SSC Notes / Reasoning / Chapter 2
Chapter 2 of 18

Classification (Odd One Out)

Grouping items by a shared property and identifying the one that breaks the pattern — word, number, and letter classification.

📖 ~11 min read 🧩 SSC Reasoning

Introduction

Classification asks you to find the shared category among most items in a group, so you can spot the one item that doesn't belong — a quick-scoring topic once you know the standard grouping categories to check.

Diagram — Spotting the Odd One Out
A B C D Odd one out A, B, C share property X

Three items share a common rule; the fourth deviates — that deviation is your answer.

Common Classification Categories to Check

CategoryCheck For
Meaning-basedCategory membership — e.g., three are fruits, one is a vegetable
Odd/even numbersThree odd, one even (or vice versa)
Prime vs compositeThree primes, one composite
Perfect squares/cubesThree perfect squares, one not
Vowel/consonant countLetters grouped by vowel-consonant pattern in a word
Alphabet position patternLetter-gap consistency (e.g., all +2 gaps except one)
Q. Find the odd one out: Mango, Apple, Potato, Banana
Mango, Apple, Banana are fruits; Potato is a vegetable → Potato is the odd one out.
Q. Find the odd one out: 8, 27, 64, 100
8=2³, 27=3³, 64=4³ (perfect cubes); 100 is a perfect square, not a cube → 100 is the odd one out.
Q. Find the odd one out: BD, FH, KM, PR
Check letter gaps: B(+2)D, F(+2)H, P(+2)R — all skip one letter, but K(+2)M also skips one (K,L,M)... check position sums instead:
BD=2+4=6, FH=6+8=14, KM=11+13=24, PR=16+18=34 — differences between consecutive pairs: 8,10,10 → BD-FH breaks the +10 pattern seen in FH→KM→PR
So BD is the odd one out.

Classification Involving Roman Numerals / Mixed Groups

Some questions mix categories deliberately (e.g., three are numbers written as words, one is a numeral) — always check the format of each item, not just its value, before assuming the category.

💡 Exam Tip: Test the "obvious" category first (meaning/type), but if all four seem similar in meaning, immediately check numerical properties (odd/even, prime, square) or letter-position patterns — SSC often hides the real rule one layer deeper.
✅ Practice Focus: Meaning-based grouping · Number property grouping (odd/even, prime, square/cube) · Letter-position gap consistency · Format-based classification (numeral vs word).

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