Here’s why you might be missing GMAT Data Sufficiency Problems – Part 1
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Let’s talk about GMAT Data Sufficiency.
Specifically, let’s talk about getting GMAT Data Sufficiency (DS) problems wrong. And I don’t mean those problems that you missed because of careless math errors, or because of concepts you hadn’t learned yet. No, I’m talking about the missed DS problems that make you want to bang your head against the wall: How on Earth did I not get that?
There are two reasons you might have this experience:
- You thought that something was sufficient, but it was actually insufficient.
- You thought that something was insufficient, but it was actually sufficient.
These two errors are actually very different from each other, and understanding which one made you miss a problem is a great way to take your DS game to the next level. Ready? As you review, use this article and the one following it to analyze which of the two mistakes you made, why it happened, and how to prevent it next time.
Type 1 Errors
Type 1 errors are known in the sciences as “false positives.” On the GMAT, they happen when you think that you have enough information to answer the question, but you actually don’t. For instance, you might have picked answer choice (D), but then found out that the right answer was (A). You thought that statement (2) was sufficient, but it actually wasn’t. Why does this happen? Here are four of the most common causes.
Your cases weren’t weird enough. Testing cases is the best way to prove that a statement is insufficient. But what happens if you test a couple of cases, and you keep getting the same answer to the question? Either the statement you’re working with is actually sufficient, or else it’s insufficient, but you didn’t test the cases that would prove it. Sometimes, all of the obvious cases will yield the same answer. It’s only when you start testing the weird stuff–fractions, decimals, zero, negatives, roots, extremely large or extremely small numbers–that you’ll start getting different results. Have you made this mistake? Then try testing unusual cases to prove insufficiency on the following problems: The Official Guide for GMAT Review, 2016 (OG 2016) DS 81, 113, and 145 and The Official Guide for GMAT Quantitative Review, 2016 (QR 2016) DS 58, 64, 68, and 112.
Statement carryover. You worked with statement (1), and found out that it was insufficient. Then you worked with statement (2), and were able to solve the problem. So, statement (2) must be sufficient…right? Well, maybe not. Did you let some information you learned from statement (1) ‘leak’ into your work on statement (2)? Often, Type 1 errors happen because you don’t keep your statements separate. If this happens to you often, improve your DS scratch work: physically separating the two statements helps you mentally separate them. Here’s one way to do it:
Issues with inequalities. Did the question or the statements include inequalities? Misinterpreting inequalities often leads to Type 1 errors. Check out this Data Sufficiency mini-question:
Is x < 2y?
(1) 2x < 5y
Take a moment to carefully prove that statement (1) is insufficient. Try testing cases! There’s a good reason to be careful here: if you speed through a statement like this on the GMAT, you’re likely to make a Type 1 error, because an inequality often gives you less information than it appears to. The solution is to slow down and test specific cases every time you see an inequality in the statements. Have you recently made a Type 1 error on a DS inequality problem? Then work slowly and carefully through these problems: OG 2016 DS 42, 111, and 68 and QR 2016 DS 70 and 104.
Too many variables. When you review a DS word problem, always translate the statements into equations on your paper. If you initially made a Type 1 error, then as you do this, you might notice that you have more variables than you expected. You may even have too many to solve the problem, meaning that the statement is probably insufficient. This type of error happens when you do a DS word problem in your head. If you don’t actually write out the math, you might find yourself reusing or forgetting about variables. Try writing out the math on these DS word problems: OG 2016 DS 96 and QR 2016 DS 51 and 52.
What now?
Type 1 (and Type 2) errors are specific to Data Sufficiency. If you’re doing better on Problem Solving than on Data Sufficiency, or if you know the math well but just can’t pull it together on the Quant section, these logical errors are likely part of the problem. Overcoming them will improve your Quant score.
Review the DS problems from a recent practice test, or a DS problem set that you recently completed. Identify your Type 1 errors by looking for problems where you mistakenly thought that a statement, or both statements together, gave you enough information to answer the question correctly. Find as many Type 1 errors as possible, and figure out why they happened. Do any of them fit into the categories described above? If so, work through the practice problems for that category.
When you finish reviewing, commit to making one good change to your DS process, based on what you’ve learned. For example:
– Always translate DS word problems into variables and equations before you start working.
– On your paper, draw a line to physically separate statement (1) from statement (2).
– Always test at least one “weird” case before concluding that a statement is sufficient.
Put this single change into action the next time you practice DS, and if you find that you’re now making fewer Type 1 errors, make it a permanent part of your routine. To learn about Type 2 errors, read on to Part 2. ?
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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.