Negative marking: when to guess and when to leave it
The arithmetic of guessing is simple, and most students get it wrong in both directions.
With one mark awarded and 0.25 deducted, a blind guess between four options has an expected value of exactly zero. That is the baseline everything else is measured against.
The rule that follows
If you can confidently eliminate even one option, guessing becomes positive expected value. If you cannot eliminate any, guessing is a coin flip that costs you nothing on average but adds variance.
Where students go wrong
Some leave everything they are unsure about, which quietly costs marks they could have earned. Others attempt everything, which is fine mathematically but eats time they needed for questions they could actually solve.
The real constraint is usually time, not the marking scheme.
Put it into practice
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