威威學長的閱讀教室：GRE邏輯推理 (GRE Logical Reasoning)

Section 1：Question Types

題目說安全帽法上路後，頭部重傷的人數下降，接著把下降歸功於新法。我們先停一下：如果騎車的人也變少了，光看傷者人數還能這樣判斷嗎？GRE 邏輯推理（Logical Reasoning）常要你檢查的，就是證據和結論之間這一段。

1.1 Strengthen（加強題）

加強題要找能讓論證更有支持的資訊。它不一定能證明結論，只要比其他選項更能補上推理缺少的部分。

常見問法：

• Which of the following, if true, most strengthens the argument?
• Which of the following provides the strongest support for the claim?

先分開前提（premise）和結論（conclusion）：前提是題幹拿出的證據，結論是作者想靠它說服你的主張。兩者之間還沒交代清楚的地方，常叫 gap，也就是推理的缺口。

選項可能補充證據、排除別的解釋，或說明因果如何發生。不要只挑「聽起來也支持這個主題」的句子，要看它有沒有碰到這個缺口。

1.2 Weaken（削弱題）

削弱題要找讓論證站不穩的資訊。先接受題幹提供的現象，再查作者從現象推到結論的那一步。

常見問法：

• Which of the following, if true, most seriously weakens the argument?
• Which of the following casts the most doubt on the conclusion?

像第 5 題，吃早餐的學生成績較高，是題幹的觀察；早餐提升成績，則是研究者的解釋。如果兩組的睡眠也不同，成績差異就有別的可能來源。

這種「還有另一個原因」叫替代解釋。有些選項則會指出因果方向可能相反、兩者沒有預想中的關聯，或樣本和資料有問題。重點是它削弱哪一段推理，不必把題幹所有內容都推翻。

1.3 Assumption（假設題）

這類問法在找論證需要、卻沒有明說的前提，也就是必要假設。缺少它時，原本的證據就不足以支持作者那一步推論。

常見問法：

• The argument depends on which of the following assumptions?
• Which of the following is an assumption required by the argument?

找出前提和結論後，試著拿掉某個候選假設，看看原來的理由還接不接得上。這叫否定測試法（Negation Test）。要檢查的是「這段論證還有沒有支持力」，不必證明結論在現實中一定為假。

拿這個簡化論證練習：「這個城市有更多警察，所以犯罪率下降。」它把警力增加當成犯罪下降的原因。若警察的存在根本不能減少犯罪，這個因果解釋就失去依據；即使犯罪率真的下降，也得找別的理由。

1.4 Paradox / Resolve（矛盾解釋題）

矛盾解釋題要讓兩個看似衝突的事實能同時成立。第 12 題就是價格上漲，營收卻下降；選項得解釋兩者如何一起發生。

常見問法：

• Which of the following, if true, most helps to resolve the apparent paradox?
• Which of the following best explains the discrepancy?

先把兩個事實各自寫清楚，再找能連起它們的新因素。這類題目不能靠否認其中一個事實過關。藥價題如果有病人轉用別的治療，銷量下降就可能抵銷漲價的效果，這才有解釋到原本的落差。

1.5 Inference（推論題）

推論題從已給的資訊往下推。先看題幹要求的確定程度，尤其別漏掉 must 這類字。

常見問法：

• Which of the following can be properly inferred from the information above?
• If the statements above are true, which of the following must also be true?

若問 must also be true，答案就得由條件推出，只有「也許如此」不夠。若問合理推論，也要找文本支持最足的選項，不能自己加原因。all、some、most、none 分別是全部、某些、多數、沒有任何一個；把 some 看成 all，結論就變了。

Section 2：Logical Reasoning Strategies by Type

2.1 Strengthen 策略

把前提和結論畫開，中間留給尚未說明的假設：

Premise → [ASSUMPTION] → Conclusion
                 ↑
         Strengthen here

用第 1 題試試看：前提是桌上有植物的員工病假較少，結論卻談到免疫功能改善。選項 (C) 提供植物影響空氣的可能機制，讓這個因果說法多一層支持。它的作用是加強，仍不能單靠這一項就排除員工原本生活習慣的差異。

