GMAT Critical Reasoning (CR) Tips – Necessary vs Sufficient

GMAT Critical Reasoning (CR) questions test the test-takers ability to read complex (sometimes intentionally tricky) paragraphs and derive rational conclusions from it. Our friends from GoGMAT, continue their GMAT preparation series for readers of MBA Crystal Ball. In this post their GMAT teachers take on another topic from the GMAT verbal syllabus that non-English speakers often find confusing. And yup, do expect sample GMAT questions to test out your newly gained knowledge.


GMAT Critical Reasoning (CR) – Necessary versus Sufficient

Critical reasoning is something we all use every day. It’s a way of thinking that allows us to draw logical conclusions based on the limited information we have at hand.  This kind of reasoning is second nature to most of us. With a little practice, you can become equally adept at answering the GMAT’s critical reasoning questions. To answer correctly one need only use their common sense, concentration and familiarize themselves with the ‘traps’ the GMAT uses to make the questions more challenging than, say, the logic problems you have to solve in your day to day life.

What are these traps and how do you learn to identify them?

There are several and they are subtly woven into the problems presented on the GMAT so that only a trained student will be able to easily pick them out. They include but are not limited to out of scope information, correlation vs. causation, extreme diction and value judgments and necessary vs. sufficient conditions.

Begin by learning about and analyzing each of these traps one at a time. Then, when you start taking practice tests, each of these challenges should already be somewhat familiar to you.

We have already shortly introduced some them in one of our previous posts. This article will introduce a detailed overview of the necessary vs. sufficient conditions trap. It will demonstrate how the GMAT complicates logical problems by deliberately equating their respective meanings.

What is a ‘necessary condition’ and what is a ‘sufficient condition?’

A ‘necessary condition’ is when a condition must be met if a particular event is to transpire. For example: Sunlight (or its artificial equivalent) must be available for plants to produce nutrients through photosynthesis. The presence of sunlight – or its artificial equivalent – is a necessary condition for photosynthesis. The process of photosynthesis cannot occur without it. However, sunlight does not automatically produce photosynthesis, several other factor need to be present as well. Therefore, we can say that sunlight is not a sufficient condition for photosynthesis.

A ‘sufficient condition’ is when a certain condition guarantees that an event will occur. For example: If a plate of food is inserted into a normally functioning oven that is turned on, the food is guaranteed to heat up. Placing the food in a hot oven is sufficient to heat it up. What is important to remember is that there other ways to heat the food up as well. That is, in order to heat up the food, it’s not necessary to place it in the oven. To get the same effect one could equally place the food into a frying pan or microwave or roast it over an open fire, or place it in the hot sun…you get the idea. A sufficient condition is not always absolutely necessary just as a necessary condition – like the sunlight for photosynthesis – is not always enough, or, sufficient.

GMAT Traps

The GMAT test contains deliberate traps that force you to pay extra attention to the logical arguments being made.  Therefore, you will have to read and think in a very precise manner in order to pick out the mistakes. Both in the given statements that introduce a critical reasoning question and in the multiple-choice answers provided, you will find statements that intentionally obfuscate the implications of necessary and sufficient conditions or assume that one is synonymous with the other.

Let’s take a closer look at exactly how this is done.

Consider the following statements:

People don’t have healthy lungs unless they don’t smoke.

This statement is somewhat confusing because it consists of a double-negative conditional argument. We can, in fact, draw several correct logical inferences based on this statement:
1. That not smoking is necessary for maintaining perfectly healthy lungs.

2. Smoking detracts somewhat from the health of the lungs.

On the GMAT, you might find a flawed argument that presumes that refraining from smoking is not only necessary for preserving health lungs but also sufficient for doing so. Perhaps you might find a statement like this: Refraining from smoking leads to having healthy lungs. This is an incorrect inference as there are many other factors (genetic, environmental etc.) that can result in less than perfectly healthy lungs. This example is simplified for the purposes of explanation, but you get the idea.

The following is an opposite case, where something sufficient can be mistakenly perceived as necessary:

If she catches the taxi, she will come on time

The statement communicates the fact that catching the taxi will be a sufficient condition for her to come on time. However, one cannot say that catching the taxi is necessary for her to come on time. Remember our example of heating up food?  If she doesn’t catch a taxi she might use other means at her disposal to avoid being late. What if our theoretical subject met a friend who had a car or what if she hopped on a bus or a bicycle? These might also prove sufficient for her being on time. None, however, is necessary. Again, on the GMAT you will find flawed answers that will try to mislead you in this direction.

GMAT Sample Problems

Now that we have deconstructed the meaning of these two kinds of conditions and learned about possible areas of confusion, let’s turn our attention to how this might be applied on actual GMAT critical reasoning questions. You’ll notice that questions do not always conform to one expected trap but demand that you be vigilant about every detail like exact word usage and syntax.

We’ll start with a simple one, in fact this is too simple to be met on the actual test, but it’s perfectly good for initial practice.

People under the age of 13 are not allowed to take the GMAT. Jason is now 10 years old.

Which of the following conclusions is best supported by the argument above?

A.        Jason will take the GMAT in three years;

B.        Jason is not allowed to take the GMAT;

C.        It is unfair that people under the age of 13 cannot take the GMAT;

D.        The youngest person who took the GMAT was 13 years old.

Let’s analyze each choice to determine the correct response and to understand why it is correct.

