GMAT Critical Reasoning: Assumption without Negation

by on February 19th, 2017

assumptionsQuick recap: we typically see four different question types on the GMAT—Strengthen, Weaken, Assumption, and Method of Reasoning. Many students find the assumption questions to be one of the toughest nuts to crack because of how difficult it is to discern what type of logic the author uses to form his/her argument.

Assumption questions can be identified by the question (or similar question):

The argument relies on which of the following assumptions?

A commonly recommended strategy is the assumption-negation technique, where we negate answer choices to determine whether the conclusion can hold without the assumption. Some test takers excel in utilizing this strategy, while others find it impossible, confusing, and time-consuming.

If you are one of the latter types of test takers, then how do you proceed? Are you just to hope that you don’t get many assumption-style critical reasoning questions on your actual exam?

As, with most anything GMAT related, there are always different ways to approach questions. It is always about the test takers figuring out what is the most efficient, thoughtful way of getting to the right answer.

On method that we can suggest is identifying parts of the critical reasoning prompt, determining the link, and eliminating answer choicest that don’t relate back to that link. Let’s break down this approach with an example:

The ancient Nubians inhabited an area in which typhus occurs yet surprisingly few of their skeletons show the usual evidence of this disease. The skeletons do show deposits of tetracycline, an antibiotic produced by a bacterium common in Nubian soil. This bacterium can flourish on the dried grain used for making two staples of the Nubian diet, beer and bread. Thus, tetracycline in their food probably explains the low incidence of typhus among ancient Nubians.

Which of the following is an assumption on which the argument relies?

A. Infectious diseases other than typhus to which the ancient Nubians were exposed are unaffected by tetracycline.

B. Tetracycline is not rendered ineffective as an antibiotic by exposure to the processes involved in making bread and beer.

C. Typhus cannot be transmitted by ingesting bread or beer contaminated with the infectious agents of this disease.

D. Bread and beer were the only items in the diet of the ancient Nubians which could have contained tetracycline.

E. Typhus is generally fatal.

After first reading the argument carefully, we should break it down line by line and identify the premise (or premises—there are often more than one in assumption questions), the conclusion, and what links the two. Test takers often find restating these pieces of information in their own words can make it even easier to understand how the argument is being formed.

Here, we have a couple of premises. Nubian skeletons show little evidence of typhus, despite the presence of typhus in the area. These skeletons also have evidence of tetracycline, an antibiotic. Tetracycline is in beer and bread, which most of the Nubian diet.

Seems much more manageable, right? So, what is the conclusion?

Well, we can infer that tetracycline, as an antibiotic, must provide some protection against typhus … though it is not directly stated. We can also assume that the beer and bread is the only place where Nubians could have picked up tetracycline. There were no pharmaceutical manufacturers in those days, right?

The roadmap for this question is Nubian skeletons > no typhus > tetracycline > lots of beer and bread > no typhus.

By now, hopefully you see that the fuzzy logic is found in either whether tetracycline combats typhus or if the Nubians’ diet is to explain for limited incidences of typhus. From there, we want to use process of elimination in our answer choices.

Quickly we should realize that A, C, and E are out of scope and not relevant to the gap in logic we are trying to resolve. That leaves us with B and D.

Many test takers will fall for D, but does it matter if there is something else in the diet of the Nubians that protects them against typhus? Whether it is bread, beer, or sushi will not matter because their diet is still the reason for tetracycline.

The better answer is (B)—if the bread and beer didn’t have this bacterium, then there is an unexplained source for tetracycline. Without the information in answer (B), the argument really not much of an argument.

In summary: it is completely possible to tackle assumption questions without the negation technique. Think about how the argument is built and what building blocks are necessary for it to have a strong foundation.

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