2.2 Weaken 策略

削弱時檢查的也是同一個位置：

Premise → [ASSUMPTION] → Conclusion
                 ↑
         Attack here (否定假設)

替代原因（Alternative Cause）可以像第 5 題的睡眠差異，替同一個結果提供另一種解釋。因果倒置（Reverse Causality）則是方向可能讀反：你以為前者造成後者，實際可能相反。

還有第三因素（Third Variable），意思是另有一個因素同時影響兩者。遇到這些選項，問它是否真的能解釋題幹的結果，別只因看到「其他原因」就勾。

2.3 Assumption 策略

必要假設（Necessary Assumption）是原論證不能缺少的條件；充分假設（Sufficient Assumption）則是加入後，已足以讓結論成立的條件。兩者不能互換。

第 8 題的博物館預期靠新訪客在商店和咖啡店消費來增加收入，至少得假設這些人會消費。但「會買東西」還不能保證多出的收入夠補掉門票，所以必要條件不等於營收增加的保證。

2.4 Paradox 策略

先用這個格式記錄兩邊事實，再把候選解釋填進去：

Fact A: ___ (矛盾的一方)
Fact B: ___ (矛盾的另一方)
Resolution: ___ (引入新因素解釋兩者為何不矛盾)

2.5 Inference 策略

遇到 must be true，就像第 13 題：全部行銷部員工都要參加會議，Sarah 是該部門員工，卻沒有出席。由這些條件能推出她違反規定；不能再加上「她已被解雇」或「主管知道」，因為題幹沒給這些資訊。

Section 3：Common Logical Fallacies

把相關性直接當因果，或只用少數例子推到全部，都會讓結論超出證據。下面列出常見的邏輯謬誤；名稱是方便你記錄錯在哪裡，不用先背完名稱才能檢查推理。

Section 4：Practice Questions

Strengthen Questions (Q1-Q3)

Question 1: A study of 500 office workers found that those who kept potted plants on their desks reported 30 percent fewer sick days than those who did not. Researchers concluded that the presence of plants improves employees' immune function.

Which of the following, if true, most strengthens the researchers' conclusion?

(A) Some office workers are allergic to certain types of plants. (B) Office workers who keep plants on their desks also tend to exercise more regularly and eat healthier diets than those who do not. (C) The plants used in the study release compounds that have been shown in laboratory tests to reduce airborne bacteria and improve air quality. (D) Office workers who take fewer sick days are more likely to receive promotions. (E) The study was conducted over a period of only three months.

Question 2: City X implemented a new traffic law requiring all cyclists to wear helmets. In the year following the law's implementation, the number of serious head injuries among cyclists in City X decreased by 40 percent.

Which of the following, if true, most strengthens the argument that the helmet law caused the decrease in head injuries?

(A) The number of cyclists in City X remained approximately the same during the period in question. (B) Some cyclists in City X were already wearing helmets before the law was passed. (C) Neighboring City Y, which has no helmet law, saw a 5 percent decrease in cyclist head injuries. (D) The cost of bicycle helmets has decreased in recent years. (E) City X also improved its bike lane infrastructure during the same period.

Question 3: A company replaced its traditional fluorescent office lighting with blue-enriched lighting, which mimics natural daylight. Over the following six months, employee productivity increased by 15 percent, and surveys showed higher job satisfaction.

Which of the following, if true, most strengthens the claim that the lighting change caused the productivity increase?

(A) The company also gave employees a small pay raise during the same period. (B) A control group of employees who continued working under fluorescent lighting showed no productivity increase. (C) Employees initially complained about the new lighting. (D) The company hired several new highly productive employees during the period. (E) Blue-enriched lighting is more energy-efficient than fluorescent lighting.

Weaken Questions (Q4-Q6)

Question 4: A newspaper article reports that Town A, which banned plastic bags in 2020, saw a 25 percent reduction in street litter by 2023. The article concludes that banning plastic bags is an effective way to reduce litter.

