Negative marking: the actual maths of when to guess

“Never guess” and “always attempt everything” are both wrong – and both cost marks. Negative marking is just arithmetic, and knowing the arithmetic turns a coin-flip into a strategy.

The expected-value ladder

With +4 for a correct answer and −1 for a wrong one, a blind guess across four options averages out to (¼ × 4) − (¾ × 1) = +0.25 marks – technically positive, but a razor-thin edge that comes with wild swings. Now watch what elimination does:

• Eliminate one option: (⅓ × 4) − (⅔ × 1) = +0.67 marks per question on average.
• Eliminate two options: (½ × 4) − (½ × 1) = +1.5 marks – half the value of simply knowing the answer.

So the rule is simple

• Nothing eliminated: the average says +0.25; the variance says one bad streak can dent your rank. Skip, and spend those seconds where you can eliminate.
• One option eliminated: attempt – the maths is now clearly on your side.
• Two options eliminated: always attempt. Hesitating here is leaving marks on the table.

Train it, don’t just know it

Under time pressure, everyone’s discipline slips – the skill is making elimination a habit, not a theory. In every mock, mark the questions where you guessed, and check afterwards which guesses were mathematically justified.