Common Data Sufficiency Mistakes - Part I

Welcome to part one of a three-part series on common mistakes that GMAT 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)^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)^2

Are we going to expand (2w – 3x)(2w – 3x) 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, 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. 

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