Unit 8
Detecting Reasoning Errors
About this unit
The most question-rich unit in the course. You will learn to spot a wide range of logical flaws: misapplying conditional rules, drawing conclusions beyond the evidence, confusing correlation with causation, and making statistical errors. Questions often present two characters debating and ask who reasons correctly.
What types of questions will you face?
- 1Two characters interpret a conditional rule (if X then Y) — identify who applies it correctly and who commits a fallacy
- 2A character draws a conclusion from a fact or observation — identify why the conclusion is flawed
- 3A statistical claim is made — identify the error (small sample, unrepresentative data, ignoring confounding factors)
- 4A "necessary vs sufficient" condition is stated — find who correctly understands the implication
- 5Multi-condition rules are given — determine who correctly or incorrectly applies them to a specific case
Skills you will build
- Correctly applying "if P then Q" rules (and knowing what can/cannot be concluded)
- Distinguishing necessary conditions (required) from sufficient conditions (enough on their own)
- Identifying correlation vs causation errors
- Recognising hasty generalisation from a small or biased sample
- Spotting "affirming the consequent" and "denying the antecedent" fallacies
- Evaluating multi-condition rules (only if BOTH conditions are met) carefully
By the end of this unit, you will be able to
- Apply conditional logic (if→then) correctly in all its forms
- Immediately recognise the 6 most common reasoning errors tested in Selective TS
- Evaluate whether a conclusion logically follows from given premises
- Score consistently on the largest and most diverse question category in the exam
Difficulty profile
Medium difficulty (avg 3.09) with a wide range. Easy questions use simple 2-person if/then disputes. The hardest involve multi-step conditional chains and complex multi-condition rules. With 122 questions, this is the biggest unit — and one of the most important.
Exam tip: Detecting Reasoning Errors
For if-then questions: the ONLY safe conclusion from "if P then Q" is (1) P is true → Q is true, and (2) Q is false → P is false. Everything else is a fallacy. Never conclude "Q is true → P must be true."
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