9.2 Odd-One-Out

Find the rule that isolates one item

Odd-one-out (also called find the unique figure) shows a small set of figures, commonly five, and asks which one does not belong. The figures are still Core Abilities analytical aptitude, not Mechanical Reasoning. You are not asked which hose would leak. You are asked which picture fails a visual rule that the others obey.

The method is stricter than pick the one that looks weird. Many sets contain several differences. The correct isolating rule is the one that leaves exactly one figure outside a group of the rest. If your candidate rule puts two figures on one side and three on the other, you have described a 3–2 split, not an odd-one-out. Keep searching.

QFD does not publish how many odd-one-out items appear on Core Abilities, whether they use four, five, or six figures, or how they are scored. This section teaches the isolation skill the pack names as analytical aptitude. Independent OpenExamPrep examples use five labelled figures because that is a convenient teaching set, not because QFD has announced a five-figure format.

OnVUE still forbids scrap paper. You cannot tick features on a printed grid. Hold a one-line rule: four have an odd number of sides; the square does not.

Work coarse to fine

Start with features that are cheap to count, then move to features that need a mental rotation.

Worked example — side-count isolate. Five polygons sit in a row:

• Figure P: outline triangle (3 sides).
• Figure Q: outline pentagon (5 sides).
• Figure R: outline heptagon (7 sides).
• Figure S: outline square (4 sides).
• Figure T: outline nonagon (9 sides).

A first glance says they are all different shapes, which is useless: that rule isolates everyone. Count sides. P, Q, R, and T have an odd number of sides. S has an even number. The isolating rule is even side count, and the isolate is the square. A competing story such as the heptagon looks more complicated isolates on taste, not on a feature that groups four together.

Worked example — fill isolate that is not the loudest difference. Five circles: four are empty outlines with a small tick at 12 o'clock; one is a filled circle with the same 12 o'clock tick. The filled circle is the isolate by fill. The ticks are an invariant. Candidates who stare at the tick and invent a rotation story will waste time, because the tick never changes.

Worked example — rotation versus reflection. Five L-shapes made of three unit squares:

• Figure U: L with the foot pointing right, standing on its long stem (call this the home L).
• Figure V: the home L rotated 90° clockwise.
• Figure W: the home L rotated 180°.
• Figure X: the home L rotated 270° clockwise.
• Figure Y: the mirror of the home L (the foot now points left if the stem is in the same standing pose).

U, V, W, and X are the same handedness. You can turn the paper (mentally) and land on each of them. Y cannot be reached by rotation; you would need to flip the figure out of the plane. The isolating rule is handedness / reflection. The isolate is Y. This is the same distinction Section 9.4 uses for 2D spatial items. On odd-one-out, it is the rule that people miss when they say they are all L-shapes, which is true and unhelpful.

Worked example — stacked shared features. Four figures share two properties at once: a filled small triangle in the top-left of a square frame, and exactly two internal dots. The fifth figure still has two internal dots, but its small triangle is empty. The isolate is that fifth figure. The isolating rule is breaks the filled-triangle half of the stack. A candidate who only counts dots finds no isolate. A candidate who only looks at triangle fill finds the isolate immediately — which is why you should test fill before you invent a more exotic story.

Isolation strategies

Use a short menu. Stop when a rule isolates exactly one figure.

Odd-one-out sets are unordered. Unlike Section 9.1, there is no next. Do not invent a sequence across the five figures just because they are printed left to right. The leftmost figure is not Frame 1 unless the rubric says so.

A method that survives time pressure

1. Name one coarse feature (side count, fill, family) and sort the five figures into buckets.
2. Check the bucket sizes. A 4–1 split is a candidate rule. A 3–2 split is not. A 5–0 split means that feature is an invariant; ignore it.
3. If every coarse feature is 5–0 or 3–2, test rotation versus reflection: can four figures be turned onto one another?
4. If four figures share two features, name both, then find the figure that breaks one of them.
5. Say the rule in a sentence before you select: four are odd-sided; the square is even-sided.
6. Reject vibe. The spikiest, darkest, or most nested figure is not automatically the isolate.

Traps on odd-one-out

• Stopping at a 3–2 split because it felt meaningful.
• Treating a mirror L as just another rotation of the L.
• Inventing a left-to-right sequence that the set does not have.
• Counting a hole and an outer outline as two shapes when the other figures were counted as one.
• Importing Mechanical Reasoning (this cog would mesh with those four).
• Changing the rule after seeing an attractive option that your first 4–1 rule did not pick.

If two different 4–1 rules seem to pick different isolates, one of the rules is sloppy. Recount. Genuine two-rule items still pick the same isolate; the second rule is a restatement (four odd-sided polygons is the same grouping as four figures that cannot tile a rectangle the way a square can, in that particular set). If they disagree, you miscounted a side or treated hatch as fill.

The pack's Core Abilities description is about reasoning ability and capacity to learn new information. Odd-one-out is that skill in miniature: you learn the set's rule from the set itself, then apply it once. You do not bring a memorised QFD figure bank, and this guide will not pretend QFD has published one.

In that labelled set the isolating sentence is short: P, Q, R, and T have odd side counts; S does not. The square is the isolate because even side count carves 4–1. A story that the nonagon is the odd one because it looks busiest carves 1–4 on taste, which is the wrong direction of attention: you want four together, not the loudest singleton.