Common Data Sufficiency Mistakes – Part I

by on February 26th, 2013

Welcome to part one of a three-part series on common mistakes that students make when answering Data Sufficiency questions.

In this article, we’ll examine two kinds of mistakes that GMAT newcomers often make, and in the next two articles, we’ll graduate to mistakes made by veteran GMAT preppers.

The mistakes we’ll examine today both stem from students’ inability to fully understand the sole objective of all Data Sufficiency questions. This sole objective is to determine whether the statements provide sufficient information to answer the target question. That’s it. No more, no less.

Students who fail to understand/ appreciate/ embrace/ kiss/ marry this sole objective are susceptible to making one of two mistakes:

  1. Over calculating
  2. Confusing the response to the target question with the response to the sufficiency question

Over Calculating

To set things up, consider this question:

Is y > -8?

(1) y = 2013^2 - 155^3 - 23^4

(2) y > (2w - 3x - 1)^2

If you remember the sole objective of all Data Sufficiency questions, this is a 10-second question.

Statement 1: y = 2013^2 - 155^3 - 23^4

Are we going to evaluate 2013^2 - 155^3 - 23^4? No. Students who evaluate this mess are asking the wrong question. They’re asking the target question, Is y greater than -8?, in which case they need to perform a lot of calculations. The question these students should be asking is, Does this statement provide sufficient information to definitively answer the target question?

If we ask this question, then we need only recognize that we could evaluate the expression and find the exact value of y, at which point we’d be able to definitively determine whether or not y is greater than -8.

Since we could answer the target question, statement 1 is sufficient.

Statement 2: y > (2w - 3x - 1)^2

Are we going to expand (3w – 2x – 1) (3w – 2x – 1) and see what we get? Absolutely not. Statement 2 is telling us that y is greater than the product of some number and itself. In other words, y > (some number)^2

Since the square of any number is always greater than or equal to zero, we can be certain that y > 0. If y is greater than zero, then it must be greater than -8.

Since we can answer the target question with certainty, statement 2 is sufficient, and the correct answer here is D.

So, never lose sight of your sole objective, which is to determine the sufficiency of the statements, not to perform tedious calculations.

Aside: In their quest to avoid over calculating, some students make the mistake of under calculating, which we’ll explore in the next article.

Now let’s examine the second mistake that stems from test-takers’ inability to fully understand the sole objective of all Data Sufficiency questions.

Confusing the Response to the Target Question with the Response to the Sufficiency Question

While examining each statement in a Data Sufficiency question, you must ask, Does this provide sufficient information to definitively answer the target question?  On occasion, this sufficiency question gets confused with the target question.

To see what I mean, consider this rudimentary (and partial) question:

Is positive integer k prime?

(1) k = 6

Statement 1: k = 6

Let’s compare the response to the sufficiency question to the response to the target question.

Sufficiency question: Does this statement provide sufficient information to definitively answer the target question?  YES (which means statement 1 is sufficient).

Target question: Is k prime? NO.

At this point, if you confuse the target question with the sufficiency question, then you will incorrectly conclude that statement is not sufficient when, indeed, it is sufficient.

The big takeaway here is to keep reminding yourself of the sole objective of all Data Sufficiency questions: determine whether or not the statements provide sufficient information to answer the target question.

In the next two articles, we’ll examine some of the more advanced mistakes (if it’s possible to have “advanced” mistakes) students make when answering Data Sufficiency questions.

6 comments

  • Hello Brent. Thank you for the article it is extremly helpful.
    Please tell me if I'm getting your article right. So what you are trying to say is that when we have a data sufficiency question. We only need to determine if the statments are suffiecient to answer the question, not if the answer to the question is Correct or NO.
    Thank you in advance for your answer.

    • Hi Youlian,
      All we need to concern ourselves is whether or not each statement is sufficient to answer the target question. 
      Do we care if the answer to the target question is YES, NO, 8, pi, or sqrt(7)? 
      No, we don't. 
      All that matters is whether or not there's one unique answer to the target question. 

      I hope that helps. 

      Cheers,
      Brent

  • Hi Brent,
                 Excellent Article. I really thought you clearly illustrated some very important concepts in Data Sufficiency that people often tend to overlook.

    • Thanks Aditya.
      In the next two articles, we'll examine some higher-level mistakes that students make on DS questions. 

      Cheers,
      Brent

  • hi brent, just wondering, in the 1st example, for statement 2, the explanation's equation is different than the actual statement 2.  is there a reason for this?  am i not seeing the factoring?  y> (2w-3x)^2  vs, y> (3w-2x-1)(3w-2x-1)

    thanks

    • Good catch - thanks!
      I've edited it accordingly.

      Cheers,
      Brent

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