GMAT邏輯中的相關和因果

相關關係：AB正相關或者負相關

因果關係：A→B

相關和因果是GMAT邏輯部分考察的一大要點。我們知道相關關係不等同於因果關係：因果關係必定是相關關係，而相關關係不一定是因果關係。

統計數據表明，2010-2019年每年的冰淇淋銷量和游泳池淹死的人數成正相關，即A（吃冰淇淋）和B（在游泳池淹死）相關，但顯然AB之間不存在因果關係，吃冰淇淋不會導致在游泳池淹死，在游泳池淹死也不會導致人們吃冰淇淋。AB之間的相關關係是因為一個混淆變量（天氣炎熱程度）的存在。

某公眾號發表了一篇閱讀量100000+的文章，攻擊某知名連鎖品牌“啃的雞”：《揭露！中國人必轉！洋快餐是如何殘害祖國花朵的！》，證據是一個抽樣調查（全國十大省會城市10000名初中生）表明，每個月吃“啃的雞”次數和初中生的肥胖程度正相關。統計數據顯示的是相關關係：A（吃啃的雞）和B（肥胖）相關，文章結論卻直接變成了因果關係：吃啃的雞導致肥胖。“啃的雞”欲反駁該文章，可以從兩個不同角度削弱：1，這些肥胖的學生對自己每日攝入的卡路里缺乏管理，攝入量大於消耗量才導致發胖，吃啃的雞並不會2，不是吃啃的雞發胖，而是胖子喜歡吃東西，不止吃啃的雞還吃麥當勞德克士漢堡王......

通過“啃的雞”的例子，我們可以抽象出解決GMAT邏輯相關和因果題目的如下思路。

削弱題，欲反駁或削弱A→B，手段有二：1，引入他因，2，因果倒置。

推及假設題和加強題，欲支持或加強A→B，手段有二：1，排除他因，2，否定因果倒置。

假設題示例1 OG2018-Q635

In Wareland last year, 16 percent of licensed drivers under 21 and 11 percent of drivers ages 21–24 were in serious accidents. By contrast, only 3 percent of licensed drivers 65 and older were involved in serious accidents. These figures clearly show that the greater experience and developed habits of caution possessed by drivers in the 65-and-older group make them far safer behind the wheel than the younger drivers are.

Which of the following is an assumption on which the argument depends?

A. Drivers 65 and older do not, on average, drive very many fewer miles per year than drivers 24 and younger.

B. Drivers 65 and older do not constitute a significantly larger percentage of licensed drivers in Wareland than drivers ages 18– 24 do.

C. Drivers 65 and older are less likely than are drivers 24 and younger to drive during weather conditions that greatly increase the risk of accidents.

D. The difference between the accident rate of drivers under 21 and of those ages 21–24 is attributable to the greater driving experience of those in the older group.

E. There is no age bracket for which the accident rate is lower than it is for licensed drivers 65 and older.

解析：排除他因

首先簡化題幹：前提是統計數據表明司機年齡A和事故率B成負相關，年輕司機事故率高，年長司機事故率低。結論是年長司機（隨年齡而來的）豐富的經驗和長期形成的開車謹慎的習慣A導致他們事故率低B。假設題需要支持因果關係，使邏輯鏈完整，手段2否定因果倒置（不是事故率低導致經驗豐富開車謹慎）在這道題行不通，手段1排除他因（不是其他原因導致事故率低），對應A選項（不是因為開得少事故率才低）。當然，A選項取非後就成了引入他因，可用來削弱原文結論。

假設題示例2

A researcher discovered that people who have low levels of immune-system activity tend to score much lower on tests of mental health than do people with normal or high immune-system activity. The researcher concluded from this experiment that the immune system protects against mental illness as well as against physical disease.

The researcher's conclusion depends on which of the following assumptions?

A. High immune-system activity protects against mental illness better than normal immune-system activity does.

B. Mental illness is similar to physical disease in its effects on body system.

C. People with high immune-system activity cannot develop mental illness.

D. Mental illness does not cause people's immune-system activity to decrease.

E. Psychological treatment of mental illness is not as is medical treatment.

解析：否定因果倒置

首先簡化題幹：前提是統計數據表明免疫系統活動水平A和精神健康程度B成正相關，結論是免疫系統活動水平高A會導致精神健康程度高B。假設題需要支持因果關係，使邏輯鏈完整，手段1排除他因（不是其他原因導致精神健康程度高B）在這道題行不通，因為精神健康可能由多個原因導致，手段2否定因果倒置（不是精神健康程度B影響免疫系統活動水平A）對應D選項。

削弱題示例1 OG2018-Q549

Editorial: The roof of Northtown’s municipal equipment storage building collapsed under the weight of last week’s heavy snowfall. The building was constructed recently and met local building safety codes in every particular, except that the nails used for attaching roof supports to the building’s columns were of a smaller size than the codes specify for this purpose. Clearly, this collapse exemplifies how even a single, apparently insignificant departure from safety standards can have severe consequences.

Which of the following, if true, most seriously weakens the editorial’s argument?

A. The only other buildings to suffer roof collapses from the weight of the snowfall were older buildings constructed according to less exacting standards than those in the codes.

B. The amount of snow that accumulated on the roof of the equipment storage building was greater than the predicted maximum that was used in drawing up the safety codes.

C. Because the equipment storage building was not intended for human occupation, some safety code provisions that would have applied to an office building did not apply to it.

D. The municipality of Northtown itself has the responsibility for ensuring that buildings constructed within its boundaries meet the provisions of the building safety codes.

E. Because the equipment storage building was used for storing snow removal equipment, the building was almost completely empty when the roof collapsed.

解析：引入他因

首先簡化題幹：前提是釘子不符合建房標準A，下大雪後房屋倒塌B。結論是A→B。欲削弱原文，可引入他因C（雪太大），C→B，對應B選項。

削弱題示例2 OG2018-Q574

People who do regular volunteer work tend to live longer, on average, than people who do not. It has been found that “doing good,” a category that certainly includes volunteer work, releases endorphins, the brain’s natural opiates, which induce in people a feeling of well being. Clearly, there is a connection: Regular releases of endorphins must in some way help to extend people’s lives.

Which of the following, if true, most seriously undermines the force of the evidence given as support for the hypothesis that endorphins promote longevity?

A. People who do regular volunteer work are only somewhat more likely than others to characterize the work they do for a living as “doing good.”

B. Although extremely high levels of endorphins could be harmful to health, such levels are never reached as a result of the natural release of endorphins.

C. There are many people who have done some volunteer work but who do not do such work regularly.

D. People tend not to become involved in regular volunteer work unless they are healthy and energetic to begin with.

E. Releases of endorphins are responsible for the sense of well

being experienced by many long distance runners while running.

解析：因果倒置

首先簡化題幹：前提是A（做志願工作）和B（壽命長）相關，做志願工作A可導致人大腦分泌可增強幸福感的物質C（A→C）。結論是C→B，根據前提A→C，結論可替換成A→B。欲反駁因果關係，因果倒置（壽命長的人才能做志願工作，而非做志願工作使人壽命長）對應D選項。

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