Here's the methodology of the Chest report: A large committee of respiratory experts read all the research papers thought to bear on each of a series of recommendations drawn from the clinical literature about cough. The committee rated how solidly the evidence supported the recommendation. It also commented on the potential benefit to patients. The committee then mashed together the solidity of the evidence and the potential benefit to come up with a letter grade (A to C) for the strength of an "evidence-based" recommendation. An "A" might be given for a recommendation that has only fair support from the research literature but, if correct, would be very beneficial. A "C" could mean that a recommendation is well-researched but of no particular consequence. D is used for negative "shall nots."