Many UPSC candidates fail to clear Prelims not due to lack of study, but because of excessive caution. Paralyzed by fear of the -0.66 negative marking penalty, they attempt only 60 to 70 questions and miss the cutoff by a couple of marks.
The Official Mathematical Formula
In UPSC Civil Services GS Paper 1, there are 100 questions for 200 marks:
- Each Correct Answer: +2.0 Marks ($200 / 100$)
- Each Incorrect Answer: -0.66 Marks ($1/3\text{rd of } 2.0$)
- Unattempted Question: 0.00 Marks
The Probability Advantage of 50:50 Eliminations
Suppose you eliminate 2 options on 20 questions, leaving you with a coin-flip choice between two answers on each. The expected value (EV) per question is:
EV = (0.5 × 2.0) - (0.5 × 0.66) = 1.0 - 0.33 = +0.67 Marks
Across 20 questions, the expected net score increase is +13.4 marks. Refusing to guess on these 20 questions leaves those 13.4 points on the table.
Risk Thresholds: When NOT to Guess
If you cannot eliminate at least one option with certainty, random guessing across four options yields an expected value of zero:
EV = (0.25 × 2.0) - (0.75 × 0.66) = 0.5 - 0.495 ≈ 0.00 Marks
Pure random guessing adds risk without statistical gain. Intelligent elimination between two choices, however, consistently yields positive returns.