Table 1.

GRADE quality assessment criteria

Quality of evidence	Study design	Lower if *	Higher if *
High	Randomised trial	Study quality: -1 Serious limitations -2 Very serious limitations -1 Important inconsistency Directness: -1 Some uncertainty -2 Major uncertainty -1 Sparse data -1 High probability of Reporting bias	Strong association: +1 Strong, no plausible confounders, consistent and direct evidence** +2 Very strong, no major threats to validity and direct evidence*** +1 Evidence of a Dose response gradient +1 All plausible confounders would have reduced the effect
Moderate
Low	Observational study
Very low

* 1 = move up or down one grade (for example from high to intermediate) 2 = move up or down two grades (for example from high to low)

** A statistically significant relative risk of >2 (< 0.5), based on consistent evidence from two or more observational studies, with no plausible confounders

*** A statistically significant relative risk of > 5 (< 0.2) based on direct evidence with no major threats to validity