Table 1.

Quality of evidence and downgrade/upgrade factors.

Quality of Evidence	Study Design	Lower if*	Higher if*
⊕⊕⊕⊕ High	Randomised trial	Study limitations - Serious - Very serious Inconsistency - Serious - Very serious Indirectness - Serious - Very serious Imprecision - Serious - Very serious Publication bias - Likely - Very likely	Large effect - Large - Very large Dose response - Evidence of a gradient All plausible confounding - Would reduce a demonstrated effect - Would suggest a spurious effect when results show no effect
⊕⊕⊕Ο Moderate
⊕⊕ΟΟ Low	Observational study
⊕ΟΟΟ Very low