Link: http://www.beatthegmat.com/mba/2009/08/ ... -questions
Applying “Necessary” And “Sufficient” To Assumption questions
by Andrea Alexander on August 25th, 2009
Andrea is a verbal test prep expert who lives and works in Michigan. Visit Grockit's Learning Center for more test prep advice.
GMAT critical reasoning questions often ask you to identify the assumption of an argument. The first step in doing that successfully is understanding what, exactly, they mean by “assumption.” An assumption in GMAT-speak is the unstated link somewhere in the chain of evidence and conclusion. Finding the assumption means, basically, finding that gap in the argument and filling it.
Assumptions can be roughly divided into “necessary” and “sufficient,” and your approach to tackling an assumption question depends in part on which kind of assumption you’re dealing with. A necessary assumption MUST be true in order for the conclusion to follow logically based on the evidence presented. Take, for example, the following simplified version of a GMAT question:
Jennie wears glasses. Jennie also gets A’s in chemistry. Therefore, Jennie must be smart.
Which of the following assumptions is necessary to support the conclusion above?
Here, you’re looking at finding the unstated idea that MUST BE TRUE in order for the argument to work logically. Take a look at the possibilities:
a) Jennie gets good grades in all of her science classes.
b) All girls named Jennie are smart.
c) Jennie wouldn’t wear glasses if she wasn’t smart.
d) Some people who get A’s in chemistry are smart.
e) Everyone who gets an A in chemistry is smart.
Now, a few of these choices support the argument’s conclusion. But only one of them is actually necessary to the argument. Let’s looks at them one at a time.
a) Jennie gets good grades in all of her science classes.
This isn’t an assumption of this argument at all. Jennie’s other science classes are outside the scope of the argument, since they are addressed in neither the evidence nor the conclusion.
b) All girls named Jennie are smart.
This choice would certainly support the conclusion; if this were true, then the conclusion would HAVE to be true. But is this statement NECESSARY to the conclusion? No. Other girls named Jennie don’t have any necessary significance to this argument. So this is not a good choice.
c) Jennie wouldn’t wear glasses if she wasn’t smart.
Again, this choice would be SUFFICIENT to make the argument’s conclusion follow from the evidence. But is it necessary? No. So we’ll bypass this one.
d) Some people who get A’s in chemistry are smart.
This is the correct choice, because it MUST be true in order for the evidence to follow logically from the conclusion. What if this wasn’t true, and no one who got an A in chemistry was smart? If that were the case, then the conclusion would not be true, based on the evidence that Jennie gets A’s in chemistry.
e) Everyone who gets an A in chemistry is smart.
Once more, this choice is sufficient to support the conclusion, but it’s not necessary. So it’s not the correct answer to the question that is being posed.
Now, hopefully you noticed that the correct answer here is the least extreme relevant statement. That doesn’t always have to be the case, but for questions that ask for necessary assumptions, it’s a good general guideline. Be wary of answer choices that are extreme; they will often be sufficient, but not necessary, and will therefore trick test-takers who aren’t careful in evaluating what exactly the question has asked them to find.
But what if the question paired with that argument looked more like this?
Which of the following assumptions, if true, best supports the conclusion above?
Well, in that case, the answer choices would look more like these:
a) Jennie gets good grades in all of her science classes.
b) All girls named Jennie are smart.
c) Jennie gets A’s in her physics class.
d) Some people who get A’s in chemistry are smart.
e) Some people who don’t wear glasses are smart.
Just as in the last example, choice a) is not relevant to the argument as an assumption. But here, choice b) is the correct answer, because if that statement is true, then the conclusion is absolutely true. Choices c) and e) are irrelevant in the same way that choice a) is, since physics and people who don’t wear glasses aren’t at issue here. Now, choice d) is NECESSARY to the argument, but it is not the BEST support to the conclusion. Even if it IS true that some people who get A’s in chemistry are smart, that doesn’t guarantee that Jennie is.
Moral of the story: keep a close eye on what the question is asking for, and read accordingly.
