Table 1.

Levels of evidence

Level of evidence	Studies on intervention	Studies on prognosis
1	Meta-analysis of randomized controlled trials Good-quality randomized controlled trial	Meta-analysis of inception cohort studies
2	Low-quality randomized controlled trial Good-quality prospective cohort study	Inception cohort study
3	Good-quality retrospective case-control or cohort study	Cohort study or control arm of randomized controlled trial
4	Expert opinion	Case series, case-control study, or poor-quality prognostic cohort study