Q25 - Roberta is irritable only when she is tired

[mshinners]

Question Type:
ID the Flaw

Stimulus Breakdown:
Irritable → Tired
Loses things → Tired
Yawning + Lost keys
Therefore, Irritable.

Answer Anticipation:
First thing I note right away is that the conclusion is that R is (almost certainly) irritable, but my first conditional has irritable as a sufficient condition. I can conlude necessary conditions if I have the sufficient condition, but nothing lets me conclude the sufficient condition (if you take the contrapositive, you can conclude the negation of the sufficient condition; here, a conclusion about not being irritable could be supported). So there's an illegal reversal.

Looking at the premises, there's also a gap between yawning and tired - one can yawn without being tired. However! the premise also establishes that Roberta lost her keys. This triggers the second conditional, letting us conclude that she's tired. So while the argument tries to get us to think there's a term shift between "yawning"" and ""tired", that's actually not a flaw here since the argument let's us infer she's tired based on her losing something.

Correct answer:
(E)

Answer choice analysis:
(A) Tricky, tricky LSAT. This answer plays off of the Term Shift I discussed in the Answer Anticipation section. However, since the argument establishes that she was yawning while losing her keys, the argument doesn't need yawning to guarantee being tired. Roberta losing her keys is enough to guarantee she's tired.

(B) Wrong flaw (Circular). None of the premises repeats the conclusion (the conclusion is a non-conditional statement of irritability; the premise about irritability is a conditional statement).

(C) Wrong flaw (Bad Generalization). The premises are general, but the conclusion is about a single instance. For this flaw to apply, that would have to be flipped (premise about a single instance; conclusion as a general rule).

(D) Tempting! This answer definitely refers to an Illegal Reversal. However, the illegal reversal happened with our first conditional about irritability, not our second conditional about losing things.

(E) Bingo. This answer choice is the normal Illegal Reversal language. If you get the abstract language for the flaws down, picking this answer should be quick. We have flashcards for this!

Takeaway/Pattern:
Work on those abstract answers! Check out our flashcard set on the topic. Also, if a Flaw question features conditional logic, there's a very solid chance the flaw is an Illegal Reversal/Negation.

#officialexplanation

[cacrv]

hoping someone will see this in the next 24 hours (yikes)!
I picked the right answer with confidence, but now I'm trying the question again, and can't find the logic in my head!

Why is D wrong?

Irritable --> tired
Lose things --> tired
------
Tired & Lose things ---> Irritable

I drew this diagram and think that both D and E apply above? What am I missing here?


Thanks in advance!

[Salvadoretorrez]

Well let's break down the argument step by step, which requires a good grasp of how to diagram only statements of course, it is simple is as follows:

Irritable ONLY when tired
I--->T
Loses things only when tired
L--->T

the two are then combined as such

I--->T
L--->T

conclusion: Yawned all day(infers being T) and has lost her keys (L) so she is most certain able irritable

Well think about what the flaw is her for a second; let's recognize that losing things is never seen as a necessary condition while in the conclusion being tired is used as a sufficient condition!

The conclusion is diagramed as such:
T and L--->I

do you follow me here? the problem is not that the necessary condition of losing things is used as a sufficient condition, because for the sake of the argument, each premise EXISTS in it's own world, you cannot combine them to make any valid inferences and as such have to be treated as they are different; so in order to make the understanding a little simpler, you have to see tired in both premises as nonidentical statements even though they are the same, but if they WERE THE SAME, BOTH D AND E WOULD BE CORRECT!!

But as such with the nature of conditional logic, they cannot be treated as the same because there is no valid inferences one can make from combining both statements.

So simply put, the correct answer would state, that the argument takes a necessary condition for being irritable to be a sufficient condition, because as seen in the argument being tired(in the context and premise of being irritable) is used as a sufficient condition when in the premise it is the necessary condition.

This is a very difficult question and requires a good fluency and understanding of conditional logic, because you have to understand what applies to certain worlds, scenarios, or etc has to be deemed as valid if you are going to make in transitive, that is not the case here.

The trick in this question is understanding that by conditional logic, tired in both premises does not have an identical understanding in the conclusion, as we have learned by studying for the LSAT; for the most part the losing of the keys is irrelevant here because multiple sufficient conditions can trigger the same necessary condition but the reversal occurs with tired for the irritable premise.

