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:

  • Levels:
    • I - systemic review of all relevant RCTs
    • II - Randomized trial or observational study with dramatic effect
    • III - Non-randomized controlled cohort/follow-up study
    • IV - Case-series, case-control studies, or historically controlled studies
    • V – mechanism-based reasoning (expert opinion, based on physiology, animal or laboratory studies)
  • Grades:
    • A – consistent level 1 studies
    • B – consistent level 2 or 3 studies or extrapolations from level 1 studies
    • C – level 4 studies or extrapolations from level 2 or 3 studies
    • D – level 5 evidence or troubling inconsistent or inconclusive studies of any level

NHMRC levels:

  • Level I: systematic review of RCTs
  • Level II: RCT
  • Level III-1: pseudorandomised trial of high quality
  • Level III-2: cohort studies or case control studies - but with a control group
  • Level III-3: cohort studies with historical controls, or no control group
  • Level IV: case series

References

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.