GMAT Tip: Critical Reasoning Strategy

Answering GMAT questions seeking the answer that strengthens an argument is easy using the author's tip Photograph by Getty Images

This tip on improving your GMAT score was provided by Brian Galvin at Veritas Prep.

Standard “strengthen the argument” problems in the GMAT’s Critical Reasoning section will involve a correct answer that helps to advance the cause, either filling in a logical gap or providing even stronger evidence for the conclusion.  But among the hardest 25 percent or so of the problems, you’ll see a high proportion for which the correct answer is actually a choice that protects the conclusion against a potential weakness. Consider the following “easy” problem:

Scientists have found reptile-like bones in fossil samples underneath Prospect Park in Brooklyn, so it is likely that dinosaurs once lived in what is now New York City.

This problem might have the following correct answer: “The bones date back to the same time as when known dinosaurs lived in other parts of the American East Coast.”

In a harder question, it might have a trickier answer: “Climate studies show that the area surrounding what is now New York City would not have been able to support reptile life even as recently as thousands of years after dinosaurs are known to have lived in North America.”

The difference? The first answer is more direct—it provides direct evidence tying the fossils to the dinosaur period.  The second serves to eliminate other sources of the fossils—it can’t have been more-recent reptiles, so it’s more likely to have originated in that prehistoric period. The first advances the cause, the second removes a potential threat (“the bones were from reptiles only a couple thousand years ago”).

How do you know to look out for these “removes the flaw” type questions? For one, they tend to occur commonly when the question stem asks for an assumption required by the argument. And they also tend to occur when answer choices use negation—terms such as “is not,” “did not,” or “not all.” Furthermore, as these questions are usually among the trickiest, the GMAT’s computer-adaptive format—which rewards correct answers with tougher questions—almost guarantees they’ll come to you when you’ve raised your verbal performance to an above-average level. Questions may look like this example:

Birds need so much food energy to maintain their body temperatures that some of them spend most of their time eating. But a comparison of Mifune, a bird of a seed-eating species, to Rossi, a bird of a nectar-eating species that has the same overall energy requirement, would surely show that Mifune spends more time eating than does Rossi, since a given amount of nectar provides more energy than does the same amount of seeds.

The argument relies on which one of the following questionable assumptions?

(A) Birds of different species generally do not have the same overall energy requirements as each other.

(B) The nectar-eating bird does not sometimes also eat seeds.

(C) The time it takes for the nectar-eating bird to eat a given amount of nectar is not longer than the time it takes the seed-eating bird to eat the same amount of seeds.

(D) The seed-eating bird does not have a lower body temperature than does the nectar-eating bird.

(E) The overall energy requirements of a given bird do not depend on factors such as the size of the bird, its nest-building habits, and the climate of the region in which it lives.

Note the symptoms here: an “assumption” question stem and some negation in multiple answer choices (lots of “does not” language). Here your job is less to advance the cause of the argument and more to protect it from a potential weakness.  To do that best, try taking the opposite of each answer choice to see if one of the answer choices directly weakens the conclusion. After all, if the correct answer removes the flaw, the opposite of the correct answer should demonstrate that exact flaw. And in this case, the argument stacks up as:

Conclusion: A seed-eating bird should spend more time eating than a nectar-eating bird.

Premise: (because) nectar produces more energy than seeds do.

If you negate the correct answer, C, look how it directly weakens the conclusion: The time it takes for the nectar-eating bird to eat a given amount of nectar is longer than the time it takes the seed-eating bird to eat the same amount of seeds.

If it takes longer to eat nectar than seeds, the conclusion (seed-eating birds spend more time eating than nectar-eating birds) is not necessarily true. The only piece of evidence that existed for the conclusion is that nectar has more energy, but if it also takes more time it’s probably a wash.

Consider what you can learn from this problem. Virtually no one would see choice C and think that it completely advances the argument or helps to prove the conclusion, but by considering the opposite of C, you should see that it preempts a direct attack on the argument. So when you see negation in answer choices and/or the assumption tag in the question stem, simply negate the answer choices to turn a very difficult “strengthen” question into a more manageable “weaken” question.

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