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Question about the Weaken the conclusion, example Kaplan CR

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Question about the Weaken the conclusion, example Kaplan CR

by reply2spg » Wed Aug 04, 2010 7:30 pm
As per the powerscore CR bible weaken the conclusion (casual relationship) type has following scenarios

Consider this

X causes Y

1. weaken in case if X, and not Y
2. weaken in case if not X, and Y
3. weaken in case if Y, then X

Now in following question

------------------------------------------------------------------

Reading skills among high school students in Gotham have been steadily declining, which can only be the result of overcrowding in the schools.

Which of the following, if true, most seriously weaken the argument above?

A) The high school system in Gotham succeeds in giving students a good education at considerably less cost than do most system.
B) Several cities have found that overcrowding in the schools is not always associated with lower reading score.
C) Gotham schools have a greater teacher-to-student ratio than most other schools.
D) Student's reading skills have not declined in other cities where the high schools are just as crowded as those of Gotham.
E) Schools are not overcrowded in many cities where high schools reading scores have declined more than they have in Gotham.

We have both types 1 and 2 as I have mentioned above. However, as per the OA type 2 is correct. So my question is if we have type 1 and 2 both present in answer choices should we go with 1 or 2?

OA E
Sudhanshu
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by KapTeacherEli » Wed Aug 04, 2010 9:46 pm
Hi reply,

While the LSAT places much more emphasis on the concept, 'sufficiency versus necessity' is nonetheless an important idea to understand for GMAT Critical Reasoning. In this case, the author's conclusion is that the decline can 'only be the result of overcrowding'; in other words, he is assuming that a overcrowding is NECESSARY for a decline in reading.

Choice D) proves that overcrowding doesn't always cause a reading drop. This proves that overcrowding does not always cause a decline. Overcrowding is not SUFFICIENT to guarantee a decline in reading. Because the author's conclusion is about the necessity of overcrowding, and this answer is about the sufficiency of overcrowding, (D), though tempting, is out of scope.

Choice E) shows us that a reading drop can occur without overcrowding. That means that overcrowding is not necessary for reading scores to drop, directly undermining the author's choice.
Eli Meyer
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www.kaptest.com/gmat

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by reply2spg » Thu Aug 05, 2010 6:38 am
Hi Eli,

Thanks for your time and insight. However, I am still confused. As per your explanation author assume that overcrowding is the only cause of a reading drop and D proves that though overcrowding is present there is no reduction in reading.

Let's say overcrowding is X and reduction in reading is Y

As per the passage Only X, then Y
D says X is present, then also no Y - why can't this weaken the passage?
E says No X, then also Y - This also weakens

Then what exactly should be the answer. In both scenarios we see that one thing is present when other is abscent. In that case how come D is out of scope?
KapTeacherEli wrote:Hi reply,

While the LSAT places much more emphasis on the concept, 'sufficiency versus necessity' is nonetheless an important idea to understand for GMAT Critical Reasoning. In this case, the author's conclusion is that the decline can 'only be the result of overcrowding'; in other words, he is assuming that a overcrowding is NECESSARY for a decline in reading.

Choice D) proves that overcrowding doesn't always cause a reading drop. This proves that overcrowding does not always cause a decline. Overcrowding is not SUFFICIENT to guarantee a decline in reading. Because the author's conclusion is about the necessity of overcrowding, and this answer is about the sufficiency of overcrowding, (D), though tempting, is out of scope.

Choice E) shows us that a reading drop can occur without overcrowding. That means that overcrowding is not necessary for reading scores to drop, directly undermining the author's choice.
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by KapTeacherEli » Thu Aug 05, 2010 7:59 pm
reply2spg wrote:Hi Eli,

Thanks for your time and insight. However, I am still confused. As per your explanation author assume that overcrowding is the only cause of a reading drop and D proves that though overcrowding is present there is no reduction in reading.

Let's say overcrowding is X and reduction in reading is Y

As per the passage Only X, then Y
D says X is present, then also no Y - why can't this weaken the passage?
E says No X, then also Y - This also weakens

Then what exactly should be the answer. In both scenarios we see that one thing is present when other is abscent. In that case how come D is out of scope?
Hi Reply,

Formal logic can be tricky here, but it's definitely a useful tool for analyzing this problem.

In this case, the author's conclusion is that the ONLY possible cause for the decline is overcrowding. This means that all cases of declines in reading must be caused by overcrowding, or:

If Decline, Then Overcrowding.

And, if this is true, we can reverse the direction of both terms, and negate both terms, to generate the Contrapositive. If X then Y is equivalent to If NOT Y the NOT X. We can confirm this makes sense with a simple real-life example: if it's Coal, it's Black, if it's Not Black, it's Not Coal. So to apply this to our current example:

If NO Overcrowding, then NO decline.


(D) gives us an example of "Overcrowding, No Decline." This doesn't contradict either of our rules! Neither of our IF's trigger--we have no 'decline', and we have no 'NO overcrowding'. (D) is compatible with the author's logic.

