question).

For a deeper discussion on sufficient vs necessary assumptions, see: https://www.beatthegmat.com/verdland-t48086.html#203040 (you'll have to scroll down a bit).
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by reply2spg » Wed Apr 21, 2010 8:22 am
what is wrong with B here?
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by lkm » Wed Apr 21, 2010 9:37 am
reply2spg wrote:what is wrong with B here?
Option B is the direct paraphrase of premise. So, it is a fact not the assumption. And you have to find the assumption here to lead to the conclusion.
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