Thus Tired from Irritable and Tired for Losing things can be seen as non identical necessary conditions because they do not possess the same relationship to the conclusion! to try to make this simple, each premise has a sub context to it in which it only applies in that world, try to understand the diagraming as such:

(I-->T1)
(L-->T2)
Conclusion (T1 and L--->I)

Take the following as another question that achieves the same flaw:

Bank Deposits are credited on the date of the transaction only when they are made before 3 PM. Alicia knows that the bank deposit was before 3 PM. So Alicia knows that the bank deposit was credited on the date of the transaction

Diagramed as follows:
Deposit Credited--->made before 3 PM
(she knows) Deposit made---> before 3 PM

conclusion: (she knows) bank deposit credited

Now in this one, it really isn't exactly the same flaw but I used to show you something; it is possible that each premise can exist and apply in its own world!; as seen with the context of her knowing something. In this case Alicia could not have known the deposit was credited because how do we know that she had the knowledge that her deposit would be credited if it was made before 3 PM in the first place?! we are only told that she knows that she made the deposit for 3 PM, not that she knows it will be credited before 3 PM!

confusing I know, but these are some of the most difficult flaws you will see on the LSAT, don't stress out, I'm taking it Saturday as well and we will both be fine!

I hope this helps!

[cacrv]

thanks so much!!! I definitely get it now - two separate worlds, two different tired's. I only thought conditionals in different worlds in the context of the games but it definitely makes sense here - thanks again!!!

[tommywallach]

Wow! Great work you guys!

-t

[zen]

I had alot of trouble with this one and many of the explanations I've read are good, but still leave me a little confused.

So here's my more simple take.

First let's get rid of answer choices A-C.

(A)- Tempted me at first, but it is wrong because he argument never mentions that yawning causes tired, nor does it indicate there is any correlation at all! It is easy to gloss over this fact because the term shift is pretty apparent and you usually want to exploit such term shifts; but this isn't a problem in this argument.

(B)- It does not assume the conclusion, a.k.a. circular reasoning. It concludes something different from what the premises give us.

(C) 1) This isn't a single instance. We have an occurrence of lost keys AND yawning. Furthermore, the premises we are given apply to ALL instances, so if we have one instance, we can validly infer something general about it.
More simply: this answer choice just doesn't describe the flaw at all

Now for the tricky answer choices:

I'll explain (D) and (E) together.

Key: L represents "losing things", I represents "irritable", T represents "tired", and Y represents "yawning".

Ok, so we are given this setup-

1) L-->T
and
2) I-->T
----------------------
Conc: Y+L-->I
(Notice how I did not equate Yawning with Tired-- it is because we don't know if they are the same; yawning is a mere indication that one could )be tired, but it does not mean the same thing. As you yawn reading this, are you really tired or just bored? )

So what's the problem here? Well, actually, it can be simple to figure out! Ask yourself "What premise allows us to get to the necessary condition of the conclusion" i.e. what premise allows us to conclude she's irritable? Well, no do, right? But what premise involves the term in the necessary condition of the conclusion? Premise 2--the only premise involving Irritable.
Now read answer choice (E)- takes a necessary for a sufficient. So that means the author thinks premise 2 actually means "T-->I"! He illegally reverses the conditional in order to suit his own nefarious reasoning ends.How does he get T? Well we know L-->T and if we make the same error as the author we could say "L-->T-->I" which allows us to draw this faulty conclusion- "L-->I".
Why is answer choice (D) wrong? Because it does not involved irritable at all! Taking the necessary for sufficient for (D) would mean "T-->L" but this does nothing for reaching the conclusion; we need to bring "I" in somehow and the only way to do that is to illegally reverse the premise involving irritable and use Tired(which we have) as a sufficient condition for Irritable; but in doing so, this violates a cardinal rule of the LSAT-- never take the sufficient for the necessary or the other way around.

I hope this helps someone. Cheers.

[Fleetwood_J_Thomas]

I have spent 2 hours trying to wrap my mind around the the conditional logic here. I understand why A-D are wrong but I am trying to understand how to correctly 'understand' the vocabulary of E.

Rule:
I-->T
L-->T

Story:
L&T(yawning) --> I

It makes more sense to me if the answer E said "Takes the Sufficient condition for Roberta being Irritable (The Rule) to be a Necessary condition (The Story)...

Please correct my thinking!

-Thomas

[SameekshaM239]

After looking at this question for ages (I hate the wording), I think I finally get it...

"Takes a necessary condition for Roberta's being irritable to be a sufficient condition"


So we are referring to this premise:

L -> T
I -> T

And this false conclusion:
L -> I
T -> I

Specifically, the error shows up at this part, which is getting illegally reversed:

I -> T
T -> I

The answer choice (E) actually refers to the T in the premise ( I -> T), the "necessary condition for Roberta's being irritable," rather than the I.

So the flawed reasoning is that they are taking the T (necessary in the premise) as sufficient.

I think a lot of us that had trouble with the wording was because we focused on the "I" (which makes the most sense to me), whereas the answer choice focused on the T.