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Complete Question Explanation

Parallel Flaw. The correct answer choice is (B)

The question stem indicates that there are two logical flaws in this argument. Before proceeding to any of the answer choices, we must clearly identify each of these flaws. The premises are that Professor Donnelly’s exams are always more difficult than Professor Curtis’s exams and that the question about dinosaurs was on Professor Donelly’s exam. According to the argument, this question must therefore be difficult.

In general terms, this conclusion mistakenly assumes that what is true of the whole must also be true of each of the parts and confuses a relative statement with an absolute statement. Because Professor Donelly’s exams as a whole are more difficult than Professor Curtis’s exams does not mean that each question on Professor Donelly’s exams is itself difficult. In fact, Professor Donelly’s exams may be more difficult because of an exam time limit rather than difficult questions. Also, we cannot determine if Professor Donnelly’s exams are truly difficult at all. Unlike this exam, both professors’ exams may be ludicrously easy and Professor Donelly’s exams may be only slightly less easy than Professor Curtis’s exams.

Therefore, the correct answer choice must incorrectly claim that some part of a group has a certain characteristic just because the group as a whole has more of that characteristic than some other group. Only answer choice (B) displays both of these flaws.

Answer choice (A): Since this answer does not conclude that Lewis’ cake must be good, it does not display the second logical flaw discussed above. It does, however, conclude that one of Lewis’ cakes will be better than most of Stockman’s cakes because Lewis is a better baker than Stockman. This is similar to the first logical flaw since Lewis might specialize in baking bread and pastries. In any case, answer choice (A) is incorrect.

Answer choice (B): This is the correct answer choice. Just as Professor Donelly’s exams are more difficult than Professor Curtis’s exams, Porter’s new book of poetry is better than any of her other books of poetry. Of course, this does not ensure that a poem from Porter’s new book is good. The selected poem might be a bad example from a good collection or the entire book may be bad, just not quite as bad her other books. Both of these mistakes match the flaws from the stimulus.

Answer choice (C): This answer does confuse a relative statement with an absolute statement, but not in precisely the same way as the stimulus does. Here, the relative statement in the conclusion (“This year’s English class will have to do more reading than last year’s class”) is erroneously drawn from an absolute statement in the premise (Professor Whitburn “always assigns a lot of reading.”) In the stimulus, the error was reversed. Furthermore, this answer does not contain the first logical flaw.

Answer choice (D): We cannot that the plot of Shield’s first novel is very complex just because it is more complicated than any other novel she has written. This flaw is identical to the second flaw discussed above, but this answer does not make the mistake of assuming that any part of a group has the same characteristic as the group itself.

Answer choice (E): While some students may argue that this answer mistakenly assumes calculus has the same characteristic as mathematics overall, must students probably agree that calculus is a difficult part of mathematics. Either way, this answer does not make an absolute claim based on a relative premise. Also, you should note that this answer choice is about tests and the stimulus is about exams. Any answer choice which addresses a similar subject as the stimulus should be a red flag. Such answer choices are occasionally correct, but they are usually traps for hurried or careless students.
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Hi Guys I am having trouble with spotting the flaws in question# 20, Section 2 LSAT 46. Supposedly the argument has 2 flaws. I think one is the flaw in the usage of evidence i.e. whole to part flaw but I cannot figure out the second one. I'd appreciate it if someone could help me with it :)
20. Professor Donnelly's exams are always more difficult than professor Curtis's exam. The question about dinosaurs was on Professor Donnelly's last exam. Therefore, the question must be difficult.
 Steve Stein
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Good question--a two-flaw stimulus can be challenging.

One flaw: The fact that D's exams are more difficult than C's doesn't mean that D's exams are difficult in general (perhaps, for example, C's are 1st grade level, and D's are 2nd grade level).

And the other: Even if the test was very hard, that would necessarily mean that every single question on the test was difficult.

I hope that's helpful! Let me know--thanks!

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Thanks Steve! It was very helpful.
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Hello ;
For this question I see both flaws , however I had trouble getting rid of answer choice A and E. ( I saw both flaws in both choices )!

1- professor Dom's exams are always more difficult than professor Curtis's exam.
2- the question about dinosaurs was on Professor Don's last exam.

Conclusion: the question must be difficult.

Flaw 1- is a whole to part : ( just because the exam a whole is more difficult doesn't make the one question difficult)
Flaw 2- relative , absolute. ( just because one profs exam is more difficult than the other , it doesn't mean that it's actual difficult, what if the other profs exams are jokes ) .

Thanks so much ,
 Clay Cooper
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Hi Sherry,

Thanks for your question, it is a good one.

I think you have fairly accurately named both of the flaws in this stimulus - the first being that it is erroneous to conclude anything about an individual question on Prof. D's exams based on evidence that only applies to those exams as a whole, and the second being that it is erroneous (as you point out) to conclude that any question of Professor D's is actually difficult simply because it is more difficult than something produced by Professor C.

The reason that A is wrong is that it lacks the second of these errors (the erroneous jump from relative comparisons to absolute judgments). Notice that the conclusion in a is entirely comparative ('better' rather than 'good').

The reason that E is wrong is precisely the same; it also draws an entirely comparative conclusion ('more difficult' as opposed to 'difficult') and avoids making the error of jumping from comparisons to absolute judgment that we see both in the stimulus and in answer choice B (the erroneous jump from 'better' to 'good').

I hope that helps! Keep working hard.
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I understand by B is correct. It makes the flawed argument that what is true of the whole is true of the individual parts. What I don't understand is why E is wrong. If mathematics is harder than history, then to assume that the calculus test is harder than the history test seems to be the same flaw. Can you explain? Thank you.


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 Jonathan Evans
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Hi, Micah,

Great question. In this question, let's prephrase by breaking down the two flaws.

First the premises:

Every one of a first group of items is more difficult than any one of a second group.

Now, the conclusion:

Therefore, a particular part of one of the first group must be difficult.

The question stem indicates that there are two flaws. Let's identify them.

First, there is a fallacy of division. What is true of the whole may not be necessarily true of one part.

Second, there is a flaw of comparison. The premises indicate relative degrees of difficulty. One test's difficulty versus another test's difficulty. The conclusion only indicates some absolute standard of "difficulty" without respect to the second group.

In other words, we could imagine that Professor Donnelly's exams are all harder than Professor Curtis's. However, relatively more difficult as they may be, they may actually be easy by some arbitrary different standard. Therefore we have no basis to claim that any of Professor Donnelly's exams is difficult unless we make reference to Professor Curtis's exams.

Answer Choice (E) makes a comparison in the conclusion which is not present in the flawed conclusion in the stimulus. Hence, it is not a complete match.

I hope this helped answer your question. Let me know if you need further help with this problem.
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Great Explanation. Thank you.
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I am not in a live class so I can't ask a question during the recorded lessons. While listening to Eric's explanation of the two flaws in the stimulus I was ultimately seeing three. It did lead me to answer correctly in (B) but wanted to know if I was seeing something that's not really there.

The flaw of division is clear, as is the relativity flaw (after the explanation in the lecture) but isn't there also a false dilemma? In stating the question must be difficult the author is assuming there is only more or less difficult as possibilities. Isn't it possible to be equally difficult? Since we haven't covered the relativity flaw concept I was looking for the whole to part and false dilemma... Luckily it's kind of the same as the relativity flaw but worried I might steer myself wrong on test day.

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