The #1 GMAT Data Sufficiency Trap

You should follow me on Twitter. While you're at it, take a moment to subscribe to GMAT Hacks via RSS or Email.


GMAT Data Sufficiency is tricky by nature. It's unfamiliar and demands abstract thinking. Fortunately, some of the mistakes you will make (and you will make them, at least in practice!) are predictable, and I can help you avoid them.

The writers of GMAT Data Sufficiency items know how most students work through DS problems. Here's a sample DS problem:

What is the value of x?
(1) x^2 = 25
(2) x is negative.

Here's the thought process of the typical test-taker:

  1. Here's the question, now I know what we're looking for.
  2. Statement (1) -- that's helpful, but not enough information. x could be positive or negative.
  3. Statement (2) -- hey, that's what (1) was missing!

So...what's the answer? Since I'm calling your attention to the process, you've probably answered it correctly: It's (C), since both statements are necessary.

But look at that thought process. The "aha!" moment comes when you look at Statement (2) the first time and realize you have enough information. Therein lies the trap.

I cannot tell you how many times I've watched students go through this process and select choice (B). Everyone makes this mistake at least once, and some people never stop making it.

So how can you avoid it? Being aware of the potential pitfall is a good first step. If you find yourself repeatedly falling into this trap, consider looking at Statement (2) first. (Either sometimes, or all the time.)

It also helps to force more rigor into your thought process. If you aren't already taking notes while doing every Data Sufficiency problem, you should start doing so. It can be simple: Just note whether each statement is sufficient or insufficient on its own.

Taking notes seems to require an additional level of self-awareness. By doing so, you'll jot down "I(nsufficient)" or "S(ufficient)" for Statement (1), then return to the question, thinking "ok, now let's consider (2) by itself."

Don't be embarrassed by the mistake--just recognize whether or not it is a problem for you. If it is, ruthlessly apply these techniques to prevent this trap from catching you ever again!



About the author: Jeff Sackmann has written many GMAT preparation books, including the popular Total GMAT Math, Total GMAT Verbal, and GMAT 111. He has also created explanations for problems in The Official Guide, as well as 1,800 practice GMAT math questions.

Total GMAT Math

The comprehensive guide to the GMAT Quant section. It's "far and away the best study material available," including over 300 realistic practice questions and more than 500 exercises!
Click to read more.