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Necessary Vs Sufficient assumption questions

Posted: Wed Oct 24, 2012 8:19 am
by nosleeptillsuccess
Okay.. My question here is... What is your approach to sufficient assumption questions?

I know the necessary assumption questions require the "necessary condition for the argument to follow" and thus can be solved via negation technique.

My concern here is for sufficient condition questions. What should be my approach? I know it asks it's needed for the conclusion to follow, but I'm just looking for a mechanical approach.

Re: Necessary Vs Sufficient assumption questions

Posted: Wed Oct 24, 2012 8:24 am
by Swimp
Maybe this isn't mechanical enough, but what I do is read each answer choice and ask myself "Would this guarantee that the conclusion is true?" If you do enough of these types of questions, you'll get used to seeing the same wrong answers again and again and you'll knock out most of the wrong answer choices in seconds.

Re: Necessary Vs Sufficient assumption questions

Posted: Wed Oct 24, 2012 10:06 am
by Theopliske8711
Michael's coffee costs $4; therefore, Michael has enough to purchase his coffee:

Sufficient: Michael has $6.
Necessary: Michael has at least $4.

It gets more detailed than that, but if you want to simplify it to the core: sufficient means there is enough to fulfill the logic; necessary is what cannot be missing.

Re: Necessary Vs Sufficient assumption questions

Posted: Wed Oct 24, 2012 10:38 am
by bp shinners
First off, in general I want a strong answer choice (since it needs to get me to 100% valid).

As far as strategy, you need to identify the new concept that shows up in the conclusion. There will (the vast majority of the time) be a new idea that pops up in the conclusion that isn't in the premises anywhere. Any answer choice that doesn't mention that new term CANNOT be correct. So you can usually eliminate 3 answer choices right there.

After that, it's a matter of figuring out which answer choice left matches up with the stimulus.

Now, in the rare case where a new term doesn't show up in the conclusion, it means the gap exists between two premises. Usually, you can diagram these and find the gap, then the AC that bridges it.

Re: Necessary Vs Sufficient assumption questions

Posted: Wed Oct 24, 2012 7:57 pm
by LSAT Hacks (Graeme)
Theopliske8711 wrote:Michael's coffee costs $4; therefore, Michael has enough to purchase his coffee:

Sufficient: Michael has $6.
Necessary: Michael has at least $4.

It gets more detailed than that, but if you want to simplify it to the core: sufficient means there is enough to fulfill the logic; necessary is what cannot be missing.
This is right. I would add that formal logic is useful for sufficient assumption questions. It is not useful for necessary assumption questions.

Often sufficient assumption questions have this form:

Evidence: A --> B --> C

Conclusion: A --> D

Possible Right Answers: C --> D OR ~D --> ~C

The questions look harder than this, because they're disguised by confusing language. But many Suf assumption questions have a structure that is that simple.