Why it matters
Look at these four words: Cricket, Hockey, Football, Chess. Three of them are played with a ball. One is not. You found the odd one in two seconds, and that is exactly the skill this topic trains.
Classification means putting things into groups by a shared property. In an odd one out question you get four items (words, numbers, letter groups or pairs). Three of them follow the same rule. One breaks it. Your job is to find the rule and pick the item that breaks it.
In the AP Police Constable written test, classification appears in the Reasoning / Mental Ability part of the paper. These questions are short, need no formula sheet, and can be answered in well under the average time of about 54 seconds per question. Because there is no negative marking, a well-trained student should never leave one blank. Fast, sure marks here give you extra time for long arithmetic questions.
Core concept
The basic method (works for every type)
- Read all four options first. Do not decide after seeing two.
- Guess a rule that three of them share.
- Test the rule on all four. A good rule picks out exactly one item.
- If your rule picks out two items or none, it is the wrong rule. Look for a stronger one.
Why step 4 matters: the paper-setter designs the question around one clear property. If your rule leaves two "odd" items, you have not found the setter's property yet.
Beginner level: word classification
Ask yourself "What kind of thing is each word?" Common bases are listed below.
| Basis | Example group | Odd one | Reason |
|---|---|---|---|
| Category | Kilometre, Metre, Kilogram, Centimetre | Kilogram | unit of mass; others are units of length |
| Function / use | Pen, Pencil, Chalk, Duster | Duster | used to erase; others are used to write |
| Source | Milk, Curd, Butter, Rice | Rice | a grain; others are dairy products |
| Status | Mumbai, Chennai, Kolkata, Kanpur | Kanpur | not a state capital; others are |
| Material / make-up | Iron, Copper, Aluminium, Brass | Brass | an alloy (copper + zinc); others are pure metals (elements) |
Intermediate level: number classification
Numbers hide their rule inside their properties. Check them in this order, from quickest to slowest:
| Check | What to look for | Example (odd one in bold) |
|---|---|---|
| Even / odd | all even except one | 14, 22, 35, 48 |
| Prime / composite | all prime except one | 17, 23, 33, 41 (33 = 3 × 11) |
| Perfect squares | n × n | 121, 169, 225, 290 |
| Perfect cubes | n × n × n | 27, 125, 343, 511 (8³ = 512) |
| Divisibility | all divisible by 3, 7, 11 … | 21, 35, 49, 54 (others are multiples of 7) |
| Digit sum | digits add to the same total | 147, 237, 327, 427 (sums 12, 12, 12, 13) |
| Square/cube ± 1 | n² + 1, n² − 1, n³ + 1 | 50, 65, 82, 99 (others are 7²+1, 8²+1, 9²+1) |
Divisibility rules to remember: a number is divisible by 3 if its digit sum is divisible by 3, by 9 if its digit sum is divisible by 9, by 4 if its last two digits form a multiple of 4, and by 5 if it ends in 0 or 5.
Intermediate level: letter classification
Turn letters into their position numbers (A = 1, B = 2 … Z = 26). Then the letters become a number problem.
| A 1 | B 2 | C 3 | D 4 | E 5 | F 6 | G 7 | H 8 | I 9 | J 10 | K 11 | L 12 | M 13 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| N 14 | O 15 | P 16 | Q 17 | R 18 | S 19 | T 20 | U 21 | V 22 | W 23 | X 24 | Y 25 | Z 26 |
Look for: equal gaps between letters (ACE has gaps of 2), vowels (A, E, I, O, U) versus consonants, and opposite letters. Opposite letters are pairs counted from both ends of the alphabet: A–Z, B–Y, C–X and so on. Their positions always add up to 27 (A 1 + Z 26 = 27).
Advanced level: pairs and mixed groups
Here each option is a pair, and the rule is the relation inside the pair.
- (3, 27), (4, 64), (5, 125), (6, 206) → second number is the cube of the first; 6³ = 216, not 206.
- C3, E5, H8, K12 → the number is the letter's position; K is 11, not 12.
At this level, two weak rules may seem to fit. Always prefer the rule that is more specific and fits all three members perfectly. In the metals example above, "they are metals" fits all four, so it is useless. "They are pure elements" separates Brass, so that is the real rule.
- ✓- Three items share one property; the odd one breaks it. A correct rule singles out exactly one option.
- ✓- Primes up to 50: 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47. 1 is not prime; 2 is the only even prime.
- ✓- Look-alike non-primes: 51 = 3 × 17, 57 = 3 × 19, 87 = 3 × 29, 91 = 7 × 13.
- ✓- Squares 11² to 20²: 121, 144, 169, 196, 225, 256, 289, 324, 361, 400.
- ✓- Cubes 1³ to 10³: 1, 8, 27, 64, 125, 216, 343, 512, 729, 1000.
- ✓- Letter positions: E 5, J 10, O 15, T 20, Y 25 (EJOTY). Opposite letters add to 27.
