In this guide
Classification questions give you four items and ask which one does not belong. Three items share a property; the fourth does not. It sounds easy, and most of the time it is. The marks are lost in the other cases, where two different properties each pick out a different item and you choose the weaker one.
So the skill is not just spotting a difference. Almost any item can be made to look different. The skill is finding the property that separates exactly one item and then checking that no other property separates a different one.
How it is asked in SSC CHSL
Classification appears in the reasoning section of both tiers, usually alongside analogy questions. It comes in four forms:
- Words: "Find the odd word: Tabla, Sitar, Veena, Sarod."
- Numbers: "Find the odd number: 8, 27, 64, 81."
- Number pairs: "(3, 10), (5, 26), (7, 48), (11, 122)."
- Letter groups: "CFI, DGJ, KNQ, PSU."
Some questions word it as "Three of the following four are alike in a certain way and one is different." The method is the same.
The method, step by step
- Name the type. Words, numbers, pairs or letters. Each has its own short list of properties to check.
- Find a rule that fits three. Say it in a sentence: "Three are string instruments."
- Check it against all four. Exactly one item must fail the rule.
- Test the rivals. Run through the common properties for that type and see whether any of them singles out a different item.
- Decide. If only your rule survives, answer. If two rules survive, prefer the more specific one: "not a cube" beats "the only odd number", because cubes are rare and odd numbers are common.
Why step 4 matters: setters build options so that the obvious difference and the intended rule coincide. When you have found a loose difference, a rival check usually shows you a tighter rule that points to the same item. When it points somewhere else, you have found the trap before it found you.
Word classification
| Grouping | Example | Odd one |
|---|---|---|
| Category | Parrot, Sparrow, Bat, Pigeon | Bat (a mammal) |
| Kind of thing | Kilogram, Metre, Litre, Thermometer | Thermometer (an instrument, not a unit) |
| Function | Knife, Scissors, Blade, Spoon | Spoon (does not cut) |
| Place or source | Ganga, Yamuna, Godavari, Dal | Dal (a lake) |
| Worker and field | Surgeon, Dentist, Cardiologist, Lawyer | Lawyer (not a medical profession) |
For words, check the category first, then the function, then the property (liquid or solid, living or non-living). Ignore spelling tricks such as first letters or word length unless every option is a meaningless word.
Number classification
Check these properties in turn for each number:
| Property | Quick test |
|---|---|
| Prime | Not divisible by 2, 3, 5, 7 (enough for numbers under 121) |
| Perfect square | Know squares up to 30² |
| Perfect cube | Know cubes up to 15³ |
| Even or odd | Last digit |
| Multiple of 3 or 9 | Digit sum divisible by 3 or 9 |
| Multiple of 5 or 7 | Last digit 0 or 5; for 7, divide |
A number that fails one property often shares another with the rest. In 8, 27, 64, 81, the number 81 is a square, but so is 64, so "square" does not separate one item. "Cube" does: 8, 27 and 64 are cubes and 81 is not.
Number pairs and letter groups
For pairs, find the rule linking the two numbers in each pair: second = first², first² ± 1, first³, first × (first + 1), or a fixed multiple. The odd pair breaks the rule.
For letter groups, write the positions (A1 … Z26) and check:
- Gaps: CFI is 3, 6, 9, with gaps of 3.
- Direction: forward (ABC) or backward (ZYX).
- Opposite letters: pairs that add to 27, such as AZ and BY.
- Vowels: how many, and where.
Six worked questions
Q1. (a) Tabla (b) Sitar (c) Veena (d) Sarod
Sitar, veena and sarod are string instruments. The tabla is a percussion instrument. No other grouping separates one item. Answer: (a) Tabla.
Q2. (a) Kilogram (b) Thermometer (c) Litre (d) Metre
Kilogram, metre and litre are units of measurement. A thermometer is an instrument that measures. Answer: (b) Thermometer. The trap is thinking "all four are about measurement", which is true but separates nothing.
Q3. (a) 11 (b) 13 (c) 15 (d) 17
All four are odd, so parity separates nothing. 11, 13 and 17 are prime; 15 = 3 × 5 is not. Rival check: 15 is also the only multiple of 3 and of 5, which points to the same item. Answer: (c) 15.
Q4. (a) 8 (b) 27 (c) 64 (d) 81
8 = 2³, 27 = 3³ and 64 = 4³. 81 is not a cube. Rival check: squares are 64 and 81, so "square" splits them two and two; odd numbers are 27 and 81, again two and two. Only "cube" leaves one item out. Answer: (d) 81.
Q5. (a) (3, 10) (b) (5, 26) (c) (7, 48) (d) (11, 122)
Rule: second = first² + 1. 3² + 1 = 10, 5² + 1 = 26, 11² + 1 = 122, but 7² + 1 = 50, not 48. Rival check: every first number is an odd prime and every second number is even. Answer: (c) (7, 48).
Q6. (a) PSU (b) DGJ (c) KNQ (d) CFI
Positions: C3 F6 I9, D4 G7 J10 and K11 N14 Q17 all have gaps of 3. P16 S19 U21 has gaps of 3 and 2. Answer: (a) PSU.
Common traps
| Trap | Fix |
|---|---|
| Picking the first difference you see | Run the rival check on all four items |
| Using real-world trivia instead of the main category | Category first, then function, then property |
| Treating 1 or 9 as prime, or 2 as not prime | 1 is not prime; 2 is the only even prime |
| Miscounting letter positions | Write A1 … Z26 once and use EJOTY (5, 10, 15, 20, 25) |
| Reading a pair backwards | Apply the rule first-to-second in every pair |
Practice
- (a) Rose (b) Mango (c) Lily (d) Lotus
- (a) Wood (b) Iron (c) Silver (d) Copper
- (a) 8 (b) 27 (c) 125 (d) 150
- (a) 23 (b) 29 (c) 31 (d) 39
- (a) (3, 12) (b) (5, 30) (c) (7, 56) (d) (9, 80)
- (a) DFH (b) UWZ (c) PRT (d) KMO
- (a) Ganga (b) Dal (c) Godavari (d) Yamuna
- (a) 121 (b) 169 (c) 225 (d) 289
Answers:
- (b) Mango. Rose, lily and lotus are flowers; the mango is a fruit.
- (a) Wood. The other three are metals.
- (d) 150. 8, 27 and 125 are cubes (2³, 3³, 5³). Parity splits two and two (8 and 150 are even), so it is not a rival.
- (d) 39. 39 = 3 × 13; the others are prime.
- (d) (9, 80). The rule is n × (n + 1): 3 × 4 = 12, 5 × 6 = 30, 7 × 8 = 56, but 9 × 10 = 90.
- (b) UWZ. D4 F6 H8, K11 M13 O15 and P16 R18 T20 have gaps of 2. U21 W23 Z26 has gaps of 2 and 3.
- (b) Dal. The Ganga, Yamuna and Godavari are rivers; Dal is a lake in Srinagar.
- (c) 225. All four are squares: 11², 13², 15², 17². The roots 11, 13 and 17 are prime; 15 is not. 225 is also the only multiple of 5, which agrees.
What to do next
- Learn squares to 30 and cubes to 15 so number sets take seconds.
- For your next 30 classification questions, write the rule and one rival check for each.
- Keep a list of word categories you get wrong (instruments, units, professions) and revise it weekly.
- Classification uses the same relationships as analogies, so practise the two together, then move on to number series.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Staff Selection Commission website .
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