IMAT Logical Reasoning · Critical Thinking

Strengthening and Weakening an Argument

Take each option as true and ask whether it makes the step from reasons to conclusion more or less believable.

Why this matters

Weaken questions were asked 12 times (2011 to 2019) and strengthen questions 10 times (2012 to 2023), including one of the two ministry questions in 2023. Both use the same stem, 'which one of the following, if true', and the same method run in opposite directions.

Concept 1 of 2: Weakening an argument: attack the link, not the facts

The words 'if true' tell you to accept every option as a fact. So you cannot weaken an argument by doubting its premises. You weaken it by showing that the premises, even if true, do not lead to the conclusion: there is another explanation, the comparison is unfair, or the plan will not achieve its aim.

Definition

Question form: 'Which one of the following, if true, would most weaken the argument?'

Treat every option as true. Ask: does it make the conclusion less likely to follow from the premises?
Alternative explanation: something else could have produced the evidence.
Unfair comparison: the case in the evidence is not like the case in the conclusion.
Plan fails: an obstacle or side effect stops the proposal achieving its own aim.
Assumption false: the option denies a gap the argument needs to close.
Ignore options that are irrelevant (on the topic but not on the claim) and options that in fact strengthen.

Worked example

'Three months ago a café replaced its menu with cheaper, healthier dishes. Since then its takings have risen by 15%. The owner concludes that customers prefer the new menu, and she plans to make the same change at her second café.' Which weakens the owner's conclusion most: (a) the new dishes take a little longer to prepare; (b) a large office opened next door three months ago, bringing many new lunchtime customers; (c) several regular customers said they liked the new dishes?

Practice this conceptself-check · 3 quick reps

Cost is rarely the weakener

If the conclusion is that a plan would work, an option saying the plan is expensive does not weaken it: an expensive plan can still work. Only options that bear on the conclusion itself count. Check what exactly the conclusion claims before judging an option.

Concept 2 of 2: Strengthening an argument: close the gap

Strengthening is weakening in reverse. Find the gap between the evidence and the conclusion, then pick the option that closes it. The strongest option usually rules out the alternative explanation you would have used to weaken the argument.

Definition

Question form: 'Which one of the following, if true, would most strengthen the argument?'

Rule out an alternative: show that nothing else could explain the evidence.
Show the comparison is fair: the groups compared were alike in every other way.
Supply the missing link: state the assumption the argument depends on.
Add direct evidence for the conclusion, not just for the topic.
Wrong options: one that repeats a premise (adds nothing), one about the topic but not the claim, one that supports a different conclusion.

Worked example

'On hot afternoons, the top floors of buildings with plant-covered roofs were about 3 °C cooler than the top floors of buildings with ordinary roofs. The city council argues that requiring green roofs on all new buildings would reduce the energy used for air conditioning.' Which strengthens most: (a) green roofs provide a home for insects and birds; (b) the buildings compared were similar in age, size and insulation; (c) green roofs can be fitted to most flat roofs?

Practice this conceptself-check · 3 quick reps

On the topic is not the same as relevant

An option can be about the same subject as the passage and still say nothing about whether the premises lead to the conclusion. Ask of each option: does it make this exact conclusion more likely? A second benefit of the plan, or a fact about how serious the problem is, does not.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Watch out for (2)

Cost is rarely the weakener
→ Weakening an argument: attack the link, not the facts
On the topic is not the same as relevant
→ Strengthening an argument: close the gap

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