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Jackie Chiles
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Venn diagram questions

by gmatalongthewatchtower Wed Aug 15, 2012 2:27 pm

I have been practicing some LSAT problems (LR). However, I tend to miss examples that require us to draw a venn diagram or at least make a mental picture of those questions. For instance, I missed PT16 S3 Q24 and PT29 S4 Q24; PT29 S1 Q18. Can you please let me know some other similar examples?

For instance, kids wearing brown shirts are more likely to buy a headphone than those who don't wear brown shirts. Kids, whose mother drive Hummer, are more likely to go to public school than those whose mother don't drive Hummer. In answer choices, the LSAT then mixes all different possible combinations of modifiers. Essentially, the LSAT mixes and jumbles all Necessary and Sufficient conditions. Do you know such questions so that I can do focused practice? I seriously need your help :(

Thanks in advance.
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Re: Venn diagram questions

by timmydoeslsat Fri Aug 17, 2012 4:48 pm

OK, this is a really good topic.

I have viewed the problems you posted about.

PT16 S3 Q24 (Flaw Question Stem) is something that I would view as testing a different skill than, say, PT29 S1 Q18 (Must be True).

Let's cover how to handle must be true inference type question stems.

PT29 S1 Q18:

PCM some SFI

PC ---> ~LS

You can just feel that this going to be testing a classic syllogism. Major premise - minor premise. We know that every single person that is a PC is a ~LS. We know that some people from the PC are SFI. We know that we can infer some SFI are ~LS.

The above is no different than:

A ---> B

A some C

So we know we have: C some B

I typically have a prephrased answer in my head for a must be true question like this so I do waste mental energy thinking about those other things. I am confident in my ability to infer from the evidence given.

From the same preptest, PT29 S4 Q24 (Weaken Question Stem):

This is, in my opinion, testing a different issue as well from the flaw question stem of PT16 S3 Q24, which will be addressed in this post as well. But for 29-4-24 first...

This is a correlation causation issue. For the conclusion to say that adequate prenatal care (APC) decreases a risk is to fundamentally say that APC is causing the decreased risk. The evidence put forward is that those with APC are more likely to have heavier weight babies. That is classic correlation language.

In my estimation, you could have totally left out the first bit of the stimulus and this argument simply suffers from the same problem. Its last line in evidence is a correlation and jumps to a cause. Notice that the first bit in the stimulus is out there not being used. Expect the test writers to bring relevance to that statement in the weakener.

Answer choice B shows that entire statement of APC is more likely to.... is not worth the paper it is typed on. You had records of babies born as prematures, and we know that they are more likely to be born underweight than ~prematures. So now we have doubt about the legitimacy of APC more likely to have heavier babies than ~APC. Yeah...that is the case due to the workers placing people in the ~APC category when we do not know that they are.

Answer choice C is the only other reasonable contender on this question, but it does not do anything for us regarding APC. We want to weaken the APC causal conclusion.

As for the first problem you addressed: PT16 S3 Q24...

I think the best way to alter your thinking about these type of stimulus is with an analogous situation.

It is easier to injure yourself by jumping off of a skyscraper than being involved in a car accident. So more injuries occur in skyscraper jumps than car accidents.

We would, obviously, need to know the occurrence of these two activities. In the same way, although women over 40 are more likely to have an ambidex child than those not over 40, can we really say that more ambidex children are born to the women over 40 than those not? We need to know how frequency of these activities.

Regarding this point you, made:
For instance, kids wearing brown shirts are more likely to buy a headphone than those who don't wear brown shirts. Kids, whose mother drive Hummer, are more likely to go to public school than those whose mother don't drive Hummer.

This is correlation language. We do not know that wearing brown shirts is causing this headphone buying. We also do not know that mothers driving hummers are causing the kids to go to public school. Perhaps something is else is causing both the public school phenomenon and the public school phenomenon.
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Re: Venn diagram questions

by ManhattanPrepLSAT1 Sat Aug 18, 2012 3:10 pm

Great explanation Timmy!

Let me just add that I would not use Venn Diagram's on the LSAT as they can easily lead to invalid inferences. Take a look at the following:


Additionally, here are some examples to work on quantified statements, good luck!

PT19, S4, Q13
PT19, S4, Q21
PT20, S4, Q19
PT21, S2, Q18
PT21, S3, Q25
PT23, S2, Q10
PT23, S2, Q12
PT24, S2, Q11
PT24, S2, Q24
PT24, S3, Q15
PT24, S3, Q19
PT25, S4, Q16
PT27, S1, Q22
PT29, S1, Q18
PT29, S1, Q23
PT29, S4, Q21
PT30, S2, Q21
PT31, S2, Q10
PT35, S1, Q20
PT36, S1, Q11
PT41, S1, Q21
PT41, S3, Q20
PT41, S3, Q25
PT42, S2, Q23
PT42, S4, Q17
PT42, S4, Q22
PT43, S3, Q9
PT43, S3, Q16
PT43, S3, Q21
PT43, S3, Q24
PT44, S4, Q26
PT45, S1, Q21
PT45, S1, Q25
PT45, S4, Q16
PT45, S4, Q22
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Re: Venn diagram questions

by TylerW235 Tue Apr 10, 2018 9:37 pm

Great explanation!

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Re: Venn diagram questions

by PalakS853 Thu Dec 06, 2018 5:05 am

Thanks for the Explaination! :)