Decision Making: Logic, Sets and Arguments
The question types
Decision Making mixes formal logic with everyday reasoning. Expect syllogisms (which conclusion must follow), logic puzzles with ordering or grouping constraints, Venn diagram and set questions, probability and combinatorics, and argument items: identify the assumption, the strongest argument, or the conclusion. Some items give five options and ask you to select two.
Concepts
- Syllogisms: 'All A are B' does not mean 'All B are A'. 'Some A are B' and 'No A are B' are reversible.
- Necessity vs sufficiency: an assumption is what the argument needs, not what would merely strengthen it.
- Probability: with or without replacement; independent events multiply.
- Sets: draw overlapping circles and place 'some' items in the overlap.
Traps
The classic error is converting a universal statement and drawing an invalid conclusion ('all doctors are graduates, so all graduates are doctors'). In argument questions choose the option that is relevant and decisive, not the one you personally agree with, and beware answers that are true but irrelevant. In puzzles, re-read the constraints after each deduction rather than assuming unstated rules.
Worked example
All nurses on the ward are trained in CPR. Some trained staff are agency workers. Must it be true that some agency workers are nurses? No - the agency workers could be other trained staff. Draw a counterexample: agency workers overlapping only the trained-staff group outside the nurses.
Rule of thumb: if you can draw one diagram in which the premises hold and the conclusion fails, the conclusion does not follow.