Choice A. Jason will take the GMAT in three years. This statement does not necessarily have to be true; the evidence itself does not provide any grounds for such an inference. According to the given conditions, Jason will be allowed to take the GMAT in three years, provided that the rules don’t change, but nothing in the argument suggests that he will or that he must. Jason may or may not end up taking the GMAT we don’t have any conclusive evidence either way thus Choice A is incorrect.

Choice B. Jason is not allowed to take the GMAT. This inference is correct. The given statement provides the information that no one under the age of 13 is allowed to take the GMAT, and Jason is only 10. At this point in time he does not meet the necessary condition of being 13 years old. Therefore, we can infer that that Jason is not allowed to take the GMAT. This is the correct answer.

Choice C. It is unfair that people under the age of 13 cannot take the GMAT. This suggested answer is completely beyond the scope of the evidence. It is a value judgment and the statement provides no information to support any inference about the fairness of the age limit.

Choice D. The youngest person who took the GMAT was 13 years old. The statement simply conveys the minimum age requirement for taking the GMAT. It contains no information about previous test-takers and thus supports no inferences like the one in this answer. Perhaps children under 13 used to be allowed to take the GMAT? Alternatively, perhaps no one as young as 13 has ever taken the exam even though the rules allow it? This is a typical incorrect choice for inference questions: it could be true, but there is no evidence to prove it.

Choice B is the correct answer.

Now for another, more complex problem:

Companies can attract a great number of new customers by discounting prices on their products. However, discounted prices usually lead to lower profit margins on sales. Therefore, companies that already have low profit margins and cannot decrease their prices any further will not be able to attract a great number of new customers.

The argument above is flawed because it fails to consider that

A)        It is possible to attract some new customers without discounting prices.

B)        It is possible for companies to attract a great number of new customers without significantly discounting prices on their products.

C)        More customers do not necessarily mean greater profits for the company.

D)        A great many new customers may be attracted by means other than price reduction.

E)        Many companies have significantly increased their profits without discounting prices on their products.

The argument above states that discounting prices can attract a great number of new customers and then concludes that companies that cannot discount their prices cannot attract a great number of new customers. However, the argument implies that price reduction is the only means of attracting numerous new customers. There is no substance to this conclusion. This is actually the same as saying that people in Florida live near the ocean, therefore people who do not live in Florida do not live near the ocean. This is absurd, but the logic is the same as in the argument at hand.

In this argument, a sufficient condition (price reduction) is assumed to be necessary. True, reducing prices is often sufficient to attract many customers, but is it always necessary for doing so (remember the hot food example)? The argument offers no evidence that price reduction is the sole means of attracting new customers. Common sense dictates that there can be other ways to attract customers other than reducing prices.

Once again, let’s analyze each answer choice to determine the correct response and to understand why it is correct.

Choice A. It is possible to attract some new customers without discounting prices. The word some in this statement is not strong enough:  the argument is concerned with attracting a great number of new customers, which is not the same as some customers. Note that the precision in word usage is sometimes a pitfall in these questions. Every detail counts!

Choice B. It is possible for companies to attract a great number of new customers without significantly discounting prices on their products. The argument above never said anything about discounting prices significantly. It was hinged on the premise that companies unable to reduce their prices by any margin, significant or otherwise, would be unable to attract customers. Therefore adding the word ‘significantly’ does not undermine the statement’s avowal of the necessity of price reduction at any rate for increasing business. This choice fails to address the major flaw in the argument.

Choice C. More customers do not necessarily mean greater profits for the company. The argument is concerned with means of increasing the number of new customers. Whether more customers always mean greater profits is not important. Again, this choice fails to address the major flaw in the argument.

Choice D. A great many new customers may be attracted by means other than price reduction. As we have already determined, the flaw is that the argument assumes that price reduction is the only means of attracting a great number of new customers. Here we correctly reinstate the condition of ‘price reduction’ as sufficient rather than a necessary condition. Furthermore, this statement directly addresses the flaw using proportional language. This is the correct answer.

Choice E. Many companies have significantly increased their profits without discounting prices on their products. The argument is concerned with attracting new customers, not with increasing profits. The fact that some companies may have increased their profits without reducing prices does not necessarily mean that their profits increased due to a greater number of new customers.

Choice D is the correct answer.

There you have it. Together we have successfully navigated through some sample Critical Reasoning GMAT problems. The difference between necessary and sufficient conditions is not all that complicated but we have demonstrated how the GMAT uses flawed statements and arguments to make it trickier.

The best parting advice we can give you (and we hope you saw the importance of it in this article) is that in your practice time you should place a lot of attention on reading questions and answer choices very carefully. After all, there is nothing more frustrating than losing score points – not because you didn’t know the answer—simply because you failed to understand the question itself.

Finally, rest assured that, if you’re like most people who prepare for the GMAT, you probably have a lot of study time ahead of you. Patiently learn about each ‘trap’ and go through problems that test your understanding and ability to apply it in various critical reasoning scenarios. Logically speaking, we can’t tell you that practice will be sufficient for GMAT critical reasoning mastery, but it is necessary if you want to become a natural.

Good Luck!

Continue learning on our GMAT discussion forum. If you have any GMAT related questions about this post or any other GMAT topic, head over to the GMAT preparation helpdesk and shoot your queries.


GMAT Preparation Tips by GoGMAT
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Sameer Kamat //
Sameer Kamat
Founder of MBA Crystal Ball. Author of Beyond The MBA Hype & Business Doctors. Here's more about me. Connect with me on Google+ | Twitter | Facebook | Linkedin

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