GMAT Critical Reasoning: Necessary and Sufficient Conditions in Assumption and Weaken Questions

Written by Kelly Granson. Posted in GMAT Study Guide

GMAT criritcal reasoning necessaryIn an earlier entry, we introduced the topic of necessary and sufficient conditions on GMAT Critical Reasoning in general and especially in Inference questions. You learned to keep in mind that what is necessary may not be sufficient, and, conversely, that a sufficient condition is not always necessary. Here, we will learn apply this principle to other Critical Reasoning question types.

1. Assumption Questions

An assumption is commonly defined as a statement that bridges the gap between an argument's premises and its conclusion. However, you can also think of it as a condition that has to be true in order for the conclusion to be true. Assumption questions will be especially challenging if you try to find a statement sufficient to fully validate the argument's conclusion. This is a mistaken effort, because the assumption does not have to be sufficient; it only has to make the conclusion more likely to be true. Consider the following example:

The new movie by director X received many negative reviews. Therefore, the film is worse than any other movie ever made by X.

For this conclusion to be true, several other statements must also be true. For example, the author must believe that reviews are a reliable indicator of a movie's quality, that the movie really deserved the negative reviews it received, that other films by X received fewer negative reviews than did the newest one, and so on.

Any of these statements could be the right answer choice for an Assumption question. If you try to deny each statement, you will see that each is necessary for the conclusion to be valid. For instance, if you found out that for some reason not all the negative reviews were objective (i.e. the movie did not deserve the criticism), you would no longer be able to judge the quality of the movie based on these reviews.

At the same time, none of the assumptions above, when taken separately, is sufficient to prove that among the works of director X, the newest movie is really the worst one. For example, even if the movie deserved all its negative reviews, another of the director's movies might have received even more criticism.

Thus, when solving Assumption questions, remember that the right answer does not need to be sufficient to confirm the conclusion completely. It just has to be necessary to the conclusion.

The logic of necessary and sufficient should guide your analysis of an argument. In Assumption questions, that logic might also be part of the author's reasoning, so that necessity or sufficiency of a certain condition is exactly what is assumed.

2. Weakening Questions and Flaw Questions

CR questions that ask you to weaken the argument or find a flaw in the author's reasoning are sometimes based on the author's failure to see the difference between necessary and sufficient conditions, mistakenly assuming that what is necessary is sufficient, or vice versa. Depending on the question type, the correct answer will either point to this confusion (identify a flaw in the author's reasoning) or show that the assumption is not necessarily true (weaken the possibility that the conclusion is sound).

Look at the following argument:

Aerobics is known to improve one's stamina significantly within a short time. Jane, who wants to increase her fatigue endurance, has opted for a yoga class instead. Therefore, she will not be able to achieve the desired result quickly and effectively.

This argument could very well be used in either a Weaken or a Flaw question. Here the author confuses what is sufficient with what is necessary. Aerobics is a way to increase stamina, but the argument does not state that it is the best or only way. Perhaps yoga or other physical activities can produce the same result. This fact is exactly what you need to introduce in order to weaken the argument or criticize the author's reasoning.

In this argument, a possible answer to a Weaken question could be Yoga is known to be as effective as aerobics in increasing one's stamina, while a Flaw question could be answered with, The argument is flawed because it fails to consider the possibility that stamina can be increased through other means with equal speed and efficiency.

Now let's look at an argument where the author is making the opposite mistake, i.e. taking what is necessary for what is sufficient.

Every chairperson ever elected has had a degree in social sciences. Alison holds a PhD in psychology. Therefore, if she chooses to take part in the election, she should become chairperson.

The author is apparently convinced that holding a degree in social sciences is sufficient for someone to be elected chairperson. However, she fails to provide any evidence that the degree alone is actually enough. Thus, you can weaken the argument by suggesting that there are other necessary qualifications, for example: All other chairpersons had been on the committee for at least 10 years before they were elected, whereas Alison only joined the committee three months ago. To describe the flaw in the author's reasoning you might want to choose the following answer: The argument is flawed because it assumes without justification that a degree in social sciences is sufficient for election.

To increase your odds on GMAT Critical Reasoning questions, be on the lookout for necessary and/or sufficient conditions stated or implied. The logic of the argument often depends on those, and being able to spot them gives you an advantage in selecting your answer.

Good luck!


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