TABLE I.

Levels of evidence¹

Level	Meaning
i	Evidence from one or more randomized controlled trials
ii-1	Evidence from one or more controlled trials without randomization
ii-2	Evidence from cohort or case–control analytic studies, preferably from more than one centre or research group
ii-3	Evidence from comparisons between times or places with or without the intervention; dramatic results in uncontrolled experiments could be included here
iii	Opinions of respected authorities, based on clinical experience; descriptive studies or reports of expert committees