Working With Causal Relationships

March 22, 2010

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If you've done very many GMAT Critical Reasoning questions, you've probably noticed that the same types of questions pop up again and again. You may not have identified the patterns, but it's likely that you recognize the familiar patterns in your CR practice.

The most common pattern of all is one of cause and effect. Let's look at an example, #40 from page 128 of The Official Guide for GMAT Verbal Review, of a CR question that fits into this pattern:

Dental researchers recently discovered that toothbrushes can become contaminated with bacteria that cause pneumonia and strep throat. They found that contamination usually occurs after toothbrushes have been used for four weeks. For that reason, people should replace their toothbrushes at least once a month.

Which of the following, if true, would most weaken the conclusion above?

The conclusion won't always be at the end of the passage, and it won't necessarily come with a big warning flag ("For that reason") but it'll always be there. Accompanying the conclusion will be another sentence or two. Altogether, the sentences will comprise an argument that, with practice, you can quickly poke holes in.

To find a way to weaken a conclusion, look at that conclusion and ask why. Be as skeptical and nitpicky and possible. In this case, you should ask:

Why should people replace their toothbrushes at least once a month?

What the passage suggests (but does not state) is that toothbrushes accumulate bacteria which, after four weeks, can give their owners pneumonia and strep throat. Because the passage doesn't make that absolutely clear, you've found a gap in the logic. It may be natural to infer that, if a toothbrush is contaminated with bacteria, it can give the owner a disease. But at this point, it's only that---an inference.

How To Weaken a Causal Argument

In more general terms, the passage is implying that X (bacteria) causes Y (diseases). When you spot such an argument, look for a few common gaps. Perhaps there's evidence that X doesn't cause Y. Maybe Z (some other cause, such as toothbrushing negligence or an alien invasion) causes Y. It may even be the case that Y causes X. Any of those claims, modified for a particular case, can weaken an argument.

Going back to the present example, the argument is based on one unstated claim, or assumption:

Toothbrushes accumulate bacteria which, after four weeks, can give their owners pneumonia and strep throat.

To weaken the argument (or, in the wording of this example, weaken the conclusion), simply contradict this assumption. You might do so as follows:

The bacteria that toothbrushes accumulate after four weeks does not give their owners pneumonia and strep throat.

Sometimes you don't even have to work that hard to contradict an assumption; you might just change "does" to "doesn't." In this case, it's just considering the possibility that X does not cause Y. That matches up nicely with the correct answer, (C):

The dental researchers found that among people who used toothbrushes contaminated with bacteria that cause pneumonia and strep throat, the incidence of these disease was no higher than among people who used uncontaminated toothbrushes.

Putting This Pattern To Use

While many GMAT questions ask you to weaken an argument or conclusion, you'll find several different question types that employ the same type of passage. If you're asked to strengthen an argument or conclusion, follow the same procedure--just don't look for a contradiction of the assumption, look for confirmation of it. If you're asked to identify a "central assumption," you don't have to confirm or contradict at all.

Recognizing cause and effect relationships will also save you time on Reading Comprehension passages and questions. Rarely will RC questions be quite so straightforward as CR examples like this one, but the arguments in persuasive passages will be similar in form.

How To Practice Identifying Cause and Effect

Any GMAT book worth its salt will have plenty of Critical Reasoning practice, and many of the questions will ask you to weaken or strengthen the argument. Always look for an underlying assumption--much of the time you'll be able to identify that by recognizing a causal relationship.

Also, realize that many journalists and editorial writers leave their arguments incomplete. Newspaper editorials are great practice for identifying poorly constructed arguments. If you find yourself angry at the supposed logic of an argument you disagree with, use it for GMAT practice. Identify what's missing, and figure out the specific way you'd contradict the assumption. You'll develop a clearer understanding of current issues and--more importantly--you'll improve your GMAT score.

Causal relationships are just one common pattern on GMAT Critical Reasoning questions. I've covered more in the Critical Reasoning section of the Total GMAT Verbal.



About the author: Jeff Sackmann has written many GMAT preparation books, including the popular Total GMAT Math, Total GMAT Verbal, and GMAT 111. He has also created explanations for problems in The Official Guide, as well as 1,800 practice GMAT math questions.

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