Negative marking: when is a guess worth it?
The simple arithmetic behind guessing in PSC Prelims with one-third, one-fourth or 0.25 minus marking.
Most state PSC Prelims papers take marks off for a wrong answer. UPPSC, MPPSC and CGPSC deduct one-third of the question's marks, UKPSC one-fourth, HPSC 0.25 marks. (Check your exam's page on this site for the current rule.)
The rule of thumb
Say a right answer gets 1 mark and a wrong one loses p marks. If you can narrow a question down to k options and pick one at random:
Expected marks = 1k × 1 − k − 1k × p
A guess is worth taking when this is above zero, which means
p < 1k − 1
With one-third negative marking (p = 13)
| Options left | Expected marks | Guess? |
|---|---|---|
| 4 (no idea) | 14 − 34 × 13 = 0 | No gain |
| 3 | 13 − 23 × 13 = 19 | Small gain |
| 2 | 12 − 12 × 13 = 13 | Yes |
What this means in the hall
- A blind guess among four options gains nothing on average and adds risk. Leave it.
- Once you can rule out two options, answering is clearly worth it.
- With one-fourth marking the blind guess is still worth about 116 of a mark, so the line moves slightly in favour of attempting.
- Some papers have their own rules. RPSC gives a fifth option for "not attempting" and penalises leaving questions blank, and HPSC can disqualify you for leaving more than 10% unanswered. Learn your paper's rule before exam day.
Quick check
With 4 options and one-third negative marking, you rule out one option and guess among the other three. Your expected marks for that question are: