When I tutor I often like to ask my students, “what makes a hard question hard from the point of view of the test writers?” Invariably I get answers that point to specific techniques that are used to dial up the level of difficulty, such as abstraction, complicated Math, etc. All true. However, at a more fundamental level hard questions are designated hard because most people get them wrong. If most people got them right then they would, by definition, be easy questions. This leads to a very, very important fact about GMAT questions: when the answer that you are about to choose seems really obvious, the question is either super easy or you are falling into a trap.
This is applicable to all GMAT question types but it is perhaps most important on Data Sufficiency questions. If you take a look in the Official Guide you will see that even on easy questions the obvious answer is usually wrong. In a previous post I wrote about the “Spectrum of Sufficiency” and made a similar point, but I want to take this one step further and describe a concept that I call “bowling with the bumpers in the gutters on Data Sufficiency.” If you have ever gone bowling with little kids you will be familiar with the bumpers that they put in the gutters to prevent the ball from falling in. Well, being aware that obvious answers are usually wrong on DS questions is kind of like bowling with the bumpers in the gutters: when you are about to fall into the gutter and pick the wrong answer you get “bumpered” back into the question.
This is powerful stuff – I personally feel very confident on DS questions in part because I know the test so well that if I happen to make a mistake and start to fall into a trap on a DS question I usually get bumpered back into the question. So it is almost difficult for me to get a DS question wrong because even when I make a conceptual or calculation error I often find myself going back to the question to reconsider – the same thing does not really happen on Problem Solving questions or at least not as often.
Let me illustrate with an example. Consider OG Data Sufficiency Question #110:
#110 Tom, Jane, and Sue each purchased a new house. The average (arithmetic mean) price of the three houses was $120,000. What was the median price of the three houses?
(1) The price of Tom’s house was $110,000
(2) The price of Jane’s house was $120,000
Now, most people spend some time on statement 1 and realize, correctly, that it is not sufficient. At this point most people blow through statement 2 and infer, incorrectly, that since it seems to provide the same kind of information as statement 1 it must also be not sufficient. That would appear to be a reasonable conclusion. But that conclusion would then lead a person to answer choice C. Most people just choose C and never look back, but anyone with a good understanding of Data Sufficiency would probably be suspicious of choice C – and with good reason. Obviously if we know the average and we know the price of 2 of the homes then we would be able to figure out the price of the third home and therefore the median.
If you were about to choose choice C, two thoughts should occur to you at this point. One, what would be the point of this question if the answer was C? There would be no clever thinking, no difficult reasoning required to come to the answer – that is very unGMAT-like. Second, would almost all people come to that same answer with the same general ease? If so the question is either really, really easy or choice C is not the right answer. I wouldn’t say that you should completely write of choice C and absolutely not pick it, but you should at least get “bumpered” back into the question and consider your options again (and here is where the “Spectrum of Sufficiency” can be helpful so please see my previous post if you haven’t already). If you really proved that statement 1 is not sufficient then you really only have one alternative (given that together the statements are definitely sufficient). You need to consider statement 2 again. Doing so and knowing in advance that it probably is sufficient (because choice C is just too easy) is very powerful since you are analyzing it with the foreknowledge of what the correct answer is already likely to be. This is the power of bowling with the bumpers in the gutters on Data Sufficiency.
Some people worry that they will not be able to tell what is truly “too obvious” to be correct. Indeed it takes some practice and most of all some repetition with DS to know what would typically be a trap answer. But again, it doesn’t mean that you absolutely cannot pick the “obvious” answer. It just means that you should question and further analyze things that seem too obvious on Data Sufficiency questions.
Remember, in order to have a high GMAT score you need to be getting hard question right. But again, a hard question is hard because most people get it wrong. So in general on the GMAT and especially on Data Sufficiency the right answer on a hard question is not the answer that most people will pick. So if you are about to choose such an “obvious” answer consider that you may be about to fall into the trap that most test takers fall into.