Weaken and Strengthen are the two most common Critical Reasoning tasks, and they are two sides of the same skill: finding the gap between an argument’s evidence and its conclusion.
Read the argument and restate the conclusion in your own words. Then ask what the author is taking for granted. Typical gaps: the group studied differs from the group the conclusion is about; a correlation is presented as a cause; a past result is assumed to hold under new conditions; a plan is assumed to have no side effects. A correct weakener attacks that gap directly, for example by showing the two groups differ in a way that matters. A correct strengthener closes it, for example by ruling out another explanation.
You do not need to destroy or prove the argument. The right answer only has to make the conclusion somewhat less likely, or somewhat more likely. That is why extreme-sounding choices are often wrong and a modest, specific choice is often right.
The main trap is the relevant-sounding option that discusses the topic but not the conclusion: customer satisfaction when the conclusion is about waiting times, or costs when the conclusion is about whether a plan will work. Test each option with one question: if this were true, would I believe the conclusion more, less or the same?
4 practice questions
Critical ReasoningWeaken ArgumentEasy
Question 1
Last year Brightwell Grocers installed self-checkout kiosks in 20 of its 40 stores. Over the following six months, the average time customers waited at checkout in those 20 stores fell by 30 percent, while waiting times in the other 20 stores did not change. Brightwell's operations director concludes that installing kiosks in the remaining 20 stores will reduce checkout waiting times there by a similar amount.
Which of the following, if true, most seriously weakens the director's conclusion?
ACustomers at the stores with kiosks reported greater satisfaction with checkout than did customers at the other stores.
BThe first 20 stores were chosen because their customers had expressed the strongest interest in scanning their own items; customers at the other stores strongly prefer staffed lanes.
CInstalling the kiosks cost Brightwell less than hiring enough additional cashiers to achieve the same reduction in waiting times would have cost.
DWhen the kiosks were first installed, some customers needed help from staff members to learn how to use them.
ESeveral of Brightwell's competitors have recently begun installing self-checkout kiosks in their own stores.
Show answer & explanation
Correct answer: B
Key Concept or Approach
The director assumes the remaining stores are like the first 20 in the ways that matter. Kiosks shorten waits only if customers actually use them.
Step-by-Step Solution
Step 1: Identify the task (Weaken Argument) and the gap in the reasoning: The director assumes the remaining stores are like the first 20 in the ways that matter. Kiosks shorten waits only if customers actually use them.
Step 2: (B) Correct. The first 20 stores were chosen for their customers' appetite for self-scanning. If customers at the other stores avoid kiosks, the kiosks will not relieve the staffed lanes there.
Step 3: (A) Satisfaction at the kiosk stores does not show whether waits would fall at the other stores. If anything, it slightly supports the plan.
Step 4: (C) Cost-effectiveness is irrelevant to whether waiting times will fall.
Step 5: (D) An early learning curve at the first stores did not stop the 30 percent reduction, so it gives no reason to expect a different result elsewhere.
Step 6: (E) What competitors do has no bearing on waiting times in Brightwell's stores.
Final Answer
Answer: B
Common trap: A is tempting because it reports a positive result from the kiosks, but it says nothing about whether waits will fall in the other stores; only B shows the two groups of stores differ.
Critical ReasoningStrengthen ArgumentEasy
Question 2
Last season, farmers in the Tolvar Valley who planted a new drought-resistant variety of sorghum harvested an average of 18 percent more grain per hectare than farmers in the valley who planted the traditional variety. On this basis, the regional agriculture agency concludes that the new variety itself, rather than any difference in farming practices or conditions, was responsible for the higher yields.
Which of the following, if true, most strongly supports the agency's conclusion?
ASeed for the new variety costs more per hectare than seed for the traditional variety.
BRainfall in the Tolvar Valley last season was above the long-term average.
CFarmers who planted the new variety had, on average, larger farms than those who planted the traditional variety.
DThe agency plans to distribute seed for the new variety to more farmers next season.
EFarmers who planted the new variety used the same amounts of fertilizer and irrigation, on land of similar quality, as those who planted the traditional variety.
Show answer & explanation
Correct answer: E
Key Concept or Approach
The agency attributes the yield gap to the seed itself. The best support rules out differences in practices or conditions between the two groups.
Step-by-Step Solution
Step 1: Identify the task (Strengthen Argument) and the gap in the reasoning: The agency attributes the yield gap to the seed itself. The best support rules out differences in practices or conditions between the two groups.
Step 2: (E) Correct. It removes the main alternative explanations (inputs and land quality), leaving the variety as the likely cause.
Step 3: (A) Seed price is irrelevant to what caused the higher yields.
Step 4: (B) If anything, a wet season makes drought resistance less likely to explain the gap, so this does not help.
Step 5: (C) Farm size is a possible difference between the groups. It raises doubt rather than removing it.
Step 6: (D) Future plans say nothing about last season's cause.
Final Answer
Answer: E
Common trap: B mentions weather and so seems relevant, but rain affected both groups alike and does nothing to rule out differences between them.
Critical ReasoningWeaken ArgumentMedium
Question 3
Six months ago the online retailer Parcelia began offering customers free returns on all clothing purchases, which had previously cost customers a fee. Since then, the number of clothing orders Parcelia receives has risen by 30 percent, while the average price of an order has stayed about the same. Parcelia's marketing director concludes that the free-returns policy has made the company's clothing business more profitable.
Which of the following, if true, most seriously weakens the marketing director's conclusion?
ASince the policy began, the share of clothing orders that customers return has tripled, and processing a return costs Parcelia more than it earns on a typical clothing sale.
BNone of Parcelia's major competitors offers free returns on clothing.
CMost customers who have used the free-returns service say they are likely to order clothing from Parcelia again.
DParcelia announced the free-returns policy through an extensive advertising campaign.
EClothing accounts for a larger share of Parcelia's revenue than any other product category.
Show answer & explanation
Correct answer: A
Key Concept or Approach
The conclusion is about profitability, but the evidence is about order volume. A weakener shows that the extra orders may not have produced extra profit.
Step-by-Step Solution
Step 1: Identify the task (Weaken Argument) and the gap in the reasoning: The conclusion is about profitability, but the evidence is about order volume. A weakener shows that the extra orders may not have produced extra profit.
Step 2: (A) Correct. Each return loses more than a typical sale earns, and returns have tripled. The added orders may therefore bring losses rather than profit.
Step 3: (B) If anything, this supports the idea that the policy drew customers.
Step 4: (C) Likely repeat business slightly supports the conclusion.
Step 5: (D) The advertising campaign is neutral. It does not show that profits did not rise.
Step 6: (E) The size of the clothing category does not show whether it became more profitable.
Final Answer
Answer: A
Common trap: D is tempting as an "alternative cause" of the rise in orders, but the conclusion is about profit, and D gives no reason to think profit did not rise.
Critical ReasoningStrengthen ArgumentMedium
Question 4
The Lindell Transit Authority installed screens showing real-time bus arrival times at every stop on 15 of its 60 bus routes. Over the following year, ridership on those 15 routes rose by 12 percent, compared with 3 percent on the other routes. The authority concludes that the screens attracted new riders and plans to install them at every stop in the system.
Which of the following, if true, most strengthens the authority's conclusion?
ARiders surveyed on routes with screens said they found the displayed information easy to read.
BFor several years before the screens were installed, ridership on the 15 routes had been growing faster than ridership on the other routes.
CInstalling screens throughout the system would cost less per stop than the initial installations did.
DThe 15 routes were chosen by random drawing, and during the year the authority changed neither service frequency nor fares on any of its routes.
ESome long-time riders on routes with screens reported that they now spend less time waiting at stops.
Show answer & explanation
Correct answer: D
Key Concept or Approach
The conclusion is causal: the screens caused the extra ridership. It is strengthened by ruling out other explanations for why the 15 routes grew faster.
Step-by-Step Solution
Step 1: Identify the task (Strengthen Argument) and the gap in the reasoning: The conclusion is causal: the screens caused the extra ridership. It is strengthened by ruling out other explanations for why the 15 routes grew faster.
Step 2: (D) Correct. Random selection rules out picking routes that were already growing, and unchanged frequency and fares rule out the obvious rival causes. That leaves the screens as the best explanation.
Step 3: (A) Screens that are easy to read are not evidence that they drew in new riders.
Step 4: (B) This points to an existing trend that could explain the gap, so it weakens the conclusion.
Step 5: (C) Installation cost is irrelevant to whether the screens caused the rise in ridership.
Step 6: (E) This concerns existing riders' convenience, not new riders.
Final Answer
Answer: D
Common trap: A and E describe benefits of the screens, but neither shows that the screens brought in new riders; D is the only choice that eliminates alternative causes.
