{"id":10725,"date":"2015-12-04T22:18:20","date_gmt":"2015-12-04T22:18:20","guid":{"rendered":"http:\/\/www.manhattanprep.com\/gmat\/?p=10725"},"modified":"2019-09-05T15:58:23","modified_gmt":"2019-09-05T15:58:23","slug":"heres-why-you-might-be-missing-gmat-data-sufficiency-problems-part-1-2","status":"publish","type":"post","link":"https:\/\/www.manhattanprep.com\/gmat\/blog\/heres-why-you-might-be-missing-gmat-data-sufficiency-problems-part-1-2\/","title":{"rendered":"Here’s why you might be missing GMAT Data Sufficiency problems – Part 2"},"content":{"rendered":"

\"Blog-DataSuff-Part2\"<\/p>\n

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In our previous article<\/a>, we divided the logical errors that test-takers make on Data Sufficiency questions\u00a0into two types:<\/p>\n

Type 1:<\/strong> You thought that something was sufficient, but it was actually insufficient.<\/p>\n

Type 2:<\/strong> You thought that something was insufficient, but it was actually sufficient.<\/p>\n

We already covered the most common reasons for Type 1 errors to occur and a few good ways to avoid\u00a0them; now, let’s cover Type 2 errors.<\/p>\n

Type 2 errors are their opposite: also known as “false negatives,” they happen when it looks like\u00a0you don’t have enough information to answer the DS question, but you actually do. The classic “(C) trap”\u00a0is one example of a Type 2 error: say that you picked (C), when the right answer was actually (B). In that\u00a0case, you thought that each statement was insufficient, and decided to combine them. But, it turned out\u00a0that (2) was actually sufficient on its own. Oops.<\/p>\n

According to data from our GMAT Navigator program (in which thousands of students have recorded\u00a0their answers to retired GMAT DS problems), Type 2 errors happen about 50 percent more often than<\/p>\n

Type 1 errors. That’s because DS problems often deliberately include statements that look useless and\u00a0irrelevant, but really aren’t. If you’re not prepared, when you see one of these problems, you’ll wrongly\u00a0assume that the statement is insufficient. Only well-prepared test-takers will see through the ruse.<\/p>\n

Here are some situations that often cause Type 2 errors:<\/strong><\/h3>\n

You mistook a yes\/no question for a value question.<\/strong> Did you see a yes\/no DS question, but treat it as if\u00a0you needed to solve for an exact value? Then you likely made a Type 2 error: you incorrectly assumed\u00a0that because you couldn’t come up with a single value, you couldn’t answer the question. Remember\u00a0that on yes\/no questions, the only answers you’re looking for are ‘yes’ and ‘no’. You don’t need to come\u00a0up with a specific number, and in fact, problems are often designed so that you can’t! Some yes\/no\u00a0questions that often lead to Type 2 errors are The Official Guide to the GMAT, 2016 (OG 2016) DS 9 and\u00a0108, and The Official Guide for GMAT Quantitative Review, 2016 (QR 2016) DS 122.<\/p>\n

Not enough math.<\/strong> One rule of thumb says that in order to solve a math problem, you need at least as\u00a0many equations as you have unknowns. If you’re pressed for time, use that rule on DS word problems\u00a0and algebra problems. But also know that in certain special circumstances, you can solve by using fewer\u00a0equations than you’d normally need. The only way to discover these special cases is to translate both\u00a0the question and the statements into math on your scratch paper, and then decide whether you can\u00a0solve. Otherwise, you’ll make a Type 2 error\u2013you’ll assume that you can’t solve, when you actually can.\u00a0Try these DS problems for practice: OG 2016 DS 106 and 140, and QR 2016 DS<\/p>\n

Combo traps.<\/strong> Some DS value questions ask you to solve for a combination of values rather than for a\u00a0single value. For instance, a DS question might ask you “What is the value of xy?” or “What percent of\u00a0John’s food budget did he spend in restaurants?” In the real world, you’d solve for each individual value\u00a0first (x and y, or John’s total budget and his restaurant spending). Unfortunately, the test writers design\u00a0combo questions that you can actually answer without knowing the individual values, and if you fail to\u00a0notice these questions, you’ll be vulnerable to Type 2 errors. That is, you might not need as much\u00a0information to solve for a combination of values as you would need to solve for the values separately.<\/p>\n

When you review a DS value question, decide whether you were asked for one value or for a\u00a0combination. If you had to find a combination of values, you may have mistakenly assumed that you\u00a0needed to solve for both values separately, making a Type 2 error. To avoid this, conclusively prove each\u00a0statement insufficient before you ever put them together. For some practice, try OG 2016 DS<\/em> 52, 63,\u00a0and 97.<\/p>\n

“Nice but not necessary.”<\/strong> When you do a DS problem, you’re looking for the minimum amount of\u00a0information that would let you answer the question correctly. That requires a different sort of thinking\u00a0than solving real-world problems does.Suppose that a friend asks you to figure out the amount of water in her backyard swimming pool. She\u00a0knows three things: the current depth of water in the swimming pool, how long it took her to fill it with\u00a0a hose, and the rate at which her hose puts out water. You’d probably respond by asking her for all of\u00a0the information she has first, and then you’d start writing equations. You might notice halfway through\u00a0that you didn’t actually need to know how deep the pool was, but who cares? You’d find the answer,\u00a0and 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\u00a0you can definitely answer the question when you use both statements together, doesn’t mean that the\u00a0statements are insufficient on their own! It might be a little more complicated to answer the question\u00a0using only the information from one statement, but it could still be possible.<\/p>\n

If you made a Type 2 error and then found yourself saying “I didn’t realize that I didn’t have to know\u00a0that!” about one of the statements, then you fell for a Nice But Not Necessary trap. These questions\u00a0often have a particular look to them: one statement will be very simple, usually giving you a single value,\u00a0like the depth of a swimming pool or the time at which a machine began working. The other statement\u00a0will be much more complex and harder to handle. Be skeptical when you see this\u2013it might be nice to\u00a0know the information from the simpler statement, but do you really need it? Or is there a more\u00a0complicated solution that only uses the info from more complex statement? To practice, check out\u00a0problems OG 2016 DS<\/em> 38, 89, and 95, and QR 2016 DS<\/em> 74 and 88.<\/p>\n

What now?<\/strong><\/h3>\n

With the information from this series of articles , you can categorize your Data Sufficiency errors into\u00a0four types: Type 1, Type 2, Careless (simple computation mistakes and miswritings), and Mathematical\u00a0(you didn’t know a rule or applied it incorrectly). Go through some DS problems you’ve done in the past\u00a0and assign each wrong answer to one of those four categories. What patterns do you notice? Are your\u00a0mistakes predictable?<\/p>\n

Then, focus specifically on your Type 2 errors. Whenever you notice one of these errors, identify the\u00a0trick that you fell for. The situations described in this article often lead to Type 2 errors, but they aren’t\u00a0the only possible causes. Can you come up with others? Based on the patterns you notice, make one\u00a0change to how you do DS problems. Some options:<\/p>\n