I’m totally fascinated by test design, and I think it’s incredibly difficult to write a good exam. A good exam should be reliable, which means that given in different forms or under different circumstances, it would yield the same results. And more importantly, it should be valid, which means (among a lot of other things) that it actually tests what it set out to test.
Unfortunately I think most teachers are not especially good at writing exams, especially the ones with a lousy theory of mind. That is, it’s hard for someone who is not the writer of the question to know what information the question writer is seeking.
And I think your grade depends a lot (and unfairly) on how well the writer of the test is able to communicate as well as his/her ability to figure out what information from the coursework is actually important.
I had one professor whose multiple choice questions were so poorly written that I got a 50% on the MC section of an exam and 100% on the written portion (which I think most people find more difficult)—I clearly knew the material, but his questions were so awful there was no way to figure out what he was looking for, or in some instances more than one answer was actually correct, or none of them were entirely correct. Picking the “best” answer is relative and it sucks to have to try to read your prof’s mind.
So…..no. Just because you don’t do well doesn’t mean you don’t know the material. But be careful not to assume that doing poorly automatically means you DO know the material—it doesn’t work that way, either.