How to Solve "Weaken the Argument" Questions in the Selective Test — Selective online tests: preparation tips

If there is one skill that separates a good Thinking Skills student from a great one, it is the ability to spot a logical flaw. In the NSW Selective High School Placement Test, you will frequently encounter questions that ask: "Which of the following, if true, most weakens the argument above?" Many students struggle with this because they naturally want to agree with the author, especially if the topic sounds reasonable. However, to ace this section in 2026, students must learn to read like a detective looking for holes in a suspect's story.

Here is the strategic framework for dismantling an argument, followed by three practice examples to test your child's critical thinking.

The 3-step strategy for "weakening" questions

To weaken an argument, you must break the chain of logic. Every argument has three links:

The evidence — The facts the author provides.

The conclusion — The point the author is trying to prove.

The assumption — The unwritten leap of logic connecting the evidence to the conclusion.

The golden rule — You almost never attack the evidence. The test-makers want you to accept the facts as true. Instead, you must attack the assumption. The correct answer will introduce a new piece of information that makes the conclusion highly unlikely, even if the original evidence is 100% true.

Practice Question 1: The "alternate cause" flaw

The argument — Last term, Oakville Primary School introduced a new Morning Fitness program where students run for 15 minutes before their first class. At the end of the term, the school's average results in the state-wide Mathematics exam improved by 12%. The principal concluded that the physical exercise directly improved the students' ability to learn and concentrate in Maths.

Question — Which one of the following, if true, most weakens the principal's argument?

A) Most students reported that they felt very tired after the morning run and needed extra water.

B) In the same term, the school hired three new specialist Mathematics tutors to assist students who were falling behind.

C) Running for 15 minutes a day has been proven by doctors to improve cardiovascular health in children.

D) The school's results in the state-wide English exam only improved by 2% over the same period.

The answer and explanation

Correct answer: B

Why it weakens the argument — The principal assumes that the only thing that caused the Maths scores to go up was the running. Option B introduces a massive alternate cause. If specialist Maths tutors were hired at the exact same time, it is highly likely the tutors caused the 12% increase, completely undermining the principal's conclusion about the running.

Why the others are wrong

A — Suggests a minor negative, but it doesn't disprove the test score data.

C — Actually strengthens the idea that the program is good, though it focuses on physical health rather than mental focus.

D — A distractor; just because English didn't improve as much doesn't prove the running failed to help Maths.

Practice Question 2: The "biased sample" flaw

The argument — The local council wants to understand if the residents of the town support a proposal to build a new multi-storey car park in the town centre. To find out, council workers surveyed 500 people who were waiting at the main bus terminal on a Monday morning. Of those surveyed, 85% said they strongly opposed building the car park. The council concluded that the vast majority of the town's residents do not want the car park built.

Question — Which one of the following, if true, most weakens the council's argument?

A) The proposed car park will cost the council over two million dollars to build and maintain.

B) People who commute to work by bus are significantly less likely to own a car or drive into the town centre than the average resident.

C) The survey was conducted by friendly council workers who offered respondents a free coffee for participating.

D) A similar car park built in a neighbouring town is rarely full during the week.

The answer and explanation

Correct answer: B

Why it weakens the argument — The council assumes that the 500 people at the bus terminal perfectly represent the views of the entire town. Option B exposes this as a biased sample. If you only ask people who use public transport (and therefore don't need parking), of course they will oppose a car park! This destroys the conclusion that the whole town shares this view.

Why the others are wrong

A — Provides a reason why the car park might be a bad idea, which actually supports the respondents' negative view.

C — Just describes how the survey was run; offering coffee doesn't necessarily change a person's opinion on infrastructure.

D — Might suggest the car park isn't needed, but it doesn't weaken the logic of how the council interpreted their survey data.

Practice Question 3: The "unintended consequence" flaw

The argument — To encourage more native birds to visit suburban gardens, the local wildlife society has recommended that residents leave bowls of high-sugar nectar out on their balconies. The society argues that because native birds rely heavily on nectar for energy, providing this easy food source will quickly increase the local bird population and improve the ecosystem.

Question — Which one of the following, if true, most weakens the wildlife society's argument?

A) Many native birds also eat insects and small seeds as part of a balanced diet.

B) High-sugar nectar left in the open attracts large swarms of invasive wasps, which aggressively chase away native birds.

C) Making homemade nectar requires boiling water and mixing specific ratios of raw sugar.

D) Suburban gardens often lack the large, mature trees that native birds prefer to nest in.

The answer and explanation

Correct answer: B

Why it weakens the argument — The society assumes that putting out nectar will only have a positive effect (feeding the birds). Option B introduces a severe unintended consequence. If the nectar actually attracts aggressive pests that drive the birds away, the society's plan will achieve the exact opposite of its goal, completely destroying the argument.

Why the others are wrong

A — Just gives extra information about bird diets; it doesn't prove the nectar idea won't work.

C — Highlights that the plan takes a bit of effort, but doesn't prove the plan is logically flawed.

D — Points out a separate problem (lack of trees), but doesn't directly attack the claim that providing food will help.

The final takeaway for 2026

When practising "weaken" questions at home, teach your child to look for the invisible variables. If an argument claims that A caused B, the most powerful way to weaken it is to find a hidden C that actually caused it instead.

Want to explore our thinking skills mock tests with weaken stems that punish vague reading? GoTestPrep Thinking Skills drills surface those hidden variables in timed sets so your child attacks assumptions—not vibes—before May.