Table 1

Overview of a simplified and traditional hierarchy for grading the quality of evidence and strength of recommendations

Levels of Evidence
     Level I
Well conducted, suitably powered RCT
     Level II
Well conducted, but small and under-powered RCT
     Level III
Non-randomized observational studies
     Level IV
Non-randomized study with historical controls
     Level V
Case series without controls
Grades of recommendations
     Grade A
Level I
     Grade B
Level II
     Grade C
Level III or lower

Levels of evidence are for an individual research investigation. Grading of recommendations is based on levels of evidence. Adapted from [1,2]. RCT, randomized controlled trial.

Bellomo and Bagshaw Critical Care 2006 10:232   doi:10.1186/cc5045