In September 2000, the Oxford (UK) CEBM Levels of Evidence published its guidelines for 'Levels' of evidence re claims about prognosis, diagnosis, treatment benefits, treatment harms, and screening. It not only addressed therapy and prevention, but also diagnostic tests, prognostic markers, or harm. The original CEBM Levels was first released for Evidence-Based On Call to make the process of finding evidence feasible and its results explicit. As published in 2009 they are:

1a: Systematic reviews (with homogeneity) of randomized controlled trials

1b: Individual randomized controlled trials (with narrow confidence interval)

1c: All or none randomized controlled trials

2a: Systematic reviews (with homogeneity) of cohort studies

2b: Individual cohort study or low quality randomized controlled trials (e.g. <80% follow-up)

2c: “Outcomes” Research; ecological studies

3a: Systematic review (with homogeneity) