GMAT的逻辑推理部分(CriticalReasoning)对于很多学生来说是难点。然而，这一部分和数学，阅读一样，是可以争取拿到满分的（在GMAT中唯一不太可能拿到满分的只有SentenceCorrection，毕竟我们不是语言学家，不可能知晓所有的idioms，但90%以上正确率是做得到的）。接下来，我们就通过几个实际的例子，来谈谈如何在GMATCritical Reasoning中拿到满分。（本文的思路也适用于LSAT的考生）

Example1 （Difficulty Level: 700）.

Whenever a major airplane accident occurs, there is a dramatic increase in thenumber of airplane mishaps reported in the media, a phenomenon that may lastfor as long as a few months after the accident. Airline officials assert thatthe publicity given the gruesomeness of major airplane accidents focuses mediaattention on the airline industry, and the increase in the number of reportedaccidents is caused by an increase in the number of news sources coveringairline accidents, not by an increase in the number of accidents.

Which of the following, if true, wouldseriously weaken the assertions of the airline officials?

A. The publicity surrounding airlineaccidents is largely limited to the country in which the crash occurred.

B. Airline accidents tend to occur far moreoften during certain peak travel months.

C. News organizations do not have anyguidelines to help them decide how severe an accident must be for it to receivecoverage.

D. Airplane accidents receive coverage bynews sources only when the news sources find it advantageous to do so.

E. Studies by government regulations showthat the number of airplane flight miles remains relatively constant from monthto month.

所有的CR题目由三个部分组成：

1.Question “Which of the following, if true, wouldseriously weaken the assertions of the airline officials? ”

2.Argument: “Whenever a … accident”(问题前面的整段话)

3. Choices

我们解题，第一步看Question。这题，问的是weaken. 90%的GMAT题目都是在问如何weaken（削弱）, support（支持）, evaluate（评估） the argument 或者是这个argument的assumption (假设)是什么

一旦题目问的是weaken, support, evaluation orassumption，我们接下来第二步就是需要阅读argument并且找出其line of reasonings (也叫作logical tree)

任何推理(reasoning)都是由前提(premises/evidence)和结论(conclusion)两个部分构成，一般我们用横线隔开，例如大家非常熟悉的假言推理（常用于数学证明）：

Ifp, then q (前提1)

P （前提2）

__________________

q（结论）

要在GMAT Critical Reasoning拿到满分，同学们应该对形式逻辑中常见的推理形式（例如简单归纳法，由因到果的推理，由果到因的推理（溯因推理，假说演绎推理，求异法，求同法），类比推理等）十分熟悉。一个argument往往由1个甚至数个reasoning构成，同学们在快速阅读理解后，应该在头脑中（也可借助草稿纸简单标注）形成作者的推理形式并分析出他用了哪一（几）种推理形式。例如本题就用到了溯因推理(Abduction)。

溯因推理是常见的由果（通常是有一个事实和现象，例如苹果从树上掉下来）找因的推理。其形式如下：

q

ifp, then q

____________________

q isbecause of p

例如：

今年的收成不好（一个现象，事实）

如果天不下雨，庄稼收成不好

———————————————————————

因此今天收成不好是因为雨量不足

注意这个推理并不是演绎推理，即所有前提为真，结论不一定为真。例如上面的这个例子，雨水过多也可能导致庄稼收成不好，这就是C à q (别的原因可以解释这个现象)，这是溯因推理5种削弱方式的一种。本题，其实和上面的例子非常类似，作者的推理如下：

q: Whenever a majorairplane accident occurs, the number of reported airplane mishaps increases fora few months

if p (publicity focuses media attention tothe industry), then q (more mishapsare reported, not a real increase in the number)

______________________________________________________________________________________________

q isbecause of p

正确答案是B，airlineaccidents tend to occur together (in peak travel months)，这解释了reported airplane mishaps在几个月中增加，B à q(别的原因可以解释这个现象)，从而削弱了原来的推理。

正如这题显示出来的，一旦你能够1）分析出argument的lineof reasonings并找出其使用的推理形式；2）掌握该推理形式的weaken的方法；那么所有weaken和assumption的题目从此就应该驾轻就熟了(assumption是用weaken来定义的，后讲)

Exercise 1 (Difficulty Level: 700):

M: It is almost impossible to find a personbetween the ages of 85 and 90 who primarily uses the left hand.

Q: Seventy to ninety years ago, however,children were punished for using their left hands to eat or to write and wereforced to use their right hands.

Q's response serves to counter any use by Mof the evidence about 85 to 90 year olds in support of which one of thefollowing hypotheses?

(A) Being born right-handed confers asurvival advantage.

(B) Societal attitudes toward handednessdiffer at different times.

(C) Forcing a person to switch from apreferred hand is harmless.

(D) Handedness is a product of both geneticpredisposition and social pressures.

(E) Physical habits learned in school oftenpersist in old age.

做出来了吗？“It is almost impossible to find aperson between the ages of 85 and 90 who primarily uses the left hand.”这是一个现象（phenomenon）。这题就是叫你找Q追溯的这个现象的原因(hypothesis = theory = model = explanation = cause!)。这个原因应该和Q指出来的原因不同，这样Q才weaken了M(counter)，正如我们上面的公式C à q。学会了溯因推理，这道只有24%正确率的题目简单吧，选A。(B,C,D,E都不能解释这个现象)

接下来，我们看一道更难的题目：

Example2 (Difficulty Level: 750) :

Roland: The alarming fact is that 90percent of the people in this country now report that they know someone who isunemployed.

Sharon: But a normal, moderate level ofunemployment is 5 percent, with 1 out of 20 workers unemployed. So at any giventime if a person knows approximately 50 workers, 1 or more will very likely beunemployed.

Sharon's argument relies on the assumptionthat

(A) normal levels of unemployment arerarely exceeded

(B) unemployment is not normallyconcentrated in geographically isolated segments of the population

(C) the number of people who each knowsomeone who is unemployed is always higher than 90% of the population

(D) Roland is not consciously distortingthe statistics he presents

(E) knowledge that a personal acquaintanceis unemployed generates more fear of losing one's job than does knowledge ofunemployment statistics

这题问的是Assumption。我们先学习下Assumption的定义：

Assumptionis an unstated premise such that if we negate it, it weakens the reasoning.

也就是说假设（assumption）必须满足两个条件：

1）没有在argument中提到过(unstated)

2) 对假设取非后变成weaken

我们还是先来看看这一题的Line of reasonings:

q: 90% of people knowsomeone who is unemployed (一个现象)

if p (5% unemployment rate) à(ifsomeone knows 50 people) 50*5%=2.5, i.e. everyone should know someone who isunemployed(q)

_________________________________________________________________________________________

q isbecause of p

仍然是溯因推理（假说演绎推理）。接下来我们来谈谈溯因推理（假说演绎推理）的别的削弱方式。If p, then q这个前提实际上包含了由因到果的推理。因此，必须符合因果关系的三大性质：

1）有因必有果

2）有果必有因

3）因先于果

因此，当我们得出p且非q（和性质1矛盾）, 非p且q（和性质2矛盾），p后于q（和性质3矛盾）时，我们就削弱了Ifp, then q，从而削弱了整个溯因推理。

这题之所以是难题，在于这里If p, then q不仅仅是从因到果的推理，同时也是统计三段论(statisticalsyllogism)：

总体A有某种统计性质P(例如平均数，中位数，众数等)

a是总体的一个样本（一部分）

————————————————————————————

a有性质P

例如：

中国人的平均年龄是45岁

小明是中国人

—————————————————————

小明是45岁

这个推理当且样本很好的代表了（typical of, representativeof）总体才成立，当样本有偏(biased, atypical of ,not representativeof)时，结论是不成立的。例如上面的例子，如果小明是小学生（样本有偏），结论明显是错误的。

回到我们的Critical Reasoning问题, Sharon实际上用了如下的推理（unstated）

5%unemployment rate – Statistical Property for the country!

A iscity of the country

________________________________________________________________________

A’sunemployment rate is also 5%

正确的选项是B, 当我们对B取非后，B变成了：

unemployment is concentrated ingeographically isolated segments of the population

换句话说，假如这个国家有10个城市，每个城市人口相同，所有失业人员全部集中在城市X（失业率50%），剩下的9个城市失业率为0%。这时我们满足了p(失业率5%)，然而生活在剩下9个城市的人，哪怕认识50个人，恐怕里面失业的也几乎应该是0%，即我们找出了统计三段论的问题，得出了p且非q，因此削弱了原文的溯因推理。

今天我用溯因推理举例，说明了GMAT逻辑推理题目应该如何拿到满分：

同学们需要具备：

1. 形式逻辑的相关知识（例如溯因推理以及其5种削弱方式）

2. 能够用好“泽宇阅读法”，即读出argument的line of reasoning并分析出其使用了什么样的逻辑推理形式

那么随着你不断练习，对1,2越发熟练掌握，你会发现，GMAT Critical Reasoning变成小菜一碟了。