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 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 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, p 388.

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

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