Brian Galvin is the Director of Academic Programs at Veritas Prep, where he oversees all of the company's GMAT prep courses.
Let's face it: Data sufficiency questions are frustrating. They're phrased in an awkward way -- we live in a results-oriented society, where we want to get the answer, solve the problem, and move on. Why would we want to spend time simply determining if we can? Those who can do, and those who can't have no business at a top business school, right?
In a way, the above statement is an underlying philosophy behind the data sufficiency setup. Think about it -- the GMAT should reward those who can solve problems with limited information more often than it rewards those who can't. By that logic, E is an "inferior" answer choice to the others -- E simply says that "the problem cannot be solved with the information given." Now, if it were always incorrect, it would be a waste of an answer choice, so it has to be a plausible answer, but it stands to reason that business schools don't want to reward the "I don't know, so I guess it can't be done" guess too often.
Think of the answer choices in this way:
- D -- with either piece of information on its own, I can solve it
- A / B -- I can solve it with one, but just can't do it with the other
- C - I can't quite do it with either alone, but I can solve it with both of them
- E -- I can't solve it with this information
The above is a loosely hierarchical structure. It's not that D is inherently a "better" choice, but you can see why the Stanford GSB is likely to be more confident about admitting someone who can solve problems in multiple ways than admitting someone who can't. Thinking strategically, then, you should approach data sufficiency problems this way: If you feel that it was relatively easy to arrive at an answer choice, be sure to check the "level" above it to ensure that you couldn't have squeezed more out of the information given.
Consider a problem such as:
Is the product of integers xy > 2?
1) x + y = 3
2) x -- y = 1
You might be inclined to select choice C, noting that, with two equations and two variables, you'll be able to solve for both variables and definitively answer the question. However, it should seem pretty obvious that you'll be able to do so, which might give you pause -- they're probably testing something deeper. If you look closer at statement 1, then, you can notice that, if x and y are both positive, the only combinations that will work are 1, 2 and 2, 1. In either case, the product is 2, which is not greater than 2. If we want either x or y to be larger than 2, our options are 3, 0 (in which case the product is 0, which is less than 2) or something greater than 3 and a negative number, in which case the product will be negative. Accordingly, there simply is no combination that could possibly give us the answer "yes," so we know statement 1 is sufficient.
This question is typical of what you'll find on the GMAT. The test exists to determine who can make the most of their resources and do more with less. Knowing this, push yourself to consider the "higher level" answer choices on data sufficiency questions, and you should put yourself in position to reap those rewards.