Sufficient/Necessary Assumption Questions
What's the difference, really?
Of the common question types, assumption questions are the toughest for many people, and a lot of students aren’t really super comfortable distinguishing them. I’m not talking about telling which is which (Necessary Assumption questions almost always have some variant of the words “requires,” “relies,” or “depends,” and Sufficient Assumption questions almost always have the word “if” in the question stem. I’m talking about being comfortable knowing why the right answer for one might be the wrong answer for the other, and vice versa. What often makes them harder is that very often, you’ll get a tempting wrong answer that would be a right answer if it were the other type of assumption question. So here’s a simplified crash course (I expect to elaborate over the weekend).
The main difference is that in a sufficient assumption question, you’re considering the answers as though they’re true, and looking for the one that guarantees the conclusion; in a necessary assumption, you’re considering the answers as though they’re false, and looking for the one that kills the argument - if the argument still works when the answer choice is false, it’s not necessary for the argument. This is sometimes called the “negation technique,” or the “denial test.” Illustrative argument:
My roommate really loves maple bars. He will buy one today if he can afford it. I happen to know that maple bars only cost 25 cents today. Therefore, he will buy a maple bar today.
Possible answer choices:
A) My roommate has at least 10 cents.
B) My roommate has 50 cents.
C) My roommate has at least 25 cents.
Take a moment and thing about how much you like or dislike the answer choices depending on whether you’re dealing with a sufficient or a necessary assumption question. Don’t scroll down yet.
A) is not a sufficient assumption. Having at least 10 cents is no guarantee that he can afford a maple bar. But it is a necessary assumption; consider it as though it’s false: If he doesn’t have at least 10 cents, then he definitely can’t afford a maple bar. The argument falls apart.
B) is a sufficient assumption. Assuming the premises of the argument are true (and you always assume the premises are true, on the LSAT), then if he has 50 cents, he can afford a maple bar. But it’s not necessary; even if he doesn’t have 50 cents, that doesn’t kill the argument. Depending on how little he has, he may or may not be able to afford one. We don’t know; lacking 50 cents isn’t a deal-breaker; the argument still works - he’ll buy one if he can afford it (which he may be able to do), and they’re really cheap today.
C) is both a sufficient and a necessary assumption. If he has at least 25 cents, he’ll buy one (i.e. sufficient), but if he doesn’t have at least 25 cents, he won’t (i.e. necessary).
Conceptually, sufficient assumptions are usually not necessary because there are multiple sufficient assumptions (if any of 3 things would suffice, then none of them is necessary. For instance, I might plan to buy a new car if I come into a lot of money. Winning the Lotto would be sufficient, but it’s not necessary, because other things would be sufficient, too, such as coming into an inheritance. It would only be necessary if it were the only thing that would suffice).
Similarly, necessary assumptions often aren’t sufficient, because there’s more than one necessary assumption (or condition, while we’re at it). If he see someone walking into a polling place, I may conclude that s/he is is a registered voter. It’s a necessary assumption that s/he is at least 18 years old, but it’s not sufficient, because being 18 isn’t the only condition necessary to be a voter - you also have to be a citizen. You also, in most place, have to have never been convicted of a felony.
Thinking of the “multiple sufficient (or necessary) assumptions” idea outlined in the previous two paragraphs is unlikely to help you on most, if any, questions, but they can help you understand how assumptions work as you study them. As always…hope this helps.