GMAT邏輯必讀bible――commonerror

1、The Value of Knowing Common Errors of ReasoningIn logic there are many more recognized forms of invalid argumentation than there are forms of valid argumentation. The test makers, being human (yes, its true, tend to repeat certain forms when creating stimuli and answer choices, and you can gain a de

2、monstrable advantage by learning the forms most often used by the test makers. Applying the knowledge you acquire in this section will take two avenues:1. Identifying errors of reasoning made in the stimulusIf you learn the mistakes that are often made by authors, then you will be able to quickly id

3、entify the error in the argument and accelerate through the answer choices to find the correct answer. Students without this knowledge will be forced to work more slowly and with less confidence.2. Identifying answer choices that describe a common error of reasoningIn Flaw in the Reasoning questions

4、, the test makers tend to use certain types of answers again and again. Depending on the reasoning used in the stimulus, these answers can describe the correct answer, but more often than not they are used as “stock” wrong answers. Familiarizing yourself with these answer choices will give you an ad

5、vantage when you encounter similar answer choices in the future. For example, “attacking the source of an argument, not the argument itself” has appeared as the correct answer in several questions. But, it has appeared in many more questions as a wrong answer choice. If you are familiar with a “sour

6、ce” argument, you can then make an immediate determination as to whether that answer is correct or incorrect.The paragraphs above help explain why test preparation works: the more you know about the exam before you walk in to take the test, the less time you have to waste during the exam thinking ab

7、out these issues. Given the immense advantage you get by knowing the flawed reasoning that appears most frequently on the GMAT, the following section will detail a variety of errors of reasoning and provide examples of answer choices that describe the error under discussion. We strongly recommend th

8、at you spend a considerable amount of time learning these forms of flawed argumentation. It will definitely help you on the GMAT!Please note that this discussion is not designed to include every possible error of reasoning, only those used most frequently by the makers of the GMAT.Common Errors of R

9、easoning ExplainedThe following classic errors of reasoning appear with some frequency. The review is given in laymans, not philosophical, terms:Uncertain Use of a Term or ConceptAs an argument progresses, the author must use each term in a constant, coherent fashion. Using a term in different ways

10、is inherently confusing and undermines the integrity of the argument. Here is an example:“Some people claim that the values that this country was built on are now being ignored by modern-day corporations. But this is incorrect. Corporations are purely profit-driven enterprises, beholden only to thei

11、r shareholders, and as such they can only assess objects based on their value.”The term “value” is used in the example above in two different senses: first in a moral or ethical sense and then in a monetary sense. This shift in meaning undermines the authors position. This type of answer choice appe

12、ars more frequently as an incorrect answer than any other type. Here are examples of how this error of reasoning can be described in answer choices:“depending on the ambiguous use of a key term”“it confuses two different meanings of the word solve ”“relies on interpreting a key term in two different

13、 ways”“equivocates with respect to a central concept”“allows a key term to shift in meaning from one use to the next”Source ArgumentAlso known as an ad hominem, this type of flawed argument attacks the person (or source instead of the argument they advance. Because the GMAT is concerned solely with

14、argument forms, a speaker can never validly attack the character or motives of a person; instead, a speaker must always attack the argument advanced by a person. Here is an example:“The anti-smoking views expressed by Senator Smith should be ignored. After all, Smith himself is a smoker!”A source ar

15、gument can take different forms, including the following:1. Focusing on the motives of the source.2. Focusing on the actions of the source (as in the above example.In the real world, you will often hear source arguments used by children and politicians (the two being alike in a number of ways, of co

16、urse.Here are examples of how this error of reasoning can be described in answer choices:“makes an attack on the character of opponents”“it is directed against the proponent of a claim rather than against the claim itself”“he directs his criticism against the person making the argument rather than d

17、irecting it against the argument itself”“it draws conclusions about the merit of a position and about the content of that position from evidence about the positions source”“assuming that a claim is false on the grounds that the person defending it is of questionable character”Circular ReasoningIn ci

18、rcular reasoning the author assumes as true what is supposed to be proved. Consider the following example:“This essay is the best because it is better than all the others.”In this example the premise and the conclusion are identical in meaning. As we know, the conclusion should always follow from th

19、e premise. In the example above, the premise supports the conclusion, but the conclusion equally supports the premise, creating a “circular” situation where you can move from premise to conclusion, and then back again to the premise, and so on. Here is another example: “I must be telling the truth b

20、ecause Im not lying.”Here are examples of how this error of reasoning can be described in answer choices:“it assumes what it seeks to establish”“argues circularly by assuming the conclusion is true in stating the premises”“presupposes the truth of what it sets out to prove”“the argument assumes what

