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