Table 2.

Levels of evidence

1++	High-quality meta-analyses and systematic reviews of randomized clinical trials with very low risk of bias; single randomized clinical trials with a very low risk of bias
1+	Well-conducted meta-analyses and systematic reviews of randomized clinical trials with low risk of bias; single randomized clinical trials with low risk of bias
1-	Meta-analyses and systematic reviews of randomized clinical trials with a high risk of bias; single randomized clinical trials with a high risk of bias.
2++	High-quality systematic reviews, related to case-control or cohort studies; high-quality case-control or cohort studies with a very low risk of confounding or bias and a high probability of causality
2+	Well-conducted case-control or cohort studies with a low risk of confounding or bias and a moderate likelihood of causality
2-	Case-control or cohort studies with a high risk of confounding or bias and a significant risk of non-causality
3	Non-analytical studies, e.g., case report and case series
4	Expert opinion