Which of the following, if true, most seriously weakens the article's conclusion?

(A) Some residents of Town A still use plastic bags brought from neighboring towns. (B) Town A simultaneously launched a major public awareness campaign about littering and installed additional public trash bins during the same period. (C) Plastic bags account for only a small percentage of all litter. (D) Several other towns have also banned plastic bags. (E) The fine for littering in Town A was increased in 2024, after the period covered by the report.

Question 5: Researchers found that students who eat breakfast score higher on morning exams than students who skip breakfast. They conclude that eating breakfast improves academic performance.

Which of the following, if true, most weakens this conclusion?

(A) Students who eat breakfast also tend to get more sleep than those who skip it. (B) The exams were administered in multiple subjects. (C) Some students who ate breakfast still performed poorly. (D) The study included students from different grade levels. (E) Breakfast was provided free of charge to all participants.

Question 6: A company found that employees who work from home three days per week complete 20 percent more tasks than those who work entirely in the office. The company concludes that remote work increases productivity.

Which of the following, if true, most weakens this conclusion?

(A) Employees who chose to work from home were already among the most productive before the policy change. (B) The company uses a task-tracking system to measure productivity. (C) Some employees prefer working in the office. (D) The company's overall revenue increased during the same period. (E) Remote workers report feeling less connected to their colleagues.

Assumption Questions (Q7-Q9)

Question 7: A nutritionist claims that drinking at least eight glasses of water per day leads to better skin health, citing a study in which participants who drank eight or more glasses of water daily reported improved skin moisture levels.

The nutritionist's claim depends on which of the following assumptions?

(A) All participants in the study had similar skin types. (B) Improved skin moisture is a reliable indicator of overall skin health. (C) Drinking more than eight glasses of water provides no additional benefits. (D) The participants did not use any skincare products during the study. (E) Eight glasses of water is the optimal amount for all body types.

Question 8: A museum director argues that offering free admission on weekends will increase overall revenue because the additional visitors will spend more money at the museum's gift shop and cafe.

The director's argument assumes which of the following?

(A) The museum currently charges more than visitors are willing to pay. (B) Free admission will attract visitors who are likely to make purchases at the gift shop and cafe. (C) Weekday attendance will not decrease as a result of the policy. (D) The museum's operating costs will not increase with more visitors. (E) Other museums that offer free admission have higher revenues.

Question 9: An educator argues that students learn vocabulary more effectively from reading books than from using flashcard apps, because a study showed that students who read extensively had larger vocabularies than those who used flashcard apps.

The educator's argument requires which of the following assumptions?

(A) Flashcard apps are not an effective way to learn vocabulary. (B) The students in the study were similar in all relevant respects except their vocabulary-learning method. (C) Reading books is more enjoyable than using flashcard apps. (D) Students who use flashcard apps do not also read books. (E) A large vocabulary indicates effective vocabulary learning.

Paradox Questions (Q10-Q12)

Question 10: Country X has a higher average income than Country Y. However, residents of Country Y report higher levels of life satisfaction than residents of Country X.

Which of the following, if true, most helps to resolve this apparent paradox?

(A) Country Y has a more equitable distribution of income than Country X. (B) Country X has a larger population than Country Y. (C) Both countries have universal healthcare systems. (D) Residents of Country X work longer hours than residents of Country Y. (E) The survey was conducted in the local language of each country.

Question 11: A certain species of bird builds its nests exclusively on cliffs overlooking the ocean, even though trees and buildings that could serve as nesting sites are abundant nearby. This behavior seems maladaptive because cliff nests are more exposed to predators and harsh weather.

Which of the following, if true, most helps to explain this behavior?

(A) The birds' primary food source is fish found in the ocean near the cliffs. (B) Other bird species in the area also nest on cliffs. (C) The birds return to the same nesting sites year after year. (D) Cliff nests provide better visibility for detecting approaching predators. (E) The population of this bird species has been declining in recent years.