- ✓- Vowels: A, E, I, O, U. All other letters are consonants.
Worked examples
Example 1 (Easy, words)
Find the odd one: Kilometre, Metre, Kilogram, Centimetre
- Step 1: What does each word measure? Kilometre, metre and centimetre measure length.
- Step 2: Kilogram measures mass.
- Step 3: Test: "unit of length" fits three and fails only for Kilogram.
Answer: Kilogram.
Example 2 (Easy, numbers)
Find the odd one: 17, 23, 33, 41
- Step 1: All are odd, so even/odd does not help.
- Step 2: Prime check. 17, 23 and 41 have no factors other than 1 and themselves.
- Step 3: 33 = 3 × 11, so it is composite.
Answer: 33. (Digit sums are 8, 5, 6, 5, so that rule gives no single item, which confirms we should ignore it.)
Example 3 (Medium, number pattern)
Find the odd one: 26, 50, 82, 120
- Step 1: All are even, so even/odd fails.
- Step 2: Look near perfect squares: 26 = 25 + 1 = 5² + 1, 50 = 49 + 1 = 7² + 1, 82 = 81 + 1 = 9² + 1.
- Step 3: 120 = 121 − 1 = 11² − 1. It is "minus 1", not "plus 1".
Answer: 120. Notice that the bases 5, 7, 9, 11 are consecutive odd numbers, which is a typical setter's design.
Example 4 (Medium, letter groups)
Find the odd one: BDF, HJL, PRT, KLN
- Step 1: Convert to positions. BDF = 2, 4, 6; HJL = 8, 10, 12; PRT = 16, 18, 20; KLN = 11, 12, 14.
- Step 2: Gaps. BDF, HJL and PRT move +2, +2.
- Step 3: KLN moves +1, +2.
Answer: KLN.
Example 5 (Hard, letter pairs)
Find the odd one: CX, DW, GT, JP
- Step 1: Positions: C 3 + X 24 = 27; D 4 + W 23 = 27; G 7 + T 20 = 27.
- Step 2: J 10 + P 16 = 26.
- Step 3: The first three are opposite-letter pairs (sum 27). The opposite of J is Q (10 + 17 = 27), not P.
Answer: JP.
Example 6 (Exam-hard, hidden rule)
Find the odd one: 147, 237, 327, 427
- Step 1: All odd, all end in 7, none is a perfect square. Surface rules fail.
- Step 2: Prime check fails too: 147 = 3 × 49, 237 = 3 × 79, 327 = 3 × 109, 427 = 7 × 61. None is prime.
- Step 3: Digit sums: 1+4+7 = 12, 2+3+7 = 12, 3+2+7 = 12, 4+2+7 = 13.
- Step 4: So 147, 237 and 327 are divisible by 3; 427 is not.
Answer: 427. Lesson: when easy checks fail, the digit sum often reveals the rule in seconds.
Real-world connection
Police work is full of classification. An officer checking CCTV footage looks for the one vehicle whose number plate format does not match the rest. While reading four witness statements, they notice the one account whose timing does not fit the others. When sorting seized items, they group them by type and quickly spot the item that does not belong. The habit you build here is the same: find the common pattern first, then the exception stands out.
- Stopping at the first difference. Counting letters or looking at first letters is a surface feature. Check meaning first, spelling last.
- Using a rule that fits two or zero items. If your rule marks two options as odd, it is not the setter's rule. Keep looking.
- Choosing a general rule when a specific one exists. "Iron, Copper, Aluminium, Brass are all metals" leads nowhere; "Brass is an alloy" is the answer.
- Prime traps. 1 is not prime, 2 is prime, and 51, 57, 87 and 91 are not prime.
- Letter-position slips. Counting from A every time wastes time and causes errors. Use EJOTY anchors.
- Personal opinion. Answer by facts that everyone agrees on, not by what you like or what is true only in your village.
- Test-on-all-four rule: a rule is correct only when exactly one option fails it.
- Number order of checks: even/odd → prime → square/cube → divisibility → digit sum → n² ± 1 or n³ ± 1.
- Letter order of checks: positions → gaps → vowels → opposite pairs (sum 27).
- Pair questions: find the relation in the first pair, then apply it to the others. The pair that fails is the answer.
- Mnemonic EJOTY: E = 5, J = 10, O = 15, T = 20, Y = 25. Count from the nearest anchor (for example, R = T − 2 = 18).
- Opposite-pair hooks: BY, DeW, GT road, HS (High School), IR (Indian Railways), JQ (Jack–Queen), KP, LO, MN.
- No negative marking: if stuck after 40 seconds, remove options you are sure about, mark your best choice and move on.
How it is asked in the exam
The common formats are:
- "Find the odd one out." Four words, numbers or letter groups.
- "Three of the following four are alike in a certain way. Which one does not belong to that group?" Same idea in longer wording.
- Pairs and clusters: number pairs like (4, 16), letter-number codes like H8, or three-letter groups like ACE.
