Table 1

Individual studies included in the guideline are formally assessed using the GRADE system1 with the quality of the body evidence rated, from ‘high’ (A) to ‘very low’ (D)

Modify the initial quality of a body of evidence
Study designInitial quality of a body of evidenceRate lower ifRate higher ifQuality of body of evidence
Randomised controlled trialsHigh★★★★Risk of bias
−1 Serious
−2 Very serious
Large effect
+1 Large
+2 Very large
High (A)★★★★
−1 Serious
−2 Very serious
Dose response
+1 Evidence of a gradient
Moderate (B)★★★
Observational studies/qualitative studiesLow★★Indirectness
−1 Serious
−2 Very serious
All plausible residual confounding
−1 Serious
−2 Very serious
+1 would reduce a demonstrated effectLow (C)★★
Publication bias
−1 Likely
−2 Very likely
+1 would suggest a spurious effect if no effect observedVery low (D) ★