This topic properly belongs in the Primary exam curriculum, and is dealt with in the chapter on Levels of Evidence from the Required Reading section for the Primary Exam. here, the topic is treated only briefly, because Question 17 from the first paper of 2012 asked for "a classification for the levels of evidence used for therapeutic studies in EBM". It is not inconceivable that this topic will continue to appear in both the Primary and the Fellowship papers.
Anyway. We have several systems of rating evidence to choose from. Here are a couple:
Oxford centre for evidence based medicine:
Cook, D. J., and M. K. Giacomini. "The integration of evidence based medicine and health services research in the ICU." Evaluating Critical Care. Springer Berlin Heidelberg, 2002. 185-197.
Kotur, P. F. "Evidence-Based Medicine in Critical Care." Intensive and Critical Care Medicine. Springer Milan, 2009. 47-57.