If you have only recently started studying for the GMAT (or even if you have been studying for a while!), you are likely annoyed by Data Sufficiency (DS). What is this weird question type, and why do they ask it? More importantly, how do we handle it?
What is Data Sufficiency?
The GMAT really is not a math test. (Neither is the GRE.) These tests are actually trying to test us on our “executive reasoning” skills—that is, how well we make decisions and prioritize when faced with too many things to do in too little time.
So, DS questions are really about quickly analyzing a collective set of data and trying to figure out which pieces you need to do the job. Imagine your boss dumping a bunch of stuff on you and saying, “Hey, our client wants to know whether they should raise the price on this product. Can you answer that question from this data? If so, which pieces do we need to prove the case?”
We do, of course, have to do some math—and sometimes that math is quite annoying. We usually do not, however, have to do as much as we usually do on regular “problem solving” questions (the normal Quant questions).
How does Data Sufficiency work?
First, we are given what is called the “question stem.” Here is an example:
How old is Oliver?
The question stem asks us a question, naturally. It can also provide information, such as the following:
If Oliver’s age is even, how old is Oliver?
Now we know that Oliver’s age is an even number. If they told me, for example, that Oliver is either 13 or 14 years old, now I know he is definitely 14, because I should only consider even numbers as possible values for Oliver’s age.
Next, the problem will give us two statements, such as the following:
(1) Oliver is 4 years older than Sam.
(2) Sam will be 11 years old in 5 years.
So, can we figure out how old Oliver is? What information would we need to do so? The first statement, by itself, does not help, because we do not know how old Sam is. The second statement, by itself, also does not help, because it does not tell us anything about Oliver.
If we put the two statements together, however, then we can actually figure out how old Oliver is. In this case, using both statements 1 and 2 together is sufficient to answer the question. (And this situation corresponds to answer choice C on the GMAT.)
DS questions have five possible answers:
(A) Statement 1 does help us to answer the question but statement 2 does not.
(B) Statement 2 does help us to answer the question but statement 1 does not.
(C) Neither statement works on its own, but I can use them together to answer the question.
(D) Statement 1 works by itself and statement 2 works by itself.
(E) Nothing works. Even if I use both statements together, I still cannot answer the question.
Okay, these are weird. How do I get better?
These are going to take some practice, yes. In addition, this was only a very short introduction; a ton of great strategies are out there that you can learn. Look for books, articles, classes, and other resources to help. (Here is one to get you started.)
You also, of course, have to learn a bunch of math. What we have presented here, though, should help you get started on this kind-of-bizarre question type in the first place!