Question 12: A pharmaceutical company raised the price of a widely used medication by 300 percent. Surprisingly, the company's total revenue from the medication decreased after the price increase.

Which of the following, if true, most helps to explain this surprising result?

(A) The medication is also available as a cheaper generic version from other manufacturers. (B) The company spent more on advertising after the price increase. (C) The medication is used to treat a rare condition. (D) The company's manufacturing costs also increased. (E) Some patients switched to alternative treatments after the price increase.

Inference Questions (Q13-Q15)

Question 13: All employees in the marketing department are required to attend a weekly meeting. Sarah, who works in the marketing department, has not attended a weekly meeting in over a month.

Which of the following can be properly inferred from the information above?

(A) Sarah has been fired from the marketing department. (B) Sarah is the only employee who has missed meetings. (C) Sarah has violated a workplace requirement. (D) The weekly meetings are not important. (E) Sarah's manager is aware of her absences.

Question 14: In a survey of 1,000 adults, 60 percent said they preferred tea over coffee. Among respondents aged 18 to 30, however, 70 percent said they preferred coffee over tea.

Which of the following can be properly inferred?

(A) Most respondents aged 31 and older preferred tea over coffee. (B) Young adults drink more beverages than older adults. (C) Coffee consumption is increasing nationwide. (D) No respondents said they drink neither tea nor coffee. (E) Adults aged 18 to 30 make up less than 50 percent of the survey respondents.

Question 15: A school district found that students who participated in after-school tutoring programs scored an average of 12 points higher on standardized math tests than students who did not participate. However, the tutoring program was optional, and students had to apply to participate.

Which of the following can be properly inferred?

(A) The tutoring program caused the increase in test scores. (B) The comparison does not rule out pre-existing differences between the two groups. (C) After-school tutoring is the most effective way to improve math scores. (D) Students who did not participate in tutoring cannot score above average. (E) The school district should make tutoring mandatory.

Answer Key with Detailed Analysis

Q1: Strengthen：Answer: (C)

Analysis: The researchers concluded that plants improve immune function, causing fewer sick days. The gap is: why would plants improve immune function? Option (C) provides a direct mechanism: plants release compounds that reduce bacteria and improve air quality, which bridges the gap between "plants on desk" and "fewer sick days."

• (A) weakens the argument
• (B) is an alternative explanation (healthier lifestyle), which weakens
• (D) is irrelevant to causation
• (E) raises concerns about study duration

Q2: Strengthen：Answer: (A)

Analysis: The argument is: helmet law → 40% decrease in head injuries. The key assumption is that the number of cyclists did not change dramatically. If the number of cyclists dropped by 40%, the decrease in injuries might simply reflect fewer cyclists, not the helmets. Option (A) eliminates this alternative explanation.

• (B) is irrelevant to the conclusion
• (C) slightly supports but does not strengthen as directly as (A)
• (D) is irrelevant
• (E) provides an alternative explanation, which weakens

Q3: Strengthen：Answer: (B)

Analysis: A control group showing no improvement directly strengthens the causal claim by ruling out other factors (e.g., general economic improvement, seasonal effects).

• (A) and (D) provide alternative explanations, weakening
• (C) is irrelevant to causation
• (E) addresses efficiency, not productivity

Q4: Weaken：Answer: (B)

Analysis: The argument assumes the bag ban caused the litter reduction. Option (B) introduces simultaneous confounding variables (awareness campaign + more trash bins), providing alternative explanations for the litter reduction.

• (A) slightly weakens but less directly
• (C) could weaken but is less specific
• (D) gives no evidence about the effect of those other bans.
• (E) happened after 2023, so it cannot explain the reduction measured by 2023. An increase between 2020 and 2023 would be a relevant alternative explanation; the timing matters.

Q5: Weaken：Answer: (A)

Analysis: The argument assumes breakfast causes better performance. Option (A) introduces "more sleep" as an alternative explanation: perhaps sleep, not breakfast, is the real cause.

