Negative marking and accuracy in CDS

Every CDS paper deducts one-third of a question's marks for each wrong answer. On its own that sounds mild. Over a full paper it is not. Twenty careless answers in the GK paper cost about 5.6 marks, and at the written cut-off a few marks can separate a call to the SSB from another six months of waiting.

This guide works through the arithmetic, shows when a guess is worth taking, and sets out habits for each paper that cut careless errors.

The penalty in each paper

Two rules from UPSC's scheme are worth knowing exactly:

• A blank answer costs nothing.
• Marking more than one option counts as a wrong answer, even if one of them is correct.

The arithmetic of guessing

Let each question be worth m marks, with four options and a penalty of m/3.

Blind guess (no option eliminated). You have a 1 in 4 chance of being right.

• Expected gain = ¼ × m − ¾ × m/3 = m/4 − m/4 = 0.

On average, a blind guess gains nothing. It only adds risk.

One option eliminated. Now 1 in 3.

• Expected gain = ⅓ × m − ⅔ × m/3 = m/3 − 2m/9 = m/9.

A small positive gain. In maths (m = 1) that is about 0.11 marks per guess.

Two options eliminated. Now 1 in 2.

• Expected gain = ½ × m − ½ × m/3 = m/2 − m/6 = m/3.

A clear gain: about 0.33 in maths, about 0.28 in English or GK.

So the rule is simple: never guess blindly; guess once you can genuinely eliminate at least one option, and do so confidently once you have eliminated two.

Why accuracy decides results

Two hypothetical candidates in the GK paper:

B attempted 20 fewer questions and scored about 11 marks more. A's 35 wrong answers cost nearly ten marks, the value of almost twelve correct answers.

The same logic in maths:

D left 30 questions blank and still beat C by more than eleven marks.

Neither example means "attempt less". It means attempt what you can get right. The goal is to raise the number of correct answers while keeping wrong answers low, and that comes from preparation and practice, not from a fixed attempt target.

Accuracy habits, paper by paper

English

• Read the whole sentence before choosing an error. Many errors only show up at the end (subject far from verb, a tense clash).
• In sentence improvement, test each option in the sentence; "No improvement" is a real answer, not a trap to avoid.
• In comprehension, answer only from the passage, not from what you know about the topic.
• In ordering questions, find the opening sentence first and check pronouns and connectors.

General Knowledge

• Read statement-based questions slowly. Mark each statement true or false separately, then match to the options.
• Watch for words like only, all, never, not. They change the answer.
• If two options feel equally right, you do not know the answer. Decide whether you can eliminate enough to guess, or leave it.

Elementary Mathematics

• Check the last step of every calculation: units, decimal point, what the question actually asked for.
• Substitute an option back into the question where possible; it is often faster than solving.
• Estimate first. If the answer must be around 40 and your calculation gives 400, recheck.
• If a question has taken 2 minutes with no end in sight, mark it and move on.

A two-round method for each paper

1. Round one: go through the paper once. Answer everything you are sure of. Mark questions where you can eliminate options for later. Skip the rest.
2. Round two: return to the marked questions. Guess where you have eliminated at least one option with confidence.
3. Final minutes: check that your OMR bubbles match your question numbers. A shifted row is the most expensive mistake of all.

An accuracy log

After every practice paper, fill in one row:

Aim for about 85 to 90 per cent accuracy in each paper. Below 80 per cent, slow down and review your error causes before attempting more. Classify each error as concept, calculation, misreading or blind guess; the pattern tells you what to fix.

Practice: check your arithmetic

1. In the English paper, a candidate gets 70 right and 18 wrong. What is the score? 70 × 0.833 ≈ 58.3; 18 × 0.278 ≈ 5.0; score ≈ 53.3.
2. In maths, a candidate gets 48 right and 12 wrong. What is the score? 48 − 12 ÷ 3 = 48 − 4 = 44.
3. You can eliminate one option in a maths question. Should you guess? Yes, on average; the expected gain is about +0.11.
4. You mark two options on one question and one of them is correct. What happens? It is treated as wrong, and the penalty applies.
5. A candidate has 30 wrong answers in GK. How many marks does the penalty cost? 30 × 0.278 ≈ 8.3 marks.

The deeper point

Negative marking rewards knowing what you know. Candidates who revise thoroughly and practise under time learn to feel the difference between "sure", "fairly sure" and "no idea". That judgement is worth more than any guessing formula. Build it through regular mock tests and previous papers, and watch the cut-off picture in how CDS merit and cut-offs work.