Strengthening, Weakening and Evaluating Claims
These items ask how new information would affect an argument, or what evidence would decide between competing explanations. The skill is locating the argument's weak joint and testing it, not judging whether the conclusion is attractive.
Attack the link, not the premise
The fastest route is usually to show that the premises can be true while the conclusion fails.
- Alternative cause: something else produced the observed effect.
- Reverse causation: the effect produced the supposed cause.
- Selection effect: the cases compared were never comparable.
- Measurement artefact: the indicator changed for reasons unrelated to the phenomenon.
- Scope gap: evidence about one group, period or region is applied to another.
How strengtheners and weakeners differ
- A strengthener supplies the missing assumption, rules out a rival explanation, or shows the mechanism operates as claimed.
- A weakener introduces a rival explanation, a counterexample, or shows the evidence is equally compatible with the negation of the conclusion.
- An option that merely restates a premise adds no support.
- Attacking a premise outright can weaken, but these items usually reward attacking the inference.
Evaluating
'Which would be most useful to know?' asks for the fact that discriminates: its answer should differ depending on whether the conclusion is true. Test each option by asking whether both possible answers remain compatible with the conclusion; if yes, it is not decisive.
Worked example
Argument: 'Firms receiving the advisory service reported 18% higher productivity two years later than firms that did not. The service therefore raises productivity.'
Which finding most weakens it? (A) The service is expensive to run. (B) Firms that opted in were already growing faster before they applied. (C) Some firms found the advice too general. (D) Productivity is difficult to measure.
Answer B: a selection effect showing the comparison group was never comparable. A concerns cost, not the causal claim; C is anecdotal; D questions measurement generally but does not discriminate between the two groups.
Traps
- Choosing a fact that is true and relevant but does not touch the inference.
- Confusing 'weakens' with 'proves false'.
- Ignoring sample size and base rates in numerical options.
- Assuming stronger wording makes an option stronger.
Quick routine
Name the conclusion, name the evidence, then ask what single missing link the option is probing.