Negative marking !!!

Negative marking in exams is part of evaluation process where in marks are deducted from the actual score for every wrong answer. It is now followed in most entrance exams. It makes more competition among the people attending the exam.

Most people are very much cautious while attending an exam that involves negative marking. They do not want to lose marks they have gained in other questions by attending questions that they dont know. I have attended few tests with negative marking and I believe that it is not a good idea to leave the question unanswered. The reason is as follows....

Negative marking scheme followed in the exams that I attended is deduction of 25% of marks assigned to the question that is wrongly answered. I assume there are four choices (1 correct and 3 wrong) to one question and each question carries one mark.

The probability that I chose the correct answer is 1/4 and for wrong answers, it is 3/4. Assume that I have four questions that I am not sure what to do or which is the correct answer. They are mutually independent of each other. So the probability that I answer all four wrongly is .75 x .75 x .75 x .75 = 0.32 and hence the probability that atleast one is correct is 1 - 0.32 = 0.68

Now if 1 question is wrong, I lose extra 0.25 marks. So total marks I lost extra if all four are wrong is 4 x 0.25 = 1 mark.

Assume I have atleast one correct. Then maximum that I will lose extra is 0.75

From this we see that we have greater probability(0.68) of gaining atleast 0.25 marks. Moreover we know atleast something about the subject. So we can eliminate options that we are sure which wont be the answer and this still increases our probability to gain marks. So I opt to attend all the questions than to leave the question unanswered. Caution to this approach is required if you are at the passing border.

I hope I am right in the probability calculation.... lol.


Anonymous said...

It's interesting you say 0.25 is deducted for every wrong answer. When I was taking Economics MCQ exam papers at Dundee University in 1998/99 we were all deducted 1 mark for every wrong answer. This resulted in a 70% (1st) being marked as 40% (3rd). I found this to be a very bad way to teach. It was very demoralising for students. There needs to be consistency or there is a wrong impression given to those that have an interest in these matters eg. employers who take these results as gospel. Education should be about encouraging to learn and motivation through reward for hard work.

August 6, 2009 3:58 PM

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Trix said...

+1 mark for correct and -1 for incorrect answer is too bad. The calculations I have done are only for the negative marking scheme that I have taken as example.

By common sense.. as the negative marks given for wrong answer increases...the probability of getting more marks reduces... and we should take less chances.... :D

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