Deductive reasoning

What it measures

Whether you can take a set of rules as given and work out what must follow from them, and only that. The rules are all you have; the skill is building every arrangement they allow and testing each option against all of them without adding a rule of your own.

How it appears

• Ordering and arrangementFour to six people or things, three or four rules about their order or positions, and a question asking which statement must be true, could be true, or cannot be true.
• Rules and conclusionsStatements such as all, some or none of one group being in another, and a conclusion to accept or reject. The same discipline applies: only what the statements force.
• InteractiveYou drag the items into an order or a grid that satisfies every rule.

Timing

18 questions in 20 minutes.

assessmentday.com gives 18 questions in 10 minutes for the same test, and the interactive version is usually 12 questions in 18 minutes. Treat 35 to 90 seconds a question as the range; the drills use the 20-minute figure.

How to tell it apart

• Inductive reasoningInductive questions give you examples and ask you to find the rule. Deductive questions give you the rules and ask what follows.
• Verbal reasoningVerbal reasoning judges a statement against a passage of prose. Deductive reasoning judges it against a short list of rules, and the rules are meant to be combined.

The method

Symbols first, arrangements second, options last.

1. Write each rule as a symbol before you think about the answer. Ana before Ben is A < B. Cara immediately after Dev is C = D + 1. Not next to is a gap of at least two.
2. Start from the most restrictive rule, a fixed position or a pair that must be adjacent, and place it first.
3. Build every arrangement that satisfies all the rules. There are usually one to four. Write them down.
4. Test each option against every arrangement. Must be true means true in all of them. Could be true means true in at least one. Cannot be true means false in all.
5. Do not add a rule the question did not give. Before does not mean immediately before, and above does not mean directly above.
6. For all, some and none statements, draw circles. Some means at least one and possibly all; all A are B says nothing about B that are not A.
7. At a minute a question, when two arrangements fit, test the options against both rather than trying to eliminate one.

Traps to know

• Before means immediately before

  The rule says Ana finished before Ben. The reader places them side by side, and every arrangement with a gap between them goes missing, along with the answer.

  Setup Ana finished before Ben; Cara finished immediately after Dev; there are five runners.

  The trap Placing Ana directly in front of Ben, which hides the arrangement where Dev and Cara come between them.

  The escape Write A < B, not B = A + 1. Only the rule about Cara and Dev fixes an adjacency.

• Reversing the rule

  All runners are fit does not mean all fit people are runners. Turning a rule round gives a new rule nobody stated, and an option that relies on it looks forced when it is not.

  Setup Every member of the team has passed the safety course. Priya has passed the safety course.

  The trap Concluding that Priya is a member of the team.

  The escape The rule runs from team to course, not back. Priya may or may not be on the team; nothing says.

• Could be true read as must be true

  An option that holds in one valid arrangement feels like the answer when it is the first arrangement you built. The question asked for what holds in every arrangement.

  Setup Two arrangements fit the rules. Tom is on floor 5 in one of them and floor 4 in the other.

  The trap Choosing Tom lives on floor 5 because the first arrangement you found has it.

  The escape Build all the arrangements before testing an option. If it fails in one, it is not must be true.

• Some means not all

  In these tests some means at least one. It does not mean only some, so it does not rule out all. An option built on "so not all of them" is not forced.

  Setup Some of the delivered parcels were damaged.

  The trap Concluding that some parcels were undamaged.

  The escape Some damaged is consistent with every parcel damaged. Nothing about undamaged parcels follows.

Worked examples