21、 it is attempting to demonstrate”“it takes for granted the very claim that it sets out to establish”“it offers, in place of support for its conclusion, a mere restatement of that conclusion”Mistaken Cause and EffectAs discussed in Chapter Seven, arguments that draw causal conclusions are inherently

22、flawed because there may be another explanation for the stated relationship. Because of the extreme causal assumption made by GMAT authors (that there is only one cause, any of the following answer choice forms could be used to describe an error of causality. Underneath each item are examples of how

23、 the error of reasoning can be described in answer choices.1. Assuming a causal relationship on the basis of the sequence of events.“mistakes the observation that one thing happens after another for proof that the second thing is the result of the first”“mistakes a temporal relationship for a causal

24、 relationship”2. Assuming a causal relationship when only a correlation exists.“confusing the coincidence of two events with a causal relation between the two”“assumes a causal relationship where only a correlation has been indicated”3. Failure to consider an alternate cause for the effect, or an al

25、ternate cause for both the cause and the effect.“fails to exclude an alternative explanation for the observed effect”“overlooks the possibility that the same thing may causally contribute both to education and to good health”4. Failure to consider that the events may be reversed.“the author mistakes

26、 an effect for a cause”Note the frequency with which the words “cause” or “effect” are used. This occurs because there are few substitutes for those two words, and thus the test makers are often forced to use those words to describe an argument containing causality. If you identify a stimulus with c

27、ausal reasoning and are asked a Flaw question, quickly scan the answers for one that contains “cause,” “effect,” or both.Straw ManThis error occurs when an author attempts to attack an opponents position by ignoring the actual statements made by the opposing speaker and instead distorts and refashio

28、ns the argument, making it weaker in the process. In figurative terms, a “straw” argument is built up which is then easier for the author to knock down.Often this error is accompanied by the phrase “what youre saying is” or “if I understand you correctly,” which are used to preface the refashioned a

29、nd weakened argument. Here is an example:Politician A: “The platform proposed by my party calls for a moderate increase in taxes on those individuals making over $20,000 per year, and then taking that money and using it to rebuild the educational system.”Politician B: “But what youre saying is that

30、everyone should pay higher taxes, and so yourproposal is unfair.”In the example above, Politician B recasts Politician As argument unfairly. Politician A indicated the tax increase would apply to those with incomes over $20,000 where Politician B distorts that to “everyone should pay higher taxes.”T

31、o determine the error of reasoning, focus on the connection between the premises and the conclusion. Remember, GMAT authors are allowed to put forth virtually any premise when making an argument; the key is how those premises are used, not whether they are factually true. Here are examples of how th

32、is error of reasoning can be described in answer choices:“refutes a distorted version of an opposing position”“misdescribing the student representatives position, thereby making it easier to challenge” “portrays opponents views as more extreme than they really are”“distorts the proposal advocated by

33、 opponents”Just a note: we did not make up the name “straw man.” The term is the proper name used in logic.Some GMAT authors misuse information to such a degree that they fail to provide any information to support their conclusion or they provide information that is irrelevant to their conclusion. H

34、ere is an example:“Some critics claim that scientific progress has increased the polarization of society and alienated large segments of the population. But these critics are wrong because even a cursory glance at the past shows that society is always somewhat polarized and some groups are inevitabl

35、y alienated.”The author provides irrelevant evidence in an attempt to refute the claim that “scientific progress has increased the polarization of society and alienated large segments of the population.” Citing facts that such a situation has always existed does not help disprove that scientific pro

36、gress has increased the severity of the situation.Note the use of the construction “some critics claim.” As usual, the authors main point is that the claim that the critics are making is wrong.Here are examples of how this error of reasoning can be described in answer choices:“The author cites irrel

37、evant data.”“draws a conclusion that is broader in scope than is warranted by the evidence advanced”“It uses irrelevant facts to justify a claim about the quality of the disputed product.”“It fails to give any reason for the judgment it reaches.”“It introduces information unrelated to its conclusion

38、 as evidence in support of that conclusion.” An internal contradiction (also known as a self-contradiction occurs when an author makes conflicting statements. The example used was:“Everyone should join our country club. After all, its an exclusive group that links many of the influential members of

39、the community.”The self-contradiction occurs when the speaker says “Everyone should join” and then follows that by saying that it is “an exclusive group.” Exclusive, by definition, means that some people are excluded.The following show how this error of reasoning can be described in answer choices:“

40、bases a conclusion on claims that are inconsistent with each other”“the author makes incompatible assumptions”“introduces information that actually contradicts the conclusion”“offers in support of its conclusion pieces of evidence that are mutually contradictory”“some of the evidence presented in su

