"Should I attempt a question I am not sure about?" is one of the most common questions before an objective exam. Coaching advice ranges from "never guess" to "attempt everything". The honest answer depends on the exam's marking scheme, and you can work it out yourself with a little arithmetic.
The Basic Idea: Expected Score
For any guess, multiply the chance of being right by the marks you gain, and the chance of being wrong by the marks you lose. If the result is positive, guessing raises your score on average; if it is negative, it lowers it.
Expected score = (chance right × marks for correct) − (chance wrong × marks deducted)
Common Marking Schemes
Most objective government exams have four options and deduct either one-quarter or one-third of the marks for a wrong answer. Typical examples from recent notifications:
- SSC exams such as CGL Tier I: 2 marks per question, 0.5 deducted for a wrong answer (one-quarter).
- Banking prelims (IBPS, SBI): one-quarter of the marks deducted.
- Railway (RRB) exams and UPSC Prelims: one-third of the marks deducted.
Patterns change, so check the scheme in your own exam's notification before relying on the numbers below.
Case 1: A Pure Random Guess
With no idea at all, each of the four options has a 25% chance.
- One-quarter penalty (SSC: +2 / −0.5): 0.25 × 2 − 0.75 × 0.5 = 0.5 − 0.375 = +0.125 marks per question. Slightly positive.
- One-third penalty (UPSC: +2 / −0.66): 0.25 × 2 − 0.75 × 0.66 ≈ 0.5 − 0.5 = 0. Pure guessing gains nothing on average.
So even blind guessing is not harmful under a one-quarter penalty, and is neutral under one-third. But "on average" hides a lot of risk: on a single paper, a run of wrong guesses can cost you several marks.
Case 2: You Can Eliminate One Option
Now each of the remaining three options has a one-in-three chance.
- One-quarter penalty: 0.33 × 2 − 0.67 × 0.5 ≈ 0.67 − 0.33 = +0.33 per question.
- One-third penalty: 0.33 × 2 − 0.67 × 0.66 ≈ 0.67 − 0.44 = +0.22 per question.
Case 3: You Can Eliminate Two Options
A 50-50 guess.
- One-quarter penalty: 0.5 × 2 − 0.5 × 0.5 = +0.75 per question.
- One-third penalty: 0.5 × 2 − 0.5 × 0.66 ≈ +0.67 per question.
This is the key lesson: every option you can confidently rule out makes a guess much more valuable. A 50-50 guess is worth roughly a third of a correct answer.
A Practical Rule
- Know it: answer.
- Can eliminate two options: almost always attempt.
- Can eliminate one option: usually attempt, especially in exams with a one-quarter penalty.
- No idea at all: skip in exams with a one-third penalty. In one-quarter exams it is a small positive bet, but only worth it if your score is near the cut-off and you need every mark.
The Hidden Costs of Guessing
- Time. Time spent agonising over a question you cannot solve is time taken from questions you can. Decide quickly, mark it, and move on.
- False confidence. The maths assumes your elimination is correct. If you "eliminate" the right answer, your odds get worse, not better. Only eliminate options you are genuinely sure are wrong.
- Sectional cut-offs. Some exams require a minimum score in each section. A few wrong guesses in a weak section can push you below that line even if your total is fine.
Use Your Mock Tests
Track two numbers in every mock: how many guesses you made, and how many were right. Guessing breaks even at 20% accuracy under a one-quarter penalty and 25% under a one-third penalty; below that, your guesses are costing you marks. If your guessing accuracy is comfortably higher, say 40% or more, your elimination is working and educated guesses are worth taking. If it is close to the break-even point, improve your elimination before relying on it in the real exam.