The Most Common GMAT Data Sufficiency Mistake

As anyone who has prepared for the GMAT would attest to, the GMAT data sufficiency is probably the trickiest component of the test. Along with requiring an abundance of abstract thought, its strange wording and structure naturally induce students to make mistakes. Here, I’m going to discuss the #1 mistake students make on data sufficiency: They don’t evaluate the statements independently!

Let’s look at a typical (albeit simple) data sufficiency question and how a student might approach it:

What is the value of x?

1) x + y = 17
2) y = 8

Our typical student will see that Statement 1 provides us with one equation and two variables and properly conclude that the statement does not provide enough information to determine the value of x.

Thus, the student will move on to Statement 2.

When our typical student sees that Statement 2 provides a value for y, this student will experience an instinctive Eureka! moment and conclude that when you plug 8 in for y into Statement 1, you’ll be able to arrive at a concrete value for x. And, of course, our student will be right.

But it’s at the next step that our student makes the mistake.

Probably driven by the excitement of that Eureka moment, our student will choose B as the answer. After all, you choose B when Statement 2 is sufficient and Statement 1 is insufficient. But a closer look shows our student’s error: Statement 2 is NOT sufficient on its own. You could only answer the original question when you combine the information in the statements, and thus our answer should be C.

This is a question that the typical GMATer should be able to solve correctly. But sometimes test anxiety converges with the excitement of arriving at a solution, and test-takers choose the wrong answer. How can you avoid it?

A good first step would be to develop as rigorous an approach as possible toward data sufficiency questions. Always write down notes for each statement, always note whether a statement is sufficient or insufficient, and, most importantly, always evaluate the statements independently. As I mentioned in a previous post, test anxiety can often get in the way of optimal performance, and it’s important to begin implementing a systematic approach from the moment you begin your preparation.


Erfun Geula

Erfun Geula is a professional GMAT tutor, based in New York City. He has scored in the 99th-percentile of the GMAT three different times, he has written and edited hundreds of practice GMAT questions, and he has over 7,000 hours of tutoring experience.

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