• (C) is anecdotal and does not weaken the general trend
• (B), (D), (E) do not provide alternative explanations

Q6: Weaken：Answer: (A)

Analysis: The argument assumes remote work caused higher productivity. Option (A) introduces selection bias: the people who chose remote work were already more productive, so the productivity difference may reflect the workers, not the work arrangement.

• (B) supports measurement reliability
• (C), (D), (E) are irrelevant to causation

Q7: Assumption：Answer: (B)

Analysis: The nutritionist equates "improved skin moisture" with "better skin health." If skin moisture is NOT a reliable indicator of skin health, the argument collapses. Negation test: "Skin moisture is NOT a reliable indicator of skin health" → argument falls apart.

Q8: Assumption：Answer: (B)

Analysis: The director assumes that visitors who come for free admission will spend money at the gift shop and cafe. If these visitors come only for the free admission and never buy anything, the revenue argument fails. Negation test: "Free-admission visitors are NOT likely to make purchases" → revenue argument collapses.

Q9: Assumption：Answer: (B)

Analysis: The study compares two groups, attributing the vocabulary difference to the learning method. This requires the groups to be comparable. If the extensive readers were already stronger students, the comparison is invalid. Negation test: "The students were NOT similar except for their learning method" → argument fails.

Q10: Paradox：Answer: (A)

Analysis: Paradox: higher income (Country X) but lower satisfaction. Option (A) introduces income distribution: if Country Y's income is more evenly distributed, residents may feel more financially secure and satisfied despite lower average income.

• (D) provides a partial explanation but is less comprehensive than (A)

Q11: Paradox：Answer: (D)

Analysis: Paradox: cliff nests seem worse (more exposed) but birds choose them. Option (D) explains that cliffs provide better visibility for predator detection, offsetting the seeming disadvantage.

• (A) explains food access but not nest site choice
• (B) and (C) don't explain why cliff nesting is adaptive

Q12: Paradox：Answer: (E)

Analysis: Paradox: higher price should increase revenue, but revenue decreased. Option (E) explains that patients switched to alternatives, meaning the quantity sold dropped enough to offset the price increase.

• (A) could explain but (E) is more direct
• (B), (C), (D) don't explain the revenue decrease

Q13: Inference：Answer: (C)

Analysis: All marketing employees must attend → Sarah is in marketing → Sarah has not attended → Sarah has violated the requirement. This is a deductively valid inference.

• (A), (B), (D), (E) go beyond what can be inferred

Q14: Inference：Answer: (A)

Analysis: 選項 (A) 限定的是受訪者，不能外推成全體人口。令年輕受訪者有 $y$ 人，其中偏好茶的至多 $0.3y$ 人；總共有 $600$ 人偏好茶，所以較年長那組偏好茶的人數至少是 $600-0.3y$。它比年長受訪者的一半 $0.5(1000-y)$ 多出 $100+0.2y>0$，因此年長組確實過半偏好茶。

不能選 (E)。例如年輕、年長各 $500$ 人：年輕組 $350$ 人偏好咖啡、$150$ 人偏好茶，年長組 $450$ 人偏好茶、$50$ 人偏好咖啡。總共仍是 $600$ 人偏好茶、年輕組 $70\%$ 偏好咖啡，但年輕組剛好占一半，已經否定「一定少於一半」。這是檢查選項用的假設分配，不是真實調查資料。

Q15: Inference：Answer: (B)

Analysis: 題目只說自願申請，沒有提供隨機分派或兩組起點相同的資料，所以這個比較沒有排除原有程度、動機等差異，選 (B)。它也沒有證明這些差異真的存在，更不能單憑平均分數差就把因果歸給課輔。

• (A) assumes causation without evidence
• (C), (D), (E) go far beyond what can be inferred

訂正時挑第 2 題，把「頭部重傷人數下降」和「安全帽法造成下降」分別寫在紙上。再問：選項 (A) 排除了哪個替代解釋？選項 (E) 又為什麼會讓歸因變得不確定？

如果只能回答一個是加強、一個是削弱，還差一步。試著把人數、法規和自行車道之間的關係說清楚，再換下一題。