Table 1. Level of evidence and strength of recommendation.

Level of evidence	Type of evidence
1++	High-quality meta analyses, high-quality systematic reviews of clinical trials with very little risk of bias
1+	Well-conducted meta-analyses, systematic review of clinical trials or well-conducted clinical trials with low risk of bias
1-	Meta-analyses, systematic reviews of clinical trials or clinical trials with high risk of bias
2++	High-quality systematic reviews of cohort or case and control studies; cohort or case and control studies with very low risk of bias and high probability of establishing a causal relationship
2+	Well-conducted cohort or case and control studies with low risk of bias and moderate probability of establishing a causal relationship
2-	Cohort or case control studies with high risk of bias and significant risk that the relationship is not causal*
3	Nonanalytical studies, such as case reports and case series.
4	Expert opinion.

NICE, National Institute for Health and Care Excellence.