Table 1.

American College of Critical Care Medicine guidelines for evidence-based medicine rating system for strength of recommendation and quality of evidence supporting the references

Rating system for references
a	Randomized, prospective controlled trial
b	Nonrandomized, concurrent or historical cohort investigations
c	Peer-reviewed, state of the art articles, review articles, editorials, or substantial   case series
d	Nonpeer reviewed published opinions, such as textbook statements or official   organizational publications
Rating system for recommendations
Level 1	Convincingly justifiable on scientific evidence alone
Level 2	Reasonably justifiable by scientific evidence and strongly supported by expert critical   care opinion
Level 3	Adequate scientific evidence is lacking but widely supported by available data and   expert opinion