41、pport of the conclusion is inconsistent with other evidence provided”“assumes something that it later denies, resulting in a contradiction”Appeal FallaciesWhile there are a number of “appeal” fallacies that appear in traditional logic (Appeal to Fear, Appeal to Force, Appeal to Tradition, etc., the

42、following three are the most applicable to the GMAT:1. Appeal to AuthorityAn Appeal to Authority uses the opinion of an authority in an attempt to persuade the reader. The flaw in this form of reasoning is that the authority may not have relevant knowledge or all the information regarding a situatio

43、n, or there may a difference of opinion among experts as to what is true in the case. Here is an example:“World-renowned neurologist Dr. Samuel Langhorne says that EZBrite Tooth Strips are the best for whitening your teeth. So, you know if you buy EZBrite you will soon have the whitest teeth possibl

44、e!”The primary defect in this argument is its use of a neurologist as an authority figure in an area of dentistry. While Dr. Langhorne can reasonably be appealed to in matters of the brain, dental care would be considered outside the scope of his expertise.Here are examples of how this error of reas

45、oning can be described in answer choices: “the judgment of experts is applied to a matter in which their expertise is irrelevant” “the argument inappropriately appeals to the authority of the mayor”“it relies on the judgment of experts in a matter to which their expertise is irrelevant” “accepts a c

46、laim on mere authority, without requiring sufficient justification”2. Appeal to Popular Opinion/Appeal to NumbersThis error states that a position is true because the majority believe it to be true. As you know, arguments are created by providing premises that support a conclusion. An appeal to popu

47、lar opinion does not present a logical reason for accepting a position, just an appeal based on numbers. Here is an example:“A recent poll states that 75% of Americans believe that Microsoft is a monopoly. Antitrust law states that monopolies have a deleterious effect on the marketplace (with the ex

48、ception of utilities, and therefore Microsoft should be controlled or broken into smaller pieces.”The author uses the results of a poll that indicate many people think Microsoft is a monopoly to conclude that Microsoft is in fact a monopoly. This type of persuasion is often used in the arguments mad

49、e by advertisements (“All the trend setters use EZBrite Tooth Strips”, politicians (“Everyone loves the environment. Vote for the Green Party!,and children (“Cmon, try this. Everyone does it.”.This type of reasoning most often appears as an incorrect answer. Here are examples of how this error of re

50、asoning can be described in answer choices:“it treats popular opinion as if it constituted conclusive evidence for a claim”“attempts to discredit legislation by appealing to public sentiment”“a claim is inferred to be false merely because a majority of people believe it to be false”“the argument, in

51、stead of providing adequate reasons in support of its conclusion, makes an appeal to popular opinion”3. Appeal to EmotionAn Appeal to Emotion occurs when emotions or emotionally-charged language is used in an attempt to persuade the reader. Here is an example:“Officer, please do not give me a ticket

52、 for speeding. In the last month Ive been fired from my job, kicked out of my apartment, and my car broke down. I dont deserve this!" Here are examples of how this error of reasoning can be described in answer choices: “attempts to persuade by making an emotional appeal”“uses emotive language i

53、n labeling the proposals”“the argument appeals to emotion rather than reason”The makers of the GMAT believe that surveys, when conducted properly, produce reliable results. However, surveys can be invalidated when any of the following three scenarios arise: 1. The survey uses a biased sample.Perhaps

54、 the most famous example of a biased survey occurred in 1936. The Literary Digest weekly magazine sent out ballots to some 10 million voters (2.3 million were returned, and returns indicated that a solid majority would vote for Republican candidate Alf Landon in the upcoming presidential election. O

55、n the basis of these results (and the size of the sample, the Literary Digest predicted that Landon would win easily. Ofcourse, when the election was held Franklin Roosevelt won in a landslide. The Literary Digest erred by sending the ballots to groups such as telephone owners and automobile owners,

56、 groups that in that era (late Depression tended to be among the wealthiest individuals and overwhelmingly Republican. The Literary Digest ended up polling a large number of Republicans and on that basis declared that the Republican candidate would win.Note that a secondary error with the polling do

57、ne by the Literary Digest is that the sample is self-selected; that is, the individuals being polled decided whether or not to respond.That opportunity introduces bias into the survey process because certain types ofindividuals tend to respond to surveys more often than others.A similar type of samp

58、ling error occurred in 1948 when the Chicago Daily Tribunepredicted Thomas Dewey would prevail over Harry Truman. The Tribune even went so far as to print the morning edition of the newspaper with that headline.2. The survey questions are improperly constructed.If a survey question is confusing or m

59、isleading, the results of the poll can be inaccurate. Questions can be confusing, such as “Do you feel it is possible that none of thecandidates would not vote to increase taxes?” (The question actually asks, “Do you feel it is possible that all of the candidates would vote to increase taxes?”. If a respondent cannot understand the qu