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Table 2 Overview of the GRADE system for grading the quality of evidence (Adapted from Reference[7]): A) Criteria for assigning grade of evidence; B) Definitions in grading the quality of evidence.

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

A)  
Criteria for assigning level of evidence
Type of Evidence  
   Randomized trial High
   Observational study Low
   Any other type of research evidence Very low
Increase level if:  
   Strong association (+1)
   Very strong association (+2)
   Evidence of a dose response gradient (+1)
   Plausible confounders reduced the observed effect (+1)
Decrease level if:  
   Serious or very serious limitations to study quality (-1) or (-2)
   Important inconsistency (-1)
   Some or major uncertainty about directness (-1) or (-2)
   Imprecise or sparse data* (-1)
   High probability of reporting bias (-1)
B)  
Definitions for levels of evidence
High Further research is not likely to change our confidence in the effect estimate
Moderate Further research is likely to have an important impact on our confidence in the estimate of effect and may change the estimate
Low Further research is very likely to have an important impact on our confidence in the estimate of effect and is likely to change the estimate
Very Low Any estimate of effect is uncertain
  1. *Few outcome events or observations or wide confident limits around an effect estimate