After answering the exam questions you know the answers to you still have time left. You spend the rest of your time answering questions you do not know the answers to, in the hope that something you write will fool the professor grading the exam into thinking you know the answer, at least in part, expressed it unclearly and deserve partial credit. Doing this is rational behavior on your part, but the result is to waste your time writing and my time reading. It also adds additional noise to the signal that exams generate, since there is a risk that I will either be fooled into giving you credit you do not deserve or interpret some other student's poorly written answer as entirely bogus when it is not.I have a solution to this problem, an economic solution, although I like to claim that it was inspired by the story of Socrates and the Delphic Oracle.
Well, one day [Chaerephon] went to Delphi, and there he had the impudence to put this question -- do not jeer, gentlemen, at what I am going to say -- he asked, "Is anyone wiser than Socrates?" And the Pythian priestess answered, "No one." Well, I was fully aware that I knew absolutely nothing. So what could the god mean? for gods cannot tell lies. For some time I was frankly puzzled to get at his meaning; but at last I embarked on my quest. I went to a man with a high reputation for wisdom -- I would rather not mention his name; he was one of the politicians -- and after some talk together it began to dawn on me that, wise as everyone thought him and wise as he thought himself, he was not really wise at all. I tried to point this out to him, but then he turned nasty, and so did others who were listening; so I went away, but with this reflection that anyhow I was wiser than this man; for, though in all probability neither of us knows anything, he thought he did when he did not, whereas I neither knew anything nor imagined I did."
On my exams, knowing what you do not know is worth something—twenty percent to be precise. That is what you get on a question for not doing it. So if you suspect that the best bogus answer you can come up with will be worth less than twenty percent, you are better off leaving the question blank or writing "I do not know," going home early, saving both of us some time and saving me the hassle of trying to figure out which answers are or are not entirely bogus.I posted on this and some related matters five years ago but thought it worth posting on it again.