Table 1

Designations of levels of evidence [1]

Level of evidence

Study design

I

Evidence obtained from a systematic review of all relevant randomised controlled trials

II

Evidence obtained from at least one properly-designed randomised controlled trial

III-1

Evidence obtained from well-designed pseudorandomised controlled trials (alternate allocation or some other method)

III-2

Evidence obtained from comparative studies (including systematic reviews of such studies) with concurrent controls and allocation not randomised, cohort studies, case-control studies, or interrupted time series with a control group

III-3

Evidence obtained from comparative studies with historical control, two or more single arm studies, or interrupted time series without a parallel control group

IV

Evidence obtained from case series, either post-test or pre-test/post-test

Merlin et al. BMC Medical Research Methodology 2009 9:34   doi:10.1186/1471-2288-9-34