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Table 1 GRADE quality assessment criteria

From: Improving the use of research evidence in guideline development: 9. Grading evidence and recommendations

Quality of evidence Study design Lower if * Higher if *
High Randomised trial Study quality:
-1 Serious limitations
-2 Very serious limitations
-1 Important inconsistency
-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
Low Observational study   
Very low    
  1. * 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)
  2. ** A statistically significant relative risk of >2 (< 0.5), based on consistent evidence from two or more observational studies, with no plausible confounders
  3. *** A statistically significant relative risk of > 5 (< 0.2) based on direct evidence with no major threats to validity