Table 1 Levels of evidence (adapted from [64])

1**

High quality meta-analyses, or systematic reviews of randomised controlled trials (RCTs).

1*

Well conducted meta-analyses, systematic reviews of RCTs.

1-

Meta-analyses, systematic reviews of RCTs or RCTs with a high risk of bias.

2**

High quality systematic reviews of cohort or case-control studies. High quality cohort or case-control studies with a low risk of confounding bias or chance.

2*

Well conducted case-control or cohort studies with a low risk of confounding bias or chance.

2

Case-control or cohort studies with a high risk of confounding bias or chance.

3

Non-analytic studies (case reports, case series).

4

Expert opinion, formal consensus.