Classification — Study Notes

Overview

Classification is a pattern-recognition topic in the SSC CHSL Reasoning section where you identify the odd one out from a group of four or five items. These items may be words, numbers, letters or figures. The examiner tests your ability to spot a common property shared by all but one element. Typically, 3–4 questions appear in Tier 1 from this topic, often mixed with similar analogy and series questions.

Mastery requires recognising standard classification patterns quickly — numerical properties (odd/even, prime, perfect square), alphabetical positions, semantic categories (fruits, animals, capital cities) and visual features (symmetry, rotation, number of sides). The real challenge is not the difficulty of individual patterns but the variety of patterns tested. A methodical approach of checking each answer option against the others will save time under exam pressure. Classification questions reward both subject knowledge (geography, general awareness) and logical thinking, so they are highly scoring if practised systematically.

Strong performance in classification builds a foundation for related topics like analogy and series. Students who struggle here often fail to check all properties before answering — they pick the first difference they notice rather than the genuine odd one out. Disciplined practice of 50–60 diverse problems will cover the vast majority of SSC CHSL patterns.

Key Concepts

• **Odd One Out principle**: Three or four items share a common property; one does not. Your task is to identify the exception, not to find a difference between any two items.
• **Number classification**: Properties include odd/even, prime/composite, perfect square/cube, multiples of a number, divisibility rules, digit sum patterns and arithmetic sequences.
• **Letter classification**: Check vowel/consonant status, positional value in alphabet (A=1, Z=26), number of letters in a word, or alphabetical patterns like every alternate letter.
• **Word classification**: Semantic grouping — all are capitals except one, all are rivers except one, all are metals except one. Requires general awareness of categories like Indian states, currencies, scientists, inventions, sports terms.
• **Figure classification**: Visual properties — symmetry (horizontal, vertical, both or none), number of sides, open/closed shapes, shading patterns, rotation angles, number of elements inside a figure.
• **Two-level classification**: Sometimes the odd one is detected only after applying two checks — for example, all numbers are two-digit primes except one, or all words are five-letter nouns except one.
• **Negative wording**: Some questions ask "Which one is NOT the odd one out?" or "All except one are similar." Read the question stem carefully to avoid careless reversal of your answer.
• **Elimination strategy**: If a pattern is not obvious, eliminate options by comparing each item to every other. The item that differs in multiple ways is rarely the answer; the subtle outlier usually is.

Formulas / Key Facts

1. **Prime numbers below 100**: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97. 2. **Perfect squares up to 400**: 1, 4, 9, 16, 25, 36, 49, 64, 81, 100, 121, 144, 169, 196, 225, 256, 289, 324, 361, 400. 3. **Perfect cubes up to 1000**: 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000. 4. **Alphabetical positions**: A=1, B=2, …, M=13, N=14, …, Z=26. 5. **Vowels in English**: A, E, I, O, U (5 vowels; rest are consonants). 6. **Divisibility by 3**: Sum of digits divisible by 3. Divisibility by 9: sum divisible by 9. 7. **Common semantic groups**: Indian states/UTs, metals (iron, copper, gold, silver, aluminium), non-metals (carbon, sulphur, oxygen), planets, Indian rivers, capitals of countries. 8. **Figure symmetry types**: Horizontal (mirror across horizontal axis), vertical (mirror across vertical axis), both axes, rotational (looks same at 180° or other angles), none.

Worked Examples

**Example 1 — Number Classification** Find the odd one: 17, 23, 37, 42, 47 *Solution*: Check if each is prime. 17 is prime, 23 is prime, 37 is prime, 47 is prime, but 42 = 2 × 21 is composite. *Answer*: 42 (only composite number).

**Example 2 — Word Classification (Semantic)** Find the odd one: Delhi, Mumbai, Kolkata, Chennai, Hyderabad *Solution*: All are major metro cities in India. Check capitals: Delhi is the national capital, but Mumbai, Kolkata, Chennai and Hyderabad are state capitals or metros but not the *national* capital. That's a weak difference. Alternative check: All are state capitals or metros. Actually Delhi is a Union Territory (now National Capital Territory), Mumbai is in Maharashtra, Kolkata in West Bengal, Chennai in Tamil Nadu, Hyderabad in Telangana. All are metros. Closer inspection — often SSC tests whether one is not a state capital. But here all serve as major administrative centres. If forced, Delhi is a UT not a state capital. *Typical SSC pattern*: One will be unrelated — say if "Agra" appeared, it is not a state capital. *Answer depends on exact options*. Always verify the grouping principle.

**Example 3 — Letter Classification** Find the odd one: B, F, H, K, P *Solution*: Check positions in alphabet. B=2, F=6, H=8, K=11, P=16. Check differences: F−B=4, H−F=2, K−H=3, P−K=5 — no clear pattern. Check even/odd position: 2 even, 6 even, 8 even, 11 odd, 16 even. *Answer*: K (position 11, the only odd-position letter).

**Example 4 — Figure Classification** Four figures: three are triangles with a dot inside, one is a triangle with a dot outside. *Solution*: Common property = triangle with internal dot. The odd one has the dot placed outside. *Answer*: The figure with the external dot.

Common Mistakes

1. **Picking the first difference noticed without checking all items** → Always verify that exactly three items share a property and only one does not. Sometimes two properties overlap; choose the more fundamental one or the one that isolates exactly one item. 2. **Confusing "odd one out" with "different from one other"** → The odd item must differ from *all* the rest on the same criterion, not just from one neighbour. If two items differ from the other three, re-examine your property. 3. **Ignoring general knowledge in word classification** → If the question says "Copper, Iron, Mercury, Aluminium," many students miss that Mercury is a liquid metal or non-metal, or that it's toxic, depending on context. Brush up on basic categories in science, geography and polity. 4. **Misreading negative questions** → "All except one are related" versus "Which one is NOT the odd one out?" These reverse your answer. Underline or highlight the question type before solving. 5. **Overlooking two-step properties in numbers** → For example, "All are two-digit primes except one" requires both checks (two-digit AND prime). A single-step check might wrongly classify 2 as the odd one (single digit prime) when the real odd one is a two-digit composite.

Quick Reference

• **Process**: Identify the common property of three items; the fourth that lacks it is the odd one.
• **Number checks**: Prime/composite, odd/even, perfect square/cube, divisibility, digit sum.
• **Letter checks**: Vowel/consonant, alphabetical position (odd/even), word length.
• **Word checks**: Semantic category (capitals, rivers, metals, scientists); requires general awareness.
• **Figure checks**: Symmetry type, number of sides, open/closed, shading, element count.
• **Verify your answer**: Confirm that exactly one item is different on the chosen property before marking.