Table 1. Levels of evidence and grades of recommendations according to the Scottish Intercollegiate Guidelines Network (SIGN) (e15).

Level of evidence		Grade of recommendation
	Requirements		Requirements
1++	High quality meta-analyses, systematic reviews of RCTs, or RCTs with a very low risk of bias	A	At least one meta-analysis, systematic review, or RCT rated as 1++, and directly applicable to the target population; or
			A body of evidence consisting principally of studies rated as 1+, directly applicable to the target population, and demonstrating overall consistency of results
1 +	Well-conducted meta-analyses, systematic reviews, or RCTs with a low risk of bias
1 –	Meta-analyses, systematic reviews, or RCTs with a high risk of bias
		B	A body of evidence including studies rated as 2++, directly applicable to the target population, and demonstrating overall consistency of results; or Extrapolated evidence from studies rated as 1++ or 1+
2++	High quality systematic reviews of case control or cohort or studies
	High quality case control or cohort studies with a very low risk of confounding or bias and a high probability that the relationship is causal	C	A body of evidence including studies rated as 2+, directly applicable to the target population and demonstrating overall consistency of results; or Extrapolated evidence from studies rated as 2++
2 +	Well-conducted case control or cohort studies with a low risk of confounding or bias and a moderate probability that the relationship is causal
2 –	Case control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal	D	Evidence level 3 or 4; or
			Extrapolated evidence from studies rated as 2+
3	Non-analytic studies, e.g. case reports, case series
4	Expert opinion