How to Think About Data Sufficiency
March 3, 2014
How to Think About Data Sufficiency
As a GMAT tutor, the most common concern I hear from new students is about data sufficiency questions. Many students, who have an otherwise fine grasp of the content tested on the quantitative section of the GMAT, break down when facing this deceptively simple question type. The reason for this is that many students, left to their own devices, fail to learn and cultivate the proper mindset toward these important questions. As a tutor, one my primary goals is to get my students to the point where they no longer get data sufficiency questions wrong simply because they are data sufficiency questions. This article details a few of the most important tips these all-important questions.
Think like a manager
Multiple choice questions are all about the grunt work of math–calculation. While you should always look for short cuts and alternative solving methods, the end result you’re seeking is still a particular quantity. Data sufficiency questions, on the other hand, force you to adopt a wider view–a manager’s view. You don’t see architects riveting girders, generals driving tanks or CEOs crunching numbers. The job of a manager is all about the appropriate allocation of resources, personnel and money to complete a job. Well, the key to managerial thinking on data sufficiency questions is to always consider not how to solve the problem but what information is necessary to do so. Consider the different ways a problem could be solved and what combination of formulas and information will allow each solution to be realized. Then evaluate your answers accordingly.
Data Sufficiency questions come in two varieties
Most DS questions are of the “solve for X” variety in which you need to isolate a single solution to have an answer. In this case, for a statement to be sufficient there must be only a single value that results from plugging in the given information. If neither statement is sufficient, C can only be the solution if both statements produce only one common result. About a third of DS questions, however, are of the “yes or no” variety. These questions give you a bit more leeway in that all you’re looking for is a definitive answer. If the question asks Is X an odd integer?, a statement will be sufficient if it can definitely show that X is an even integer, an odd integer or not an integer at all. In other words, “NO” is just as sufficient as “YES” is. Only “MAYBE” answers are insufficient–answers that could be true depending on the numbers chosen.
Know your rules
Recognizing sufficiency is much easier when you have a good grasp on the theory underpinning questions. For example, in advance of the GMAT, you should know that you need as many variables as you have equations in order to solve a system of equations. You should know that you can find the area of a circle if you know the radius, diameter or circumference. You should know the formulas for overlapping sets problems, combined work problems, permutation questions, combination questions and many others. The better you know the rules that govern a particular problem, the easier it will be to determine sufficiency and the less work you will have to do in that process.
Give each statement a chance
This is one of the toughest adjustments you have to make on DS questions but it’s a critical one. When you evaluate the first statement, you have only that statement and the information given in the question. However, when you’re looking at statement 2, somewhere in the back of your mind, you remember statement 1 and whether or not that statement was sufficient. It’s extremely important, however, that you disregard that information when evaluating statement 2: statement 2 must get its own opportunity to answer the question. In fact, if statement 1 is sufficient, statement 2 is on its own. Be especially careful of problems that seem to have an “obvious C” answer at a glance. If the statements seem like they need each other right away, it’s often a trap and either one statement, the other or both are sufficient in ways you may not have considered.
Rich Carriero is the Academic Manager for GRE and GMAT at Next Step Test Preparation, which matches students with one-on-one GMAT tutorsnationwide. He has 15 years experience in the test Preparation industry.