The first thing to note is what question stem pertains to what type of assumption question. If the stem is,
"What makes the conclusion logically follow"
or
"The conclusion logically follows if which of the following is assumed"
It is a sufficient assumption question. Any stem that asks us to make the argument completely valid is a sufficient assumption question.
If the stem is,
"Which one of the following is an assumption required by the argument"
or
"The argument depends on assuming that"
It is a necessary assumption question.
Alright, so now you need to know how to approach each type. Lets look at Question 9 in LR2 from the 2007 free LSAT:
http://lsac.org/JD/pdfs/SamplePTJune.pdf
We break down the argument just as we would any other stimulus, but in this case we are looking for something which is required by the argument or else it falls apart.
Premise
The
natural habitat of the Tasmanian has been taken over by sheep farming. There is no physical evidence that they exist in the area of their
natural habitat.
Conclusion
The Tasmanian is
extinct and sighting to the contrary are incorrect.
Well...the evidence offered in support of the conclusion talks about the natural habitat of the Tasmanian...Which is great. But the conclusion is that the Tasmanian does not EXIST ANYWHERE (they are extinct)! Clearly, there is a disconnect here. What if the Tasmanian moved away from its natural habitat or what if the Tasmanian is being bred in captivity!? In that case the evidence used to support the conclusion would be incorrect and the author could not make the conclusion.
The assumption doesn’t mean the argument is valid but without the assumption, the argument is no good AT ALL. Clearly, answer choice D is the one we want.
As for sufficient assumption questions, lets look at Question 6 LR 1
Premise
Appointed to Board --> Undergraduate Degree + No Felony Convictions
Conclusion
Murry cannot be an Executive Administrator because he has a felony conviction
So you'll notice a massive theme shift in most sufficient assumption questions. Here, the question jumps from conditions about board members and applies them to Murry in being accepted as an Executive Administrator...well, we dont know anything about being an executive administrator! Are they subject to the same requirements of the board? Are they a position on the board itself!? We just dont know.
So what were are doing is looking for the answer choice that links up the premise with the conclusion. In this case, answer B wraps this up nicely. If candidates for the Executive Administrator position are subject to the same requirements as the board, then Murry cannot be accepted for that position!
BAM!