Explain why an economist might be required to use compensating variation to calculate consumer surplus. In which situation would an economist need to worry for bias?

HINT: this is a hard question, there is a clear and definite answer to this - you can't be vague or you will receive a nonvague amount of point : 0. You need 2 paragraphs to answer this question. (i'm joking of course, but try your best, start from what Consumer surplus is, what compensating variation is .... )