Negative marking and accuracy in NDA

Every NDA paper has a hidden third option on every question: leave it. Because a wrong answer costs marks, the number of questions you attempt is a decision, not a target. Candidates who treat it as a target — "I must attempt 100 in maths" — often score less than those who attempt fewer and get more right.

This guide works through the rule, the arithmetic of guessing, and the habits and exam-hall method that turn it into marks.

The rule

The NDA notification sets out three points:

• For each wrong answer, one-third of the marks for that question is deducted.
• If you mark more than one option, it counts as a wrong answer, even if one of them is correct.
• A question left blank carries no penalty.

In recent papers, that works out as follows:

The last column is the easiest way to remember the rule: three wrong answers wipe out one right answer, in either paper.

The arithmetic of guessing

Every question has four options. Let m be the marks for a question. The expected marks from an attempt are the chance of being right times m, minus the chance of being wrong times m/3.

Blind guess (one chance in four):

• Expected marks = (1/4 × m) − (3/4 × m/3) = m/4 − m/4 = 0.

A blind guess gains nothing on average. It only adds risk.

One option ruled out (one chance in three):

• Expected marks = (1/3 × m) − (2/3 × m/3) = m/3 − 2m/9 = m/9, slightly positive.

Two options ruled out (one chance in two):

• Expected marks = (1/2 × m) − (1/2 × m/3) = m/2 − m/6 = m/3, clearly positive.

In marks, per question:

For the last row: 0.8 × 2.5 − 0.2 × 0.833 = 2.0 − 0.17 ≈ 1.83 in Maths, and 0.8 × 4 − 0.2 × 1.333 = 3.2 − 0.27 ≈ 2.93 in GAT.

The break-even point

Put the formula in general form. If your chance of being right is p, the expected marks are p × m − (1 − p) × m/3, which simplifies to m × (4p − 1) ÷ 3. This is positive whenever p is more than 1/4.

So, on average, any attempt where you are genuinely better than a blind guess adds marks. Two cautions keep this honest:

1. Candidates overrate their eliminations. "I'm sure it isn't option (a)" is often wrong under exam pressure. If your elimination is shaky, your real chance is closer to one in four.
2. Time is also a cost. A maths question you spend four minutes on for a 50% chance may cost you two easier questions later in the paper.

Why accuracy decides results

Consider two candidates in the maths paper (made-up figures to show the arithmetic):

B attempted 15 fewer questions and scored about 39 marks more. Candidate A's 30 wrong answers cost 25 marks — the value of 10 correct answers.

The same logic applies to GAT, where the stakes are higher per question. In GK especially, questions you half-remember are where most penalties come from.

A three-pass method for the paper

1. First pass (about half the time): go through the whole paper and answer every question you can solve with confidence. Mark the answer sheet as you go. Skip anything that looks long or uncertain.
2. Second pass: return to skipped questions. Solve those that need more working, and attempt those where you can eliminate at least one option with certainty, ideally two.
3. Final minutes: check that every marked bubble matches the question number, and that no question has two bubbles. Do not start new guesses in the last five minutes.

This protects you from the most expensive mistake of all: running out of time with easy questions still unseen at the end of the booklet.

Accuracy habits

In Mathematics

• Read what is asked. Degrees or radians? The value, or the number of solutions? The sum of roots, or the roots themselves?
• Substitute an option back when solving directly is slow. For example: if 2^x + 2^(x+1) = 24, which of 2, 3, 4 or 5 is x? Try x = 3: 8 + 16 = 24. Done. (Directly: 2^x × 3 = 24, so 2^x = 8 and x = 3.)
• Watch the last step. Signs, a factor of 2, and simple arithmetic cause more wrong answers than hard concepts.
• Estimate first. If a probability comes out as 1.2, or a length as negative, something has gone wrong.

In General Knowledge and English

• Read "not", "incorrect", "except" and "which of the following" twice.
• In statement-based questions, judge each statement separately, then look at the options.
• In English error-spotting, confirm the error by fixing it in your head; if you cannot fix it, you have not found it.
• If two facts are fighting in your memory and you cannot eliminate either, leave the question.

Keep an accuracy log

Aim for accuracy of 85–90%. If yours falls below that, reduce attempts and slow down for the next two mocks. If it is consistently above 90% but your attempts are low, you are being too cautious: push your second pass harder.

Practice set

1. In Maths, a candidate attempts 90 questions and gets 72 right. What is the score?
2. In GAT, a candidate attempts 130 questions and gets 100 right. What is the score?
3. What is the expected gain from a GAT question where two options have been eliminated with certainty?
4. A candidate marks two options on a maths question, and one of them is correct. What happens?
5. You are about 60% sure of a maths answer. Is attempting worth it, and by how much on average?

Answers:

1. 165. 18 wrong. 72 × 2.5 = 180; 18 × 0.833 = 15; 180 − 15 = 165.
2. 360. 30 wrong. 100 × 4 = 400; 30 × 1.333 = 40; 400 − 40 = 360.
3. About 1.33 marks. m/3 with m = 4.
4. It counts as a wrong answer, so about 0.83 is deducted.
5. Yes, about 1.17 marks. 0.6 × 2.5 = 1.5; 0.4 × 0.833 = 0.33; 1.5 − 0.33 ≈ 1.17.

For how accuracy feeds into mocks, read mock tests and previous papers for NDA, and for the full marking scheme, see the NDA exam pattern explained.