Levels of evidence

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 pseudo-randomised controlled trials (alternate allocation or some other method).

III-2   evidence obtained from comparative studies with concurrent controls and allocation not randomized (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 and post-test.

These levels of evidence ratings have been adapted from US Preventive Services Task Force (1989), Guide to clinical preventive services: an assessment of the effectiveness of 169 interventions, (ed M Fisher), Williams and Williams, Baltimore, Appendix A, p388.

Source: NHMRC A guide to the development, implementation and evaluation of clinical practice guidelines.