The formula
score = (correct − wrong ÷ (options − 1)) ÷ questions × maximum
Where it comes from
The penalty exists to cancel out guessing. Answer at random on a four-option test and you get one in four right: for every lucky correct answer there are three wrong ones. Taking a third of a mark off per wrong answer leaves the expected value of guessing at zero, and the exam measures knowledge rather than nerve.
How to work it out by hand
- Divide the wrong answers by the number of options minus one
- Subtract that from the correct answers
- Divide by the total number of questions
- Multiply by the maximum score
What is worth knowing
From that comes the correct strategy: if you can rule out at least one option, answering pays, because your odds already beat the guessing the penalty is set to cancel. If you have no idea and can rule out nothing, leaving it blank is statistically neutral. Always check the exact formula in the exam rules: some deduct a quarter of a mark and others a third.