PT14.S4.Q09 - anyone who supports the new tax plan

ccwilso4-1ccwilso4-1 Free Trial Member
edited December 2015 in Logical Reasoning 30 karma
Hi! I am having difficulty with predicting descriptive flaw questions. If anyone has done this question can you please explain to me how the correct answer is D.

Comments

  • David WayneDavid Wayne Free Trial Member
    edited May 2014 571 karma
    Hi CCwilso,

    The LSAT knows that a lot of test takers like yourself understand that there's some sort of sufficiency/necessity confusion going on in this problem, but the answer choices aren't simply going to say "Confuses a sufficient condition for a necessary condition" or vice versa, because that'd be too easy (although some questions do). What this question is doing in the presentation of the correct answer choice is ensuring you understand the exact conditional relationship that the argument has botched.

    Let's start by diagramming our given information. Sentence 1 contains two conditional statements, which can be diagrammed as follows (i use the ~ to imply the negation of an idea)

    Support new tax plan -> ~Chance of being re-elected
    Truly understand economics -> ~ Support new tax plan

    While doing this problem I rewrote my information by expressing the second statement as a contrapositive:

    Support new tax plan -> ~Chance of being re-elected
    Support new tax plan -> ~Truly understand economics

    And now let's diagram the conclusion:
    Chance of being reelected -> Truly understand economics

    What's the flaw here? Intuitively we know its some sort of Sufficiency and necessity problem, but you need to pinpoint the exact conditional statement that has been mistakenly reversed (meaning, they interpreted A->B as B->A). So where is it?

    Well, it will become clear to you if you rewrite the conclusion as a contrapositive

    ~Truly understand economics -> ~ Chance of being reelected

    Now you must ask yourself: Where is the botched conditional relationship? If you can't figure that out, then ask yourself: Which conditional relationship can we reverse in order to get a valid argument? Well, if we had the conditional relationship {Support new tax plan -> ~Truly understand economics} replaced with the arrow reversed it would read as follows: {~Truly understand economics -> Support new tax plan}.
    If we had that, our argument would be valid. If ~Truly understand -> Support new tax plan, and Support new tax plan -> ~Chance of being reelected, then we can infer that chance of being reelected -> truly understand economics (express that as its contrapositive to see more clearly why it is inferable. Its essentially A->B->C, therefore A->C)

    But alas, we do not have that. So having identified the conditional relationship that was botched (Support new tax plan -> ~Truly understand economics) and understanding that the argument, with its faulty logic, has ASSUMED that A-> B implies B-> A, we merely need to identify the answer choice that pinpoints that error.

    The LSAT very commonly does this by stating the answer choice in terms of the possibility that the assumed conditional relationship may not in fact occur. So if the argument illicitly assumed B->A, then the answer choice can express the denial of that relationship. How do we deny a conditional relationship? We do so by saying that B even if ~A. If that were a possibility in our world, then B->A would not necessarily be a fact. In fact, it would be denied.

    Answer choice D tells us this. It says that the relationship that has been illicitly assumed (namely, ~Truly understand - > Support new tax plan) may not necessarily be true. Some people who don't understand economics may not support the tax plan. If that's true, the conditional relationship that has been assumed is actually denied, and the argument falls apart. Again, A->B was assumed, and answer choice D says that in our world, A and ~B can occur, and if that occurs, the conditional relationship cannot possibly be true.

    When doing sufficiency necessity problems that have a flaw, try to ask yourself what conditional relationship was illicitly reversed, and find the answer choice that expresses the denial of that relationship. Remember, we can deny a conditional relationship (For instance, A->B) by showing that i can have A, even if ~B. If i demonstrate that, i've demonstrated my conditional relationship isn't true of the world. Answer choice D identifies that the illicitly assumed conditional relationship may not occur in our world by denying the conditional relationship in this exact way.

    Please reply if you need clarification on anything i've said. Also, check out preptest 53 section 1 question 18 for another exercise utilizing this exact framework.
  • taboadaj21-1taboadaj21-1 Free Trial Member
    edited May 2014 3 karma
    Thank you so much!
  • ccwilso4-1ccwilso4-1 Free Trial Member
    30 karma
    Thank You! I have reread your post multiple times which has finally lead to me understanding. The only point that I am a little unclear about is, how do we know which sentence in the premise is sufficient and which is necessary.


    I have negated all statements to what I feel flows. This is what I have so far.

    Premises:
    chance -> ~support
    econ -> ~support

    Conclusion:
    econ -> chance

    At this point I can't figure out how exactly to find the flaw?
  • David WayneDavid Wayne Free Trial Member
    edited May 2014 571 karma
    Your conclusion is reversed. Only someone who truly understands economics means that 'econ' would be the necessary condition. So your conclusion should be:

    Chance->Econ.

    Flawed method of reasoning questions, like all arguments - as JY often notes - require you to think about how the premises INTEND to support the conclusion. We take the same approach whether we weaken or strengthen, or do NA or SA questions. Here, we understand that the argument intends to support the conclusion through formal logic. Again, if you think about what would make the argument valid - in other words, what the author mistakenly thinks his argument is proving - it becomes more clear.

    chance -> ~support
    econ -> ~suppor

    these can be translated into:

    Support -> ~Chance
    Support -> ~Econ

    ___

    Now let's express our conclusion as a contrapositive:

    ~Econ-> ~Chance

    Let me rewrite this in a simpler form.

    A->~B
    A->~C
    ________
    conclusion: ~C -> ~ B

    This would only work if ~C -> A. Then i could infer the conclusion because that would mean that ~C->~B, since if ~C -> A and A->~B, then ~C->~B.

    In our argument, we'd need Support new tax plan -> ~Truly understand economics to have its arrow reversed, in order to use lawgic to infer the conclusion. If we had ~Truly understand economics -> Support new tax plan, then we could infer ~econ->~Chance because we already have Support - > ~Chance.

    Here's what the valid argument would look like, with the fixed conditional relationship written in caps lock:


    ~ECON->SUPPORT
    Support -> ~Chance

    Conclusion:
    ~Econ-> ~Chance

    See how that's just A->B.>C , therefore A-> C? If it's still not clear to you, you need to revisit the lawgic lessons and spend more time with the problem. It took me a while until i was consistently comfortable with these.

    Do more of these and they'll become more clear. There are tons and tons of examples that commit the flaw but require you to pinpoint the exact conditional relationship that was botched. The only way you'll achieve mastery over the topic is if you see it in other problems and need to utilize your skills again and again

    If you can't determine how to translate sentences and how to determine what elements are sufficient or necessary, review your 7sage lessons, as i felt they provide a perfect understanding of how to translate english statements into lawgic quickly and accurately, far better than i could do here.



  • ccwilso4-1ccwilso4-1 Free Trial Member
    30 karma
    I understand completely now, thank you so much!!! Your second explanation really filled in the gap for me. I really struggle with Flawed method of reasoning for some reason, but I just realized that I still have more flawed lessons, so hopefully they help me understand each type! Good luck with your exam!!!
  • LSATislandLSATisland Free Trial Inactive Sage
    edited May 2014 1878 karma
    @David Michael-1-1

    I didn't read this whole conversation but your LSAT ability is visible from a cursory reading. The referencing you do to other LSAT questions reflects a thorough familiarity with the LSAT. I'm sure you'll do awesome on the coming LSAT.

    Capitalize your "I"s . You deserve it.
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