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GMAT Tip: Data-Sufficiency Land Mines

GMAT Tip: Data-Sufficiency Land Mines

Photograph by Nick Hannes/Hollandse Hoogte/Redux


Most test-takers fear the GMAT’s data-sufficiency questions for their myriad traps and mind-bends; because these are not your standard math problems, many are beside themselves over the countless ways to make mistakes or get questions wrong. If this describes you, there’s a cure for that anxiety: Simply remember that because of how data-sufficiency questions are structured, there are only two ways you can get a wrong answer:

1. You think you have enough information, but you don’t.

2. You think you don’t have enough information, but you do.

Think about it: Data-sufficiency questions offer five answer choices—A through E—that require test-takers to determine whether one or both statements, alone or together, are sufficient to answer the question. If you think you have enough information and you do, you’re right, and if you think you don’t have enough and you don’t, you’re also right. So there are only two ways you can go wrong. Knowing this, you can start to further investigate why you make those mistakes.  So let’s break down the common culprits.

Mistake One: You think you have enough information, but you don’t.

Usually people make this mistake because they assume something. Say a question asks, “What is the value of x?” and one of the statements provided is that “x2 = 16.” If you assume that x is positive (or, put another way, if you forget that x could be -4 and doesn’t have to be 4), you’d get this one wrong.  So be sure not to make assumptions—some of the hardest questions on the GMAT rely directly on this trap, but they hide it by making the rest of the question seem involved so that you let down your guard. Since no one is immune from these assumptions, be certain to check for them each time.

The other common mistake in this field is the “false B” answer, in which you incorporate information from statement 1 as you read statement 2. With the previous example, if statement 2 were to say “x > 0,” you might then be tempted to think, “Oh, it’s the +4 and not the -4, so statement 2 tells me what I need to know and is sufficient.” But since answer choice B requires that statement 2 ALONE is sufficient, you cannot bring in any previous information. This trap is one that make you think you have enough information when you don’t … yet.

Naturally, there are other ways the GMAT can get you to think you have more information than you really do, but the dominant strategy to avoid this trap altogether is to play devil’s advocate. When you think you have enough information, cross-examine your answer by checking for assumptions. “Would this change if x were negative?” “Would this change if x were a non-integer?” “It says x is greater than 3, but did I consider 3.00001 or something just a shade over?”

Mistake Two: You think you don’t have enough information, but you do.

This mistake is a little more subtle; typically it involves people leaving valuable information on the table. In the previous example, were the question phrased differently—“If x represents the number of books on a shelf, what is the value of x?”—the reference to books might seem unimportant. But it’s actually telling you that x cannot be negative. Or say the statement said: x2 – 4x = 4x – 16. On the surface you might just say, “It’s a quadratic equation so there will be two solutions; therefore it’s not sufficient.” But the key to many questions is to make the best use of the information you have. If the GMAT gives you a piece of algebraic information, usually you can turn it into another. By manipulating this statement (subtracting 4x and adding 16 to both sides), you can get it to x2 – 8x + 16 = 0, which is (x -4)2 = 0, meaning that x must be 4. The given statement might look insufficient, but if you leverage that asset by rephrasing the algebra in a potentially more convenient way, you’ll often find that a statement is surprisingly sufficient.

More important than these examples is the strategy they represent. If you know the two ways to get a wrong answer, you can check yourself:

1. If you think you have enough information (if you’re picking “sufficient”), play devil’s advocate to make sure that you’re not missing a threat to your answer. To do this most effectively, try using numbers that have different properties—particularly negative numbers, fractions, and zero.

2. If you think you don’t have enough information (if you’re picking “not sufficient”), see if you can better leverage assets. Is there a way to rework the algebra? Does the story line of the problem guarantee a positive number or an integer?

When you make mistakes on data-sufficiency homework and practice test problems—rest assured, you will—try to fit them to these common traps and strategies to avoid them. The data-sufficiency structure is more regimented than most think. There are only two ways to get an answer wrong, so focus on these two strategies to ensure you get more questions right.


For more GMAT advice from Veritas Prep, watch “2 Ways You Can Go Wrong on GMAT Data Sufficiency Questions”


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