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.

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

*Few outcome events or observations or wide confident limits around an effect estimate