Time: the average is about 54 seconds per question. Aim for 15–25 seconds on easy classification and under 45 seconds on hard ones, so you bank time for arithmetic.
Difficulty levels:
| Level | What it looks like | Example |
|---|---|---|
| Easy | one obvious category | Venus, Mars, Jupiter, Moon |
| Medium | one number or letter property | 8, 27, 125, 150 |
| Hard | hidden rule or relation within pairs | (2, 5), (3, 10), (4, 17), (5, 24) |
In the medium example, 8, 27 and 125 are cubes (2³, 3³, 5³); 150 is not. In the hard example, the second number is first² + 1, and 5² + 1 = 26, not 24.
- Tiger, Lion, Leopard, Elephant — which is odd? Answer: Elephant. The others are big cats.
- 27, 125, 343, 511 — which is odd? Answer: 511. The others are cubes of 3, 5, 7; 8³ = 512.
- ACE, GIK, LNP, QSV — which is odd? Answer: QSV. Positions 17, 19, 22 give gaps of +2, +3; the others move +2, +2.
- 21, 35, 49, 54 — which is odd? Answer: 54. The others are multiples of 7.
- Ganga, Yamuna, Brahmaputra, Chilika — which is odd? Answer: Chilika. It is a lake (a lagoon in Odisha); the others are rivers.
- ✓- Classification = three items share a property; the fourth does not.
- ✓- Read all four options, form a rule, then test it on every option.
- ✓- A valid rule picks out exactly one item; otherwise look for a stronger rule.
- ✓- Words: check category, use, source, status, material.
- ✓- Numbers: even/odd, prime, square/cube, divisibility, digit sum, n² ± 1.
- ✓- Letters: positions (EJOTY), gaps, vowels, opposite pairs that add to 27.
- ✓- Pairs: find the relation inside one pair and test the rest.
- ✓- Aim for under 30 seconds on most of these; never leave one blank.
తెలుగు సారాంశం (Telugu summary)
వర్గీకరణ (Classification) ప్రశ్నలో నాలుగు అంశాలు ఇస్తారు. వాటిలో మూడు ఒకే లక్షణాన్ని పంచుకుంటాయి, ఒకటి మాత్రం భిన్నంగా ఉంటుంది. ఆ భిన్నమైన దాన్ని గుర్తించడమే మన పని.
ముందుగా నాలుగు ఆప్షన్లనూ చదవండి, మూడింటికి సరిపోయే నియమాన్ని ఊహించండి, ఆ నియమాన్ని నాలుగింటిపైనా పరీక్షించండి.
సరైన నియమం కచ్చితంగా ఒక్క ఆప్షన్ను మాత్రమే వేరు చేస్తుంది. రెండింటిని వేరు చేస్తే లేదా దేన్నీ వేరు చేయకపోతే, మరింత బలమైన నియమం వెతకండి.
పదాల ప్రశ్నల్లో వర్గం, ఉపయోగం, మూలం, తయారీ పదార్థం వంటి ఆధారాలను చూడండి.
సంఖ్యల ప్రశ్నల్లో సరి/బేసి, ప్రధాన సంఖ్యలు, వర్గాలు, ఘనాలు, భాజనీయత, అంకెల మొత్తం అనే క్రమంలో పరీక్షించండి.
1 ప్రధాన సంఖ్య కాదు; 2 మాత్రమే సరి ప్రధాన సంఖ్య. 51, 57, 91 ప్రధాన సంఖ్యలు కావు.
అక్షరాల ప్రశ్నల్లో అక్షర స్థానాలు (A=1 … Z=26), అక్షరాల మధ్య అంతరం, అచ్చులు, వ్యతిరేక అక్షరాలు (వాటి స్థానాల మొత్తం 27) చూడండి.
EJOTY గుర్తుంచుకోండి: E=5, J=10, O=15, T=20, Y=25.
ఒక్కో ప్రశ్నకు సగటున సుమారు 54 సెకన్లు ఉంటాయి. ఇలాంటి ప్రశ్నలను 30 సెకన్లలోపు పూర్తి చేస్తే లెక్కల ప్రశ్నలకు సమయం మిగులుతుంది. నెగటివ్ మార్కులు లేవు కాబట్టి ఏ ప్రశ్ననూ వదలకండి.
Key terms (English — తెలుగు):
- Classification — వర్గీకరణ
- Odd one out — భిన్నమైనది / సరిపోనిది
- Common property — సామాన్య లక్షణం
- Even / Odd number — సరి సంఖ్య / బేసి సంఖ్య
- Prime number — ప్రధాన సంఖ్య
- Perfect square — పరిపూర్ణ వర్గం
- Perfect cube — పరిపూర్ణ ఘనం
- Divisibility — భాజనీయత
- Digit sum — అంకెల మొత్తం
- Vowel / Consonant — అచ్చు / హల్లు
- Opposite letters — వ్యతిరేక అక్షరాలు