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Table 1 Summary of a simplified evidence hierarchy, A) Levels of evidence across clinical research studies, and B) Grading of recommendations based on levels of evidence (Adapted from [2, 3])

From: The need to reform our assessment of evidence from clinical trials: A commentary

A)  
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
B)  
Grades of Recommendations
Grade A Level I
Grade B Level II
Grade C Level III or lower