Classification (Odd One Out)

high importance~3 Q in Tier 19 formulas⚡ 9 shortcuts6 subtopics

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The idea of odd one out

Classification gives four/five items that look similar and asks which one does not belong. Every question hides a rule that three options obey and one breaks. Your whole job is to find that rule fast.

The rule can sit at any of these levels:

Category (sparrow, crow, parrot, bat — bat is a mammal, the rest are birds)
Function/property (keyboard, mouse, scanner, monitor — monitor is output)
Number property (17, 23, 29, 33 — 33 is not prime)
Letter structure (ACE, BDF, GIK, MOP — MOP does not keep a constant +2 step)
Relation in a pair (three creator-creation pairs, one creator-material pair)

Strategy order for CGL: test number/letter properties first (they are objective), then function, then category. If two options look odd, you are at the wrong level — go one level deeper (all squares? then odd vs even squares).

📖 Notes 🧩 4 question types 🎯 12 practice Q

Detailed notes

What is classification?

Classification means putting things into groups. In the exam it appears as "odd one out": you get four (sometimes five) items. Three of them share one common feature. One item does not. You must pick that one item.

Everyday example: apple, mango, banana, potato. Apple, mango and banana are fruits. Potato is a vegetable. So potato is the odd one out. The exam works exactly like this, only the hidden feature is a little harder to see.

The golden rule: find what the THREE share

Do not hunt for "something strange" in one option. Every option can look strange in some way. Instead, find a rule that exactly three options obey. The fourth option, which breaks that rule, is the answer. Say the rule in one short sentence: "these three are fruits", "these three are prime numbers", "in these three the letters jump by +2".

Levels where the rule can hide

Category – what kind of thing it is (bird, metal, state, planet).
Function or use – what it does (input device, writing tool, measuring instrument).
Property – a quality it has (liquid or solid, living or non-living, natural or man-made).
Number property – prime, square, cube, multiple of 7, digit sum.
Letter structure – steps between letters, vowels, opposite letters.
Word structure – the spelling itself: letters in alphabetical order, double letters, a word that reads the same backwards (palindrome).

Which level to test first

Test objective rules first, because they can be checked with certainty: number properties, letter steps and spelling. Then test function, then category. If a rule gives you two odd options, the rule is wrong or too shallow – go one level deeper. Example: 121, 169, 225, 289 are all perfect squares (level 1 gives no answer). Go deeper: 11, 13, 17 are prime but 15 is not, so 225 is the odd one.

The four question formats

Find the odd one out – the standard form: "Three of the following four are alike... find the one that is different."
Find the one that belongs – three items are given as a group (e.g. Mercury, Venus, Mars) and you choose the option that fits the same group (Jupiter). This is odd-one-out turned upside down: now three options break the rule and one obeys it.
Two-level (tie-break) questions – all four share a first property; three share a second one.
Meaning-free word questions – the words have no common meaning; the rule sits in the spelling. Example: BEFIT, GHOST, ALMOST, DREAM – in the first three the letters are already in A-to-Z order; in DREAM they are not.

Worked example

Find the odd one: 15, 21, 35, 43.

Parity: all odd – no help.
Prime check: 15 = 3 × 5, 21 = 3 × 7, 35 = 5 × 7, but 43 is prime.
Rule: three are products of two odd primes; 43 is itself prime. Answer 43.

Exam habits that save marks

Write the rule for the three before you mark the answer.
Check the rule on all four options, not just the one you suspect.
Use only school-level facts. The exam never needs rare knowledge; if you need a rare fact, you are on the wrong rule.
In "which belongs to the group" questions, remember the answer is the one that fits, not the one that is different.

Quick revision

Step	What to do
1	Read all four items once, fast
2	Try objective rules: numbers, letter steps, spelling
3	Then function, property, category
4	Rule must fit exactly three items
5	Two odd items? Go one level deeper
6	"Belongs to the group" = pick the one that FITS

Types of questions asked

Every way this subtopic shows up in exams — how to recognise it, the formula or logic to use, and a solved example.

Type 1: Standard odd one out (three alike, one different)very common3 practice Q

How to spot it:

The question says 'three of the following four are alike in a certain way'. The items can be words, numbers or letter groups.

Read all four options.
Try objective rules first (number or letter properties), then use, property and category.
Write the rule the three share in one sentence.
Mark the item that breaks it.

Why it works: the setter builds the question around one rule that fits exactly three items.

Example: Find the odd one out: Square, Rectangle, Rhombus, Cube.

Square, rectangle and rhombus are flat (2-D) shapes. A cube is a solid (3-D) shape. Answer: Cube.

Type 2: Select the item that belongs to the same groupcommon3 practice Q

How to spot it:

Three items are given together (for example 'Mercury, Venus, Mars') and you must choose the option that fits with them.

Name the group the three given items form.
Test every option against that group.
Exactly one option fits; the other three are outsiders.

Why it works: it is odd-one-out turned upside down; the rule of the given three decides.

Example: Mercury, Venus, Mars belong to a group. Which of these also belongs? Moon, Sun, Jupiter, Sirius.

The three are planets of our solar system. Moon is a satellite, Sun and Sirius are stars. Answer: Jupiter.

Type 3: Two-level (tie-break) odd one outcommon3 practice Q

How to spot it:

The first rule you try fits all four items (all squares, all animals, all consecutive letters).

Note the property all four share - it cannot decide the answer.
Look one level deeper inside that property (square of what? which kind of animal?).
The item that breaks the deeper rule is the answer.

Why it works: setters make the obvious rule common to all four to trap hasty students.

Example: Find the odd one: 121, 169, 225, 289.

All are squares: $11^2, 13^2, 15^2, 17^2$ . Level two: 11, 13 and 17 are prime, 15 is not. Answer: 225.

Type 4: Word-structure (spelling) odd one outoccasional3 practice Q

How to spot it:

The words have no common meaning. The rule is in the spelling: letters in alphabetical order, double letters, palindrome, first and last letter.

Check if the letters of each word are in A-to-Z order.
Check for double letters and palindromes (same forwards and backwards).
Check first/last letters and vowel count.
The word that fails the spelling rule is the answer.

Why it works: when meaning gives no single odd item, the setter has used the letters themselves.

Example: Find the odd one: LEVEL, RADAR, CIVIC, MOTOR.

LEVEL, RADAR and CIVIC read the same backwards (palindromes). MOTOR backwards is ROTOM. Answer: MOTOR.

Shortcut tricks

⚡ Write the majority rule, not the exception

Instead of hunting for the odd item, write the rule the other three follow ('three are primes'). Then confirm each of the three obeys it and the remaining one breaks it. This prevents choosing an option that is merely unusual rather than breaking the rule.

Example: Q. 21, 35, 49, 63, 56

Sol. Majority rule: multiples of 7 (21, 35, 49, 63 ✓). 56 is also a multiple of 7 — so this rule makes nobody odd; go deeper: 21, 35, 49, 63 are all odd; 56 is even → 56 is the odd one.

If your first rule makes two items odd, the real rule is one level deeper.

Where students lose marks

Stopping at the first difference found instead of testing it on ALL options — the difference must separate exactly 1 from the rest.
Assuming the odd item must be the 4th/5th option; CGL shuffles positions freely.
Using loose categories ('all are animals') where a tighter one exists ('all are birds') — always prefer the tightest rule that still groups 3+ options.

Practice sets — 12 questions

Sets of 10, mixed across the question types above. Each answer comes with a step-by-step explanation.

Topic test · 12 questions

Suggested time 7 min · wrong answers go to your mistake notebook automatically.