Here’s why you might be missing GMAT Data Sufficiency problems – Part 2
Did you know that you can attend the first session of any of our online or in-person GMAT courses absolutely free? We’re not kidding! Check out our upcoming courses here.
In our previous article, we divided the logical errors that test-takers make on Data Sufficiency questions into two types:
Type 1: You thought that something was sufficient, but it was actually insufficient.
Type 2: You thought that something was insufficient, but it was actually sufficient.
We already covered the most common reasons for Type 1 errors to occur and a few good ways to avoid them; now, let’s cover Type 2 errors.
Type 2 errors are their opposite: also known as “false negatives,” they happen when it looks like you don’t have enough information to answer the DS question, but you actually do. The classic “(C) trap” is one example of a Type 2 error: say that you picked (C), when the right answer was actually (B). In that case, you thought that each statement was insufficient, and decided to combine them. But, it turned out that (2) was actually sufficient on its own. Oops.
According to data from our GMAT Navigator program (in which thousands of students have recorded their answers to retired GMAT DS problems), Type 2 errors happen about 50 percent more often than
Type 1 errors. That’s because DS problems often deliberately include statements that look useless and irrelevant, but really aren’t. If you’re not prepared, when you see one of these problems, you’ll wrongly assume that the statement is insufficient. Only well-prepared test-takers will see through the ruse.
Here are some situations that often cause Type 2 errors:
You mistook a yes/no question for a value question. Did you see a yes/no DS question, but treat it as if you needed to solve for an exact value? Then you likely made a Type 2 error: you incorrectly assumed that because you couldn’t come up with a single value, you couldn’t answer the question. Remember that on yes/no questions, the only answers you’re looking for are ‘yes’ and ‘no’. You don’t need to come up with a specific number, and in fact, problems are often designed so that you can’t! Some yes/no questions that often lead to Type 2 errors are The Official Guide to the GMAT, 2016 (OG 2016) DS 9 and 108, and The Official Guide for GMAT Quantitative Review, 2016 (QR 2016) DS 122.
Not enough math. One rule of thumb says that in order to solve a math problem, you need at least as many equations as you have unknowns. If you’re pressed for time, use that rule on DS word problems and algebra problems. But also know that in certain special circumstances, you can solve by using fewer equations than you’d normally need. The only way to discover these special cases is to translate both the question and the statements into math on your scratch paper, and then decide whether you can solve. Otherwise, you’ll make a Type 2 error–you’ll assume that you can’t solve, when you actually can. Try these DS problems for practice: OG 2016 DS 106 and 140, and QR 2016 DS
Combo traps. Some DS value questions ask you to solve for a combination of values rather than for a single value. For instance, a DS question might ask you “What is the value of xy?” or “What percent of John’s food budget did he spend in restaurants?” In the real world, you’d solve for each individual value first (x and y, or John’s total budget and his restaurant spending). Unfortunately, the test writers design combo questions that you can actually answer without knowing the individual values, and if you fail to notice these questions, you’ll be vulnerable to Type 2 errors. That is, you might not need as much information to solve for a combination of values as you would need to solve for the values separately.
When you review a DS value question, decide whether you were asked for one value or for a combination. If you had to find a combination of values, you may have mistakenly assumed that you needed to solve for both values separately, making a Type 2 error. To avoid this, conclusively prove each statement insufficient before you ever put them together. For some practice, try OG 2016 DS 52, 63, and 97.
“Nice but not necessary.” When you do a DS problem, you’re looking for the minimum amount of information that would let you answer the question correctly. That requires a different sort of thinking than solving real-world problems does.Suppose that a friend asks you to figure out the amount of water in her backyard swimming pool. She knows three things: the current depth of water in the swimming pool, how long it took her to fill it with a hose, and the rate at which her hose puts out water. You’d probably respond by asking her for all of the information she has first, and then you’d start writing equations. You might notice halfway through that you didn’t actually need to know how deep the pool was, but who cares? You’d find the answer, and your friend would walk away happy.On Data Sufficiency, you can’t ask for all of the information and then see what happens. Just because you can definitely answer the question when you use both statements together, doesn’t mean that the statements are insufficient on their own! It might be a little more complicated to answer the question using only the information from one statement, but it could still be possible.
If you made a Type 2 error and then found yourself saying “I didn’t realize that I didn’t have to know that!” about one of the statements, then you fell for a Nice But Not Necessary trap. These questions often have a particular look to them: one statement will be very simple, usually giving you a single value, like the depth of a swimming pool or the time at which a machine began working. The other statement will be much more complex and harder to handle. Be skeptical when you see this–it might be nice to know the information from the simpler statement, but do you really need it? Or is there a more complicated solution that only uses the info from more complex statement? To practice, check out problems OG 2016 DS 38, 89, and 95, and QR 2016 DS 74 and 88.
What now?
With the information from this series of articles , you can categorize your Data Sufficiency errors into four types: Type 1, Type 2, Careless (simple computation mistakes and miswritings), and Mathematical (you didn’t know a rule or applied it incorrectly). Go through some DS problems you’ve done in the past and assign each wrong answer to one of those four categories. What patterns do you notice? Are your mistakes predictable?
Then, focus specifically on your Type 2 errors. Whenever you notice one of these errors, identify the trick that you fell for. The situations described in this article often lead to Type 2 errors, but they aren’t the only possible causes. Can you come up with others? Based on the patterns you notice, make one change to how you do DS problems. Some options:
- As soon as you read a DS question, write either “value” or “yes/no” on your paper.
- Always translate DS word problems into variables and equations before deciding that a statement is insufficient.
- Before you begin solving a DS value problem, determine whether it’s a combo problem or not.
Now that you know what these two types of logical errors look like, start trying to recognize them as you review DS problems. With time and practice, you’ll start to notice problems that would normally lead you to make Type 1 or Type 2 errors as you see them. Understanding why you made a mistake is the first step to avoiding it next time! ?
Want full access to Chelsey’s sage GMAT wisdom? Try the first class of one of her upcoming GMAT courses absolutely free, no strings attached.
Chelsey Cooley
is a Manhattan Prep instructor based in Seattle, Washington. Chelsey always followed her heart when it came to her education. Luckily, her heart led her straight to the perfect background for GMAT and GRE teaching: she has undergraduate degrees in mathematics and history, a master’s degree in linguistics, a 790 on the GMAT, and a perfect 170/170 on the GRE. Check out Chelsey’s upcoming GMAT prep offerings here.