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Flaw Questions: Sufficiency/Necessary distinction issues

til354567til354567 Member
in Logical Reasoning 53 karma

In questions where it is appears that there is a Sufficiency/Necessary issue, are there examples of questions where the SA/NA answer is a trap? I find myself spending way too much time trying to decipher if the stimulus is mistaking sufficiency for necessity or mistaking necessity for sufficiency even when there is only one Answer Choice involving SA/NA.

Does anyone have an example where the distinction matters? My gut tells me to recognize the cookie cutters where you can find them and move on.

For example in PT 80 - Sec 4 - Q16, I knew B was right but for some reason was thinking the order of SA/NA in AC B matters (even though it does not) which made me think that maybe there was a sneaky trap (though I knew there really wasn't).

I go -1 or -2 on LR so don't think this really a foundations issue, more of a confidence thing. Thanks for any thoughts!

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Comments

  • danielbrowning208danielbrowning208 Alum Member
    531 karma

    There are definitely SA/NA traps, usually where the flaw is stated in the opposite terms. For example, if the argument confuses sufficient for necessary, the trap answer would say the argument confused necessary for sufficient.

    So you certainly want to make sure the relevant answer choice is descriptively accurate and that it matches what the argument is actually doing. But at your score range you should be able to do this quickly. However, once you ensure that the answer choice does match the SA/NA flaw, you can confidently move on.

  • Jonathan WangJonathan Wang Yearly Sage ⭐
    edited June 2020 6866 karma

    Reiterating OP's ask - can someone please give a citation of a question where it does matter (and preferably several, if you have them)? I've seen several people at various points say that they've seen questions with this trap but I can't remember any off the top of my head.

  • BinghamtonDaveBinghamtonDave Alum Member ⭐ 🍌🍌
    8694 karma

    @"Jonathan Wang" @til354567 I am not aware of a single question where the concept of "confusing necessary for sufficient" is conflated in a way to trick us into choosing "confusing sufficient for necessary" in an answer choice or vice versa.

    The only thing that I believe comes close to what OP is asking from my reading is PT 51 Section 1 Question 20, where the question gives us:
    A---->B
    then concludes B
    On the basis of A
    Answer choice (B) fits this pattern: the denial of the sufficient bringing about the denial of the necessary, but answer choice (E) provides us with:

    A--->B
    In front of us we have a B
    Therefore in front of us we have an A

    This is a question (the only one to my knowledge but I could be wrong) where the distinction makes a difference. I did a write up on this question on the comment section found here awhile back:
    https://7sage.com/lsat_explanations/lsat-51-section-1-question-20/

    I should also note here as a tutor and soon-to-be 1L that we have a few small caveats in how the LSAT describes/the vocabulary around the the suff/necess flaw that we should be aware of:

    -The first comes from PT 75 Section 1 Question 12 where answer choice (B) looks like it is describing a flaw but is actually describing the function of a valid contrapositive! Which isn't really even the flaw here for this tough question.

    -The LSAT can really hide the Suff/Necess flaw well: please see PT 22 Section 2 Question 25. Here we might be focused on the survey/sample and be diverted from the core flaw.

    -If you ever encounter a flaw question where the logic presented looks valid: ie they did an operation with a conditional statement in a way that the logic allows us, then look to the concepts/words for what we could call a "detail creep."

    Hope this helps
    David

  • til354567til354567 Member
    53 karma

    @BinghamtonDave said:
    @"Jonathan Wang" @til354567 I am not aware of a single question where the concept of "confusing necessary for sufficient" is conflated in a way to trick us into choosing "confusing sufficient for necessary" in an answer choice or vice versa.

    The only thing that I believe comes close to what OP is asking from my reading is PT 51 Section 1 Question 20, where the question gives us:
    A---->B
    then concludes B
    On the basis of A
    Answer choice (B) fits this pattern: the denial of the sufficient bringing about the denial of the necessary, but answer choice (E) provides us with:

    A--->B
    In front of us we have a B
    Therefore in front of us we have an A

    This is a question (the only one to my knowledge but I could be wrong) where the distinction makes a difference. I did a write up on this question on the comment section found here awhile back:
    https://7sage.com/lsat_explanations/lsat-51-section-1-question-20/

    I should also note here as a tutor and soon-to-be 1L that we have a few small caveats in how the LSAT describes/the vocabulary around the the suff/necess flaw that we should be aware of:

    -The first comes from PT 75 Section 1 Question 12 where answer choice (B) looks like it is describing a flaw but is actually describing the function of a valid contrapositive! Which isn't really even the flaw here for this tough question.

    -The LSAT can really hide the Suff/Necess flaw well: please see PT 22 Section 2 Question 25. Here we might be focused on the survey/sample and be diverted from the core flaw.

    -If you ever encounter a flaw question where the logic presented looks valid: ie they did an operation with a conditional statement in a way that the logic allows us, then look to the concepts/words for what we could call a "detail creep."

    Hope this helps
    David

    Very helpful and great examples.

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