## How to distinguish necessary v. sufficient assumption in LR

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iowalum

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Joined: Mon Oct 25, 2010 5:15 pm

### How to distinguish necessary v. sufficient assumption in LR

I'm having trouble being able to read an LR stimulus and know if I'm supposed to be looking for the correct answer as necessary or sufficient. I have done pretty well with connecting previously unrelated pieces of the premise(s) and conclusion, but I still don't think I can distinguish between the two types and some assumption Q's are still getting me.

Strategy?

foxtrottortxof

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Joined: Fri Nov 05, 2010 10:47 pm

### Re: How to distinguish necessary v. sufficient assumption in LR

The quick and dirty way for me was just repeating to myself really simple truths. It's sufficient to say that if I live in San Francisco, I necessarily live in California (sf --> CA). It is not sufficient to say that if I live in California, I necessarily live in San Francisco (CA --> SF). It's sufficient to say that if I don't live in California, I necessarily don't live in San Francisco (~CA --> ~SF). And so forth.... Starting with very concrete examples helped me develop an intuition for it.

Once you can internalize that, you're golden, though I don't know how much of a strategy that is...lol.

kaiser

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### Re: How to distinguish necessary v. sufficient assumption in LR

There are very specific ways of asking for a sufficient assumption. The wording will almost always be something like this:

"The conclusion logically follows if which of the following is assumed" --> Key phrase here is "logically follows". If you see that phrase, you know you are looking for a sufficient condition, since it is something that makes it so the conclusion HAS TO follow, much in the way that a necessary condition HAS to follow from the sufficient condition

"Which of the following guarantees the truth of the conclusion" --> I'm not sure if this is an actual phrasing, or just my way of interpreting it, but you can see the common trend. You are looking for the only answer choice that absolutely, 100% guarantees that the conclusion. You are NOT looking for something that merely makes the conclusion possible. You are looking for an answer choice that makes it so the conclusion MUST be the case

Compare this to necessary assumption questions, in which you are merely looking for an answer choice that makes the conclusion POSSIBLE. In a necessary assumption question, think of it like throwing a life jacket to a argument that would otherwise drown. You are merely trying to keep it above water. It will often have phrasings like "necessary", "required", "essential", etc. These words imply that the assumption is required for the argument to make sense at all.

LSAT Blog

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### Re: How to distinguish necessary v. sufficient assumption in LR

Great question, iowalum.

I like both of these responses. Kaiser really nailed it.

If the verb in the question stem is synonymous with necessity (depends upon, requires, assumes), it's a necessary assumption question. If the verb is synonymous with sufficiency (allows, enables), or you have one of these phrases: "follows logically if...assumed", "properly inferred if...assumed", "properly drawn if...assumed", it's sufficient assumption.

Here are my detailed thoughts on how to tell the difference.

bp shinners

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Joined: Wed Mar 16, 2011 7:05 pm

### Re: How to distinguish necessary v. sufficient assumption in LR

The one tricky one is the Necessary Assumption prompt that reads:
"The argument assumes which of the following?"

Usually, the Nec. Assumption questions have a synonym for necessary (must, depends, requires, etc.). That prompt only gives you 'assumes', but it's asking for a necessary assumption.