GMATers, especially newcomers to the test, tend to spend on average less time on Data Sufficiency questions than on the other Data Insights question types. This is in part because Data Sufficiency questions don’t require an “answer” in the same way that the other questions do, so it is a little harder to achieve the same level of certainty when doing a DS question. Test takers, therefore, often “go on a hunch” and select an answer that they “think” is right or that they are “pretty sure” is right. This is a big no-no! As this post will explain, guessing and knowing that you are guessing on DS questions is fine, but selecting an answer because you are “pretty sure” it is right (and believing that you will most of the time be right) is a great way to get a lot of Data Sufficiency questions wrong.
The first thing that you need to understand is that one of the prime features of Data Sufficiency questions, perhaps even their raison d’etre, is to lure people into making false assumptions. In other words, the questions are designed with that very purpose in mind, because one of the things that Data Sufficiency tests is the willingness of a person to rest on unproven assumptions vs their desire to try to draw conclusions based on proven facts. Obviously in the real world, business decisions should be made based on real evidence and should be arrived at with as high a level of certainty as possible, so this is part of what the GMAT is getting at through its use of Data Sufficiency. With this in mind, DS questions are often designed so that what SEEMS to be the case is often not the case and the reward is there for the person who attempts to prove out his or her hunches.
In my opinion (and this is something I will elaborate on in a future post), there are 3 main ways that you can evaluate the statements on a Data Sufficiency question. You can take a conceptual approach, an algebraic approach, or a picking numbers approach. The approach that you take depends of course on the question itself and it should also be noted that in some cases you may use a combination of these approaches and in other cases perhaps something else entirely. But the point is that many people are comfortable taking a conceptual approach when they really shouldn’t be. When I am tutoring someone, I will often hear, “I am pretty sure statement 2 is sufficient,” to which I will usually respond, “can you prove that” or “what is your level of certainty?”
This latter question is important because if you are 99% certain, then it is ok to stay conceptual since at that point you are not really assuming – conceptually you really understand why the statement is sufficient or not. However, if you are less that 99% certain, you have a decision to make: should you spend the time to prove that you are right or should you guess and move on. Again, the key here is that many people don’t even realize that they are in fact guessing when they are not close to 100% certain in their answer – they think it is just the nature of DS and that it is fine to be pretty sure on a question and then move on. It IS ok to “go on a hunch” as long as you recognize that you are essentially guessing and that there is a chance, perhaps a good chance, that you are wrong. For the sake of time management, sometimes it is indeed better to guess and move on since in many cases it would take a very long time to prove that a statement is sufficient or not sufficient, but you need to accept and understand that when you pick an answer on DS without a very high level of certainty, you are essentially guessing.
If you have the time and you believe that it is within your ability to go further than the conceptual understanding that you think you have, the next step is to try to actually PROVE that a statement is sufficient or not. Again, I will come back to this in a future post because it will require a fair amount of explanation and some examples to really illustrate how that would work exactly, but for now suffice it to say that in most cases your choices are algebra and/or picking numbers. Questions that are algebraic in nature (either they give you equations or they give you words that could easily be translated into equations) are obviously approachable in an algebraic way, though it must be said that even in these cases sometimes picking numbers is a better approach. And on other questions that have less of an algebraic structure, especially number property questions, are often best approached by picking numbers (and often can’t be done algebraically).
So to summarize, try to force yourself to reach a high level of certainty in your judgment about the statements on a DS question. “Going on a hunch” or being “pretty sure” is just not enough and will often lead to many wrong answers. Sometimes proving things out would be too time consuming or difficult, but at the very least understand that when you are not sure on a Data Sufficiency question you are essentially guessing.
In a future post I will discuss the specifics of “proving” statements to be sufficient or not and will give some examples, so stay tuned!