GMAT Critical Reasoning (CR) questions can be tough nuts to crack in the GMAT Verbal section. The GMAT preparation series on MBA Crystal Ball, written by the GMAT teachers at GoGMAT, continues with some tips on finding logical flaws in the Critical Reasoning questions with samples to explain the underlying concept.
Several types of GMAT questions can include arguments based on flawed logic. They test your ability to recognize logical errors in the author’s reasoning and they require you to avoid making logical errors of your own. Being familiar with basic logical errors—confusing numbers and percentages, mistaking correlation for causation, and failing to differentiate between necessary and sufficient conditions—is essential if you are to pick the right answers when making inferences, identifying assumptions, finding flaws in reasoning, and weakening arguments.
Numbers vs. Percentages
Confusing numbers and percentages is a flaw in reasoning that appears very frequently on GMAT. To recognize it, you must keep in mind that you cannot make judgments about percentages if all the information you have is raw numbers. Therefore, remember that larger numbers do not always indicate larger percentages, and smaller percentages do not necessarily produce smaller numbers. The reverse is also true: if you only have percentage data, you can make no certain conclusions about raw numbers. Consider the following example:
While College X provides financial support to 60% of its students, College Y gives financial aid only to 30% of those enrolled. Clearly, twice as many students of College X receive financial aid.
If you are used to spotting statistical flaws, you will see immediately that the author’s conclusion about numbers of students is unwarranted, since the only evidence available pertains to percentages of students. The author’s conclusion would be right only if about the same number of students attended both colleges. The test makers are giving you the opportunity to assume incorrectly that the two colleges are of similar size and thus falsely accept that the author’s conclusion is true.
Causation vs. Correlation
The distinction between causation and correlation is another common source of logical flaws on GMAT. If events X and Y occur simultaneously, it is possible that one makes the other happen and the relationship between X and Y is causal. However, it is also possible that X and Y occur together simply by happenstance, or both result from Z. In both cases, the X-Y relationship is a correlation. Assuming causation where there is only correlation is a common error. So is assuming that an event has only one possible cause. Consider the following statement:
Most graduates of School X get jobs in management. Clearly, the reputation of the school gets them to their positions.
Here, a causal relationship is assumed where there might be no connection between the school’s reputation and its graduates getting management jobs. Perhaps most graduates of all similar schools are also employed as managers. Alternatively, even if the percentage of graduates employed in management is actually higher for School X, the school’s reputation may be irrelevant; perhaps the skills possessed by the graduates earn them their managerial positions, or perhaps the cause is some other factor that the author fails to mention. If the argument offers no evidence of causation, it is always false to assume causal relationship.
Another flaw common in causal reasoning is assuming that causation operates in a certain direction (X causes Y), when in fact it might operate in the opposite direction (Y causes X). Look at the following example:
A study has shown that many employees in managerial positions are good at critical thinking. Clearly, the responsibility of being a manager fosters development of the ability to think critically.
Here, the author assumes that managers who excel at critical thinking have developed this ability because of the nature of their work. It is, however, equally possible that the managers in question got their jobs because they were already good critical thinkers.
Necessary vs. Sufficient Conditions
Many types of Critical Reasoning arguments deal with conditional statements. Here’s an example of such a statement in its most basic form:
If a baby is hungry, it starts crying.
This means that a hungry baby will cry; hunger is sufficient to cause crying. It does not mean that a crying baby is hungry; babies cry for all sorts of other reasons, and crying is not necessary to bring a baby to tears. Suppose the argument tells you that the baby is, in fact, hungry.
If a baby is hungry, it starts crying. The baby is hungry. Therefore, it is crying.
You can safely infer that the baby is crying, since hunger is sufficient for crying to begin. Since hunger is not a necessary condition for a baby to start crying, the following arguments are wrong:
If a baby is hungry, it starts crying. The baby is crying. Therefore, it is hungry.
If a baby is hungry, it starts crying. The baby has just eaten. Therefore, it is not crying.
These last two examples falsely assume that a sufficient condition is also necessary. Other CR arguments can include the opposite reasoning flaw: assuming that what is necessary is also sufficient for an event to occur.
Logical Flaws and Different Question Types
In addition to questions that directly ask you to identify flaws in the author’s reasoning, you also have to recognize faulty reasoning when you are making inferences, finding assumptions, and strengthening or weakening arguments. Distinguishing between numbers and percentages will help you avoid typical wrong answers on Inference questions, especially those asking “what must also be true if the statements above are true.” Assumption, Strengthen, and Weaken questions that deal with numbers and percentages often assume information about the size of the overall total that you need to find a correct conclusion.
Knowing this, you will be able to identify the necessary assumptions more readily. If you need to find an assumption or strengthen an argument whose conclusion is about a causal connection, you will look for proof that it goes in the right direction or that there are no possible alternative causes. If, conversely, you need to weaken an argument, you will look for the answer that even indirectly proves the opposite.
Virtually any Critical Reasoning question type can test your ability to avoid succumbing to the logical flaws discussed above, so learn to spot those flaws in GMAT argument and answering virtually any type of Critical Reasoning questions will become much easier.