EXCEPT questions in logic reasoning

michaelt
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EXCEPT questions in logic reasoning

Postby michaelt » Thu Oct 17, 2013 1:27 pm

Some questions of LR are formulated with the word EXCEPT, e.g.

"Each of the following, if true, would help to resolve the apparent discrepancy EXCEPT:"

The obvious approach is this:

1) if it is relevant and helps, the answer is incorrect
2) if it is irrelevant or doesn't help, the answer is correct

Now, there is a weird case in PT30, section 2, question 19:

"Each of the following principles is logically consistent with conclusion EXCEPT:"

The first answer (A) is out of scope, so I chose it and moved on without even reading the other answers. It turned out this was a wrong way to solve it. Out-of-scope means that it is unknown whether it is consistent or not. And unknown means it might be consistent. So the answer was incorrect. The correct answer must be clearly inconsistent, so it's (C). The answer (E) is even more clearly out of scope and, by the same logic, is not correct.

The approach of eliminating answers with irrelevant information has always worked so far. Why is it different here? Is there anything special about the wording "logically consistent"?

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neprep
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Re: EXCEPT questions in logic reasoning

Postby neprep » Thu Oct 17, 2013 2:26 pm

Well the approach you listed might be correct apropos of Resolve-EXCEPT questions, but the question here isn't a Resolve-EXCEPT question, so it changes the way you approach it. I never treated EXCEPT as a separate question type with a unique approach; weaken-EXCEPT, strengthen-EXCEPT, resolve-EXCEPT etc. can't all be solved with the same strategy.

Out-of-scope means that it is unknown whether it is consistent or not.


The answer choices that are "irrelevant" are indeed known to be logically consistent with the conclusion; that is, any AC in the question that doesn't directly contradict the the stimulus is logically consistent with it. It is not unknown whether it is or isn't consistent. See what LSAC is saying: Each of the incorrect answer choices is consistent with the conclusion.

For example, if my argument is "Since Sally is 4 feet tall at the age of three, she should be considered unusually tall for her age," then each of the following is known to be logically consistent with my conclusion:

1. Sally's last name is Mathis
2. Sally sells sea shells
3. There can only be two mauve dinos in the display

While this is clearly inconsistent:
4. People shouldn't consider a girl unusually tall based on her age and height alone

michaelt
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Re: EXCEPT questions in logic reasoning

Postby michaelt » Thu Oct 17, 2013 3:08 pm

neprep wrote: ... any AC in the question that doesn't directly contradict the the stimulus is logically consistent with it.

in other words: "Not inconsistent, therefore consistent". Let's compare it to "true EXCEPT:" questions:

1) "the following could be true EXCEPT:" (i.e. the right answer MUST be false)
2) "the following must be true EXCEPT:" (i.e. the right answer COULD be false).

Notice that LSAT makers never use "the following is true EXCEPT:", they always add "must" or "could". So in "logically consistent" case it should be the same: either "must be logically consistent", or "could be logically consistent". From your explanation it seems that "logically consistent" somehow intrinsically means "could be". Is it so?

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neprep
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Re: EXCEPT questions in logic reasoning

Postby neprep » Thu Oct 17, 2013 3:17 pm

michaelt wrote:
neprep wrote: ... any AC in the question that doesn't directly contradict the the stimulus is logically consistent with it.

in other words: "Not inconsistent, therefore consistent". Let's compare it to "true EXCEPT:" questions:

1) "the following could be true EXCEPT:" (i.e. the right answer MUST be false)
2) "the following must be true EXCEPT:" (i.e. the right answer COULD be false).

Notice that LSAT makers never use "the following is true EXCEPT:", they always add "must" or "could". So in "logically consistent" case it should be the same: either "must be logically consistent", or "could be logically consistent". From your explanation it seems that "logically consistent" somehow intrinsically means "could be". Is it so?


Well, no, because in PT30, Section 2, Number 19, the stem clearly does say "is logically consistent...except." When you're talking about "true EXCEPT," your task is distinct from the task of finding logical consistency or the lack thereof. So I wouldn't extrapolate from trends about "true EXCEPT" questions any guiding principles to help with "logically consistent EXCEPT" questions. They are two different question types with different ideal approaches.

michaelt
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Re: EXCEPT questions in logic reasoning

Postby michaelt » Thu Oct 17, 2013 4:08 pm

Ok, let's not compare to "must/could be true" questions. Let's compare to paradox:

the following helps to resolve paradox EXCEPT
the following contributes to a resolution of discrepancy EXCEPT
etc.

If the answer claims something irrelevant, we say it does not help, does not contribute, therefore the answer is correct. We take it as "irrelevant, therefore not helpful". Yet why do we take "irrelevant, therefore not inconsistent, therefore consistent" ?


