Necessary and Sufficient Assumption

bullfrogwart

I am having many problems with this, please help

bullfrogwart

Ya that still isn't helping

tsamvelyan

Honestly, the way I approach these questions is through the mechanical method. For sufficient assumptions, I look at the elements introduced in the premise/premises and if there is a new element introduced in the conclusion. If there is a new element that's introduced in the conclusion but it was not present in the premise/premises, then that element must appear in the correct answer choice. I also try to cross out answers that are "soft" and irrelevant. That works for me - especially since during timed sections I really don't have the time to work through them or diagram them.

For Necessary Assumptions, I keep two things in mind, the answer has to be 1) relatively soft and 2) it cannot bring in outside/new information. I usually avoid answers that say "this was the most important factor". Remember, for Necessary Assumptions, the answer just needs to make the argument "stand". If I'm ever stuck between two answer choices, I use the negation technique. The correct answer choice should hurt/destroy the argument.

I hope this helps. I can totally see why these question types can be frustrating.

bullfrogwart

so, for a necessary assumption, you negate the given premise and if that destroys the argument then it is a necessary assumption.

tsamvelyan

I negate the given answer choice and if that affects/destroys the stimulus, then that's the correct answer choice. For example:
Person A is a great skater. Person A has won skating championships. Therefore, Person A will be very competitive in the upcoming skating championship.

Answer choice:
In order to be competitive in a skating championship one has to know how to skate. Try negating this statement. If this statement was negated and applied into the answer choice, it would destroy our argument, because it's claiming (negated version) that one does not have to know how to skate in order to be competitive in a skating championship.

bullfrogwart

I think I might just give up. I get it figured out then I go to my lsat book, I try a big passage and I cant do it.

tanes256

@tsamvelyan and @jasonawdriscoll your explanations are golden for me!!! Thx for this. I'll still have to apply this when working these question types to make sure I have it but this is sweet! I read every post about SA and NA questions because they are definitely my weakest two question types. @bullfrogwart you certainly cannot give up! You're already defeating yourself. When I get fed up with these two question types I just move on to something else. I know I'll eventually have to conquer them but no need discouraging me from continuing studying for that day. Just stay at it and keep rereading these posts and keep reviewing the lessons. Eventually it'll click for you. I'm still waiting on it to click for me as well so don't get discouraged. It'll come to you. I also have Manhattan LR book to reenforce what I'm learning here. I use it only for those question types that I didn't quite understand here. 7Sage is a beast but for some reason I'm not understanding the SA and NA material given. Naturally you do well in certain areas than others so I'm not stressing. LG was once my weakest area but with continued practice it is now my strongest. Long story short, just keep grinding. You got this!!!

tsamvelyan

@Tanes25 - Glad to hear it's helping. @bullfragwart - if you apply the method I suggested, I guarantee you that you can narrow down your answer choices to two (one of them being the correct answer choice), from there, if you can't narrow it down any further, you have a 50/50 chance.

James Ray

BullFrog
Let me see if I can address the difference in a different way. Please keep in mind that I am taking a different tack here to try and highlight the difference between these questions in a qualitative way that may help you learn how to tackle these questions. Okay, so!

Phase 1 - Missiles and Bombs
For NA questions, what are they asking for is something that is absolutely vital in order for the conclusion to be true. So what are they really asking you for? Reassurance! They want you to say something that is absolutely vital in order for the argument to be valid.

For SA Questions they are not asking that AT ALL!!!!!!! What they want is not something vital. They want something that will do in a pinch!

Though there are a million analogies you can use, try this one. NA questions are looking for a cruise missile and SA need a carpet bomb. A cruise missile will target a specific thing and take that out, carpet bombing will level an entire area. And this really is what you are doing.

Phase 2 - What You are Doing

I find it helpful to identify exactly what it is you need to do. For NA questions, what you need to do is defend the argument. How?

NA Question Objective: Defend!
An NA question wants you to identify something that has to be taken as granted for something to be true. If that something is not taken, then the argument simply cannot work. It just can't. So they want you to find that thing. There will only be one in the answer choices and that answer choice would work to satisfy teh relationship established in the argument.

An SA Question doesn't want that.

SA Question Objective: Nuke Em'
An SA Q is not concerned with figuring out any one thing, or defending the argument. What it wants is for you to name something, anything, no matter how outlandish, that lets it be true. It is a Nuke they are looking for! They want you to say something that if true, forces their conclusion to be true. You are not defending a specific relationship, you are defending all relationships of that type!

Phase 3 - How to get it done

In the end this is really up to you and to your thought process. Giving you tips on how I do it wouldn't necessarily work for you and besides, everyone has done that already. The way I do it is really about deciding how to approach it when I see each question. The best method, is the one that you understand very well. That being said, here are tips that I find helpful to breaking down the questions to get them right.

NA Questions:
1. You need to break down the argument. You are dealing with how to defend a relationship. Do this first.
2. Think of a few things that could possible be NAs. This will help frame your mind to hunt questions.
3. DO NOT start negating things right away. It takes a while to do it that way.
4. If you are not clear on each part of the argument, DO NOT NEGATE ANYTHING! If you start doing that without really understanding the relationships you are going to be confused
5. I hate the notion of looking for certain types of language ( i.e. some, most , all) to indicate incorrect answers for these types of questions. I can think of arguments where there are NAs that would use any of those. Instead, once you understand the argument and do tip 2, you will know which of these should not appear. THEN you can eliminate based on those words

SA Questions:
1. You need to break down the argument. The question wants you to find a panacea, a Nuke that will wipe everything out. You cannot possibly know what it is you need if you don't know EXACTLY what it is you are trying to affirm. You need a carpet bomb answer that will envelop all possibilities, to include this question while not excluding others.
2. Preform answers. Think about what sorts of things would force the conclusion to be true while also including other things. If it pertains only to this argument then it is likely a necessary assumption. That's not a panacea, that's a vaccine for HepC.
3. Again, same thing as tip 5 above. Understand the argument before seeking anything out vis-a-vis language but keep in mind that they want something pretty general so if the answer is getting into a lot of details about the stim, it probably will not work. You need something more general.

I hope that this, in concert with the other great comments herein, will help you with your journey. Don't give up on SA and NA questions, they are really tough but you can do it. If you have further issues I recommend reviewing the core logic lessons. That will help.