Table 1.

Determination of the Quality of Evidence

• Underlying methodology
A	RCT
B	Downgraded RCT or upgraded observational studies
C	Well-done observational studies
D	Case series or expert opinion
• Factors that may decrease the strength of evidence
1.	Poor quality of planning and implementation of available RCTs suggesting high likelihood of bias
2.	Inconsistency of results (including problems with subgroup analyses)
3.	Indirectness of evidence (differing population, intervention, control, outcomes, comparison)
4.	Imprecision of results
5.	High likelihood of reporting bias
• Main factors that may increase the strength of evidence
1.	Large magnitude of effect (direct evidence, relative risk (RR) > 2 with no plausible confounders)
2.	Very large magnitude of effect with RR > 5 and no threats to validity (by two levels)
3.	Dose response gradient

RCT, randomized controlled trial; RR, relative risk