Table 7– Grading of evidences for recommendations according to the Scottish Intercollegiate Guidelines Network

Study rating	Study design	Requirements	Target population	Grades of recommendation
1++	High-quality meta-analyses, systematic reviews of RCTs or RCTs with a very low risk of bias	≥1 study	Directly applicable	A
1+	Well-conducted meta-analyses, systematic reviews, or RCTs with a low risk of bias	Studies with overall consistency	Directly applicable	A
1++/1+	As above	Studies with overall consistency	Extrapolated	B
2++	High-quality case–control/cohort studies or their systemic reviews, with very low risk of confounding/bias and high probability of causal relationship	Studies with overall consistency	Directly applicable	B
2++		Studies with overall consistency	Extrapolated	C
2+	Well-conducted case–control/cohort studies with low risk of confounding/bias and a moderate probability of causal relationship	Studies with overall consistency	Directly applicable	C
2+		Studies with overall consistency	Extrapolated	D
3	Nonanalytic studies, e.g. case reports, case series
4	Expert opinion