## Solving Necessary and Sufficient Assumption Questions

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slingshot_fuel

Posts: 5
Joined: Tue Jul 16, 2013 8:37 pm

### Solving Necessary and Sufficient Assumption Questions

Firstly, let me start off by extending my gratitude to all of the forum members for posting their immensely helpful information regarding the preparation for the LSAT! I read tons of topics on this site and have learned methodologies and reasoning that seem to have completely escaped most other LSAT sites. This brings me to my question: I have been studying for about 3 months and am making some very nice gains in my scores. During my preparation, I continually notice and make notations of a conditional logic trend that may lend itself to my solving various LR problems with more confidence. My possible misunderstanding of this common trend, or my sheer lack of intelligence, has caused me to often choose the wrong answer choice. Finally, please let me qualify this post and my question by apologizing in advance - In case my post is inherently stupid and/or the answer is so obvious... please disregard if this is the case.

The best way I can explain is by looking at this particular question: PT 45 Section 1 #21. It is a rather simple assumption question. The conclusion states "Thus, the geological record suggests that there is no consistent causal link between major meteor impacts and mass extinctions." The problem I have faced numerous times while drilling is that I can usually eliminate 3 of the 5 answer choices without much effort. The remaining 2 answer choices are typically an extremely similar conditional statement, however, they are reversed, negated, or slightly altered in some other way. So, since the conclusion as stated above is the main point of the argument, is it wrong for me to attempt to solve this by visualizing the following: "--> the geological record suggests that there is no consistent causal link between major meteor impacts and mass extinctions"? I deliberately left the sufficient side of the arrow blank. So, since we are trying to prove this conclusion, and lets pretend that the correct answer choice would have to contain the concluded statement (either in the form as stated or negated), is it correct to immediately go the only answer choices that have either of the following 2 scenarios:
1) If "there is a consistent causal link..." then "some relevant premise of the argument";
2) If "some relevant premise of the argument" then "there is not a consistent causal link...".

I notice that the LSAT frequently will have answer choices that seem attractive which state (as in answer choice D): "If there is not a consistent causal link... then ... blalbalbla". Are these answer choices inherently wrong since they attempt to prove the conclusion by placing the same in the sufficient assumption slot of the wrong answer choice? I feel that in order to prove a conclusion, you MUST either place the negation of the conclusion into the sufficient slot OR place the identical (logically-speaking) conclusion into the necessary slot. Is this correct? Any feedback would be much appreciated! Also, I apologize for the lengthy post... it's sometimes tough to explain through a computer...

foggynotion

Posts: 46
Joined: Sun Nov 29, 2009 4:19 am

### Re: Solving Necessary and Sufficient Assumption Questions

Yes, what you describe will work on many sufficient assumption questions. Most of the time with these types of questions, you have to link something in the conclusion to something in the premises. Just as you describe, there will often be two answer choices that both have the two ideas you want to link together, but the choices will have the two ideas switched around. A lot of time, what you'll see is something like this:

A -> B

therefore A -> C

So, just like you said, you want to follow the direction of the arrow and add in an assumption that would allow to you start with A and wind up with C, so in this case you want as assumption that reads ... -> C, or it's contrapositive, ~C -> ... (I tend to think of it like this: A -> ... -> C). So for this one, it would be B -> C, or ~C -> ~B.

Or you might see something like this:

A -> B
B -> C

therefore D -> C

So here, you want your assumption to START with D and take you to C. So you would be looking for something like D -> ... or
... -> ~D.

There are probably some questions this might not work on, but I think it would be a good strategy for most sufficient assumption questions.

slingshot_fuel

Posts: 5
Joined: Tue Jul 16, 2013 8:37 pm

### Re: Solving Necessary and Sufficient Assumption Questions

Thank you very much for your detailed response! I thought I was going bonkers due to over-analysing this particular issue! I think this is a key tool and/or technique that SHOULD be included in every LSAT prep material... however, neither Powerscore nor Manhattan mention this.

LSAT Blog

Posts: 1257
Joined: Mon Dec 07, 2009 9:24 pm

### Re: Solving Necessary and Sufficient Assumption Questions

I agree - it's an important technique. I talk about it quite a bit on my site here:

http://lsatblog.blogspot.com/2009/12/su ... -tips.html

Jeffort

Posts: 1888
Joined: Wed Jun 18, 2008 4:43 pm

### Re: Solving Necessary and Sufficient Assumption Questions

slingshot_fuel wrote:Thank you very much for your detailed response! I thought I was going bonkers due to over-analysing this particular issue! I think this is a key tool and/or technique that SHOULD be included in every LSAT prep material... however, neither Powerscore nor Manhattan mention this.

What you have noticed is a pattern of a common type of attractive incorrect answer choice offered in sufficient assumption questions: an answer choice that simply gives you a premise that is the reverse of the conditional statement that is sufficient to justify the conclusion.

It's mentioned in the books, at least in the explanation(s) for a sample question or two, but may not be highlighted as a common type of offered trap answer as well as it could be emphasized based on how common such trap answers actually are in terms of how often they are offered as an incorrect answer choice. Just like attractive incorrect answers on must be true questions that offer an incorrect/mistaken reversal of a given premise in the stimulus, these trap answers on SA questions are there to test your understanding of how conditional reasoning works in terms of distinguishing sufficient from necessary conditions and understanding that the relationship only logically operates in one direction ( suff ---> necc ).

Nevertheless, it's good that you noticed this so that you will be careful with answer choices that mention the right elements so that you double check to make sure they are presented in the proper S --> N form to justify the argument rather than in reverse form.

Another common tactic in SA questions to throw people off from picking the correct answer is to phrase the premise in its contrapositive form so to see that it does in fact link up with and connect the premises to the conclusion, you must consider the contrapositive form of what the answer choice says in order to see how it properly links things together. By stating it in contrapositive form, it gives the superficial appearance that the conditions are presented in reverse form from what you need if you don't think deeper and examine the contrapositive, thus getting many people to incorrectly reject it based on only superficial analysis rather than full understanding of the what the premise actually establishes.

It is also common in SA questions to encounter attractive trap answers that simply just restate a premise already given in the argument, so watch out for those too. They are easy to get suckered into when in the heat of battle under timed conditions if you aren't being careful since they jump off the page due to recognition that it is stating something that is important to the argument, too bad it's just redundant with what's already in the argument. Sometimes these traps will restate the premise with the same wording used in the stimulus, other times they will paraphrase a stated premise with different wording to make it look like it is something different than what is already presented in the premises to make it sound attractive as a new premise.