(E) Gives us an example of "Decline, but NO Overcrowding". This contradicts the author's logic. According to the author's conclusion that the 'only' possible cause of a reading decline is overcrowding, if there is a decline, the must be overcrowding; if there is no overcrowding, there can't be a decline. We've undermined his logic, and (E) is correct.


Of course, this may seem arcane to you. That's okay too--formal logic can be a useful tool on the GMAT, but it's by no means required. We can also realize that this is a sufficiency versus necessity trap just from the pattern--that 'only' language.

Again, he's saying the only possible cause is overcrowding, but says nothing about overcrowding guaranteeing a decline. Picking Numbers is a Kaplan strategy normally used on the Quantitative section, but it also applies to the CR section when paraphrasing would be useful; in this case, we'll pick the probability of overcrowding causing trouble. "Overcrowding, the only cause of reading declines, has a 20% chance when present of hurting reading scores."

This is consistent with the author's reasoning--he says that only overcrowding could cause a decline, and our hypothetical rule says the same.

Take a look at (D). Some schools have overcrowding but no decline. In our hypothetical, (D) is no only allowed, but expected! if 20% of overcrowded schools have declines, 80% don't, so we knew all along we'd see schools with overcrowding but no decline.

(E), meanwhile, is impossible under the author's assumption. Because the author assumes that the only cause of reading declines is overcrowding, a decline without overcrowding should be impossible--and if it is possible, the author's assumption is undermined.
Eli Meyer
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www.kaptest.com/gmat

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by reply2spg » Thu Aug 05, 2010 8:27 pm
I think I got it, though I need to read your explanation 2-3 times.

I am just checking my understanding whether it is correct. So as you explained decline is 'only and only' because of overcrowding. So If I have overcrowding then it doesn't matter whether I have decline or not. However, If I have decline then overcrowding must be there. If it is not then that option is weakening the argument.

I hope I understood correctly. hmmmm....this is interesting!!!!!

Thanks a lot for your superb explanation Eli.
KapTeacherEli wrote:
reply2spg wrote:Hi Eli,

Thanks for your time and insight. However, I am still confused. As per your explanation author assume that overcrowding is the only cause of a reading drop and D proves that though overcrowding is present there is no reduction in reading.

Let's say overcrowding is X and reduction in reading is Y

As per the passage Only X, then Y
D says X is present, then also no Y - why can't this weaken the passage?
E says No X, then also Y - This also weakens

Then what exactly should be the answer. In both scenarios we see that one thing is present when other is abscent. In that case how come D is out of scope?
Hi Reply,

Formal logic can be tricky here, but it's definitely a useful tool for analyzing this problem.

In this case, the author's conclusion is that the ONLY possible cause for the decline is overcrowding. This means that all cases of declines in reading must be caused by overcrowding, or:

If Decline, Then Overcrowding.

And, if this is true, we can reverse the direction of both terms, and negate both terms, to generate the Contrapositive. If X then Y is equivalent to If NOT Y the NOT X. We can confirm this makes sense with a simple real-life example: if it's Coal, it's Black, if it's Not Black, it's Not Coal. So to apply this to our current example:

If NO Overcrowding, then NO decline.


(D) gives us an example of "Overcrowding, No Decline." This doesn't contradict either of our rules! Neither of our IF's trigger--we have no 'decline', and we have no 'NO overcrowding'. (D) is compatible with the author's logic.

(E) Gives us an example of "Decline, but NO Overcrowding". This contradicts the author's logic. According to the author's conclusion that the 'only' possible cause of a reading decline is overcrowding, if there is a decline, the must be overcrowding; if there is no overcrowding, there can't be a decline. We've undermined his logic, and (E) is correct.


Of course, this may seem arcane to you. That's okay too--formal logic can be a useful tool on the GMAT, but it's by no means required. We can also realize that this is a sufficiency versus necessity trap just from the pattern--that 'only' language.

Again, he's saying the only possible cause is overcrowding, but says nothing about overcrowding guaranteeing a decline. Picking Numbers is a Kaplan strategy normally used on the Quantitative section, but it also applies to the CR section when paraphrasing would be useful; in this case, we'll pick the probability of overcrowding causing trouble. "Overcrowding, the only cause of reading declines, has a 20% chance when present of hurting reading scores."

This is consistent with the author's reasoning--he says that only overcrowding could cause a decline, and our hypothetical rule says the same.

Take a look at (D). Some schools have overcrowding but no decline. In our hypothetical, (D) is no only allowed, but expected! if 20% of overcrowded schools have declines, 80% don't, so we knew all along we'd see schools with overcrowding but no decline.

(E), meanwhile, is impossible under the author's assumption. Because the author assumes that the only cause of reading declines is overcrowding, a decline without overcrowding should be impossible--and if it is possible, the author's assumption is undermined.
Sudhanshu
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by KapTeacherEli » Fri Aug 06, 2010 7:26 am
reply2spg wrote:I am just checking my understanding whether it is correct. So as you explained decline is 'only and only' because of overcrowding. So If I have overcrowding then it doesn't matter whether I have decline or not. However, If I have decline then overcrowding must be there. If it is not then that option is weakening the argument
Bingo!
Eli Meyer
Kaplan GMAT Teacher
Cambridge, MA
www.kaptest.com/gmat

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