Intro
One very important distinction on the LSAT is the difference between sufficient and necessary assumptions. These are two separate question types and confusing them is a common mistake students make. Before diving into the specific approaches for each, it's worth making sure that the underlying concept is clear.
What's the Difference?
Consider the example below:
"Jones is at the t-shirt store and the t-shirt he wants costs $20. Therefore, Jones can afford the t-shirt he wants."
The conclusion above is that Jones can afford the t-shirt. The evidence is that he's at the store and the shirt costs $20.
Notice that the argument jumps from "the shirt costs $20" to "Jones can afford it." This is a gap in the reasoning. See the assumption? We need to assume that Jones has $20! Otherwise how would Jones afford the t-shirt?
But this example involving Jones actually goes even deeper than that. Consider: wouldn't we also need to assume that Jones has at least $10? Or at least $5? Or just that Jones isn't completely broke and has some amount of money? These are all necessary assumptions. Even though the ones involving less than $20 don't prove the conclusion, they are still needed for the conclusion to be true.
On the other hand, what if we assume that Jones has $30? This proves the conclusion, so it's a sufficient assumption. But it's certainly not necessary to have $30, because the t-shirt only costs $20.
In the example there is one assumption (that Jones has $20) that is both sufficient and necessary. But there are also many assumptions that are either sufficient or necessary. We need to assume that Jones is not flat broke, but that doesn't prove he can buy the shirt. It would be sufficient to assume that Jones has a $100 bill, but that's not necessary considering the t-shirt is only $20.
This difference between sufficient and necessary conditions trips many students up early on in their studying journey. Hopefully this example helps make a distinction between the two types of assumptions.
Applying this to LSAT questions
When you are faced with a Necessary or Sufficient Assumption LR question, you need to make sure you're evaluating the answer choices correctly. For Necessary assumption questions ask yourself "is this answer choice needed for the conclusion to be true?" And for Sufficient Assumption questions ask yourself "Does this answer choice prove the conclusion?"
Watch out for trap answers. For example, you'll often see answer choices that are very strong and would prove the conclusion of a Necessary Assumption stimulus, but are not needed for that conclusion. An answer like this would not be correct for a Necessary Assumption question.
For real LSAT examples and additional tools like the negation test, check out the Sufficient Assumption and Necessary Assumption lesson pages.