If "not inconsistent = consistent", then "not unhelpful = helpful", and I could say "irrelevant, therefore not unhelpful, therefore helpful" :)

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neprep
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Re: EXCEPT questions in logic reasoning

Postby neprep » Thu Oct 17, 2013 4:16 pm

michaelt wrote:Ok, let's not compare to "must/could be true" questions. Let's compare to paradox:

the following helps to resolve paradox EXCEPT
the following contributes to a resolution of discrepancy EXCEPT
etc.

If the answer claims something irrelevant, we say it does not help, does not contribute, therefore the answer is correct. We take it as "irrelevant, therefore not helpful". Yet why do we take "irrelevant, therefore not inconsistent, therefore consistent" ?


Irrelevant facts do not help resolve discrepancies, therefore irrelevant answer choices on resolve-EXCEPT questions are correct.

So the reason we take different approaches to resolve-EXCEPT and logical consistency questions is that they are completely different question types.

Basically, just because an AC seems irrelevant and you see "EXCEPT" in the question stem does not allow you to just select it and move on. Each EXCEPT question is different, and those kinds of shortcuts will usually always lead you to the wrong answer.

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neprep
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Re: EXCEPT questions in logic reasoning

Postby neprep » Thu Oct 17, 2013 4:40 pm

michaelt wrote:"


If "not inconsistent = consistent", then "not unhelpful = helpful", and I could say "irrelevant, therefore not unhelpful, therefore helpful" :)


No, in the context of Resolve-EXCEPT questions, irrelevant does not equal "not unhelpful."

An AC that is irrelevant = unhelpful.

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Jeffort
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Re: EXCEPT questions in logic reasoning

Postby Jeffort » Thu Oct 17, 2013 6:22 pm

This question stem is asking for the answer choice that MUST BE FALSE according to the information in the stimulus, simple as that.

The word EXCEPT in a stem is always asking for something that constitutes the logical opposite or extreme opposite of what is otherwise being described.

Consistent with something else is just a way of saying that something is possible/could be true/can logically co-exist with the other information. The logical opposite of could be true = must be false.

The logical opposite of consistent can also be described as inconsistent/NOT consistent, which means that it contradicts, which are all just synonymous ways of saying must be false, so, in other words, the stem is asking for something that must be false.

This is a particular type of LR question, not really an EXCEPT question to a normal type. It's just a tricky way of phrasing the question stem to mask that the question is simply asking for something that must be false. You also see questions of this type with stems that specifically ask for something that 'cannot be true'/'must be false'. Some stems for this type also use the word compatible to throw people off by asking 'each of the following is compatible with the information above EXCEPT'. Same thing, just different way to paraphrase the same logical criteria for the answer. LSAC just likes to engage in some fun word play with the various different stems for this must be false question type to mess up unprepared students and get them to freak out on test day and go into a mild panic due to not immediately recognizing the task of the question.

michaelt
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Re: EXCEPT questions in logic reasoning

Postby michaelt » Thu Oct 17, 2013 7:49 pm

This is exactly what I have been looking for: "consistent = could be true". I mistakenly assumed "consistent = must be true"

The answers could be:
1) True
2) False
3) neither true, nor false; could be true and could be false; irrelevant; unknown; etc.

--------------------------------------
must be true = 1 True
must be true EXCEPT = 4 True
--------------------------------------
could be true = 4 False
could be true EXCEPT = 1 False
--------------------------------------

helps to resolve = MUST BE TRUE
contributes to the conclusion = MUST BE TRUE
consistent with the conclusion = COULD BE TRUE

Since this is my first time I ran into "consistent with" question, I am a little worried if there are other ways. Does anyone know of any other ways to imply "could be true"?

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neprep
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Re: EXCEPT questions in logic reasoning

Postby neprep » Fri Oct 18, 2013 4:44 am

michaelt wrote:This is exactly what I have been looking for: "consistent = could be true". I mistakenly assumed "consistent = must be true"

The answers could be:
1) True
2) False
3) neither true, nor false; could be true and could be false; irrelevant; unknown; etc.

--------------------------------------
must be true = 1 True
must be true EXCEPT = 4 True
--------------------------------------
could be true = 4 False
could be true EXCEPT = 1 False
--------------------------------------

helps to resolve = MUST BE TRUE
contributes to the conclusion = MUST BE TRUE
consistent with the conclusion = COULD BE TRUE

Since this is my first time I ran into "consistent with" question, I am a little worried if there are other ways. Does anyone know of any other ways to imply "could be true"?



Answer choices in "Helps to resolve" and "Contributes to the conclusion" are not always MUST BE TRUEs. They are simply potential ways in which a set of seemingly contradictory evidence could all be true, or are one of the many ways in which the conclusion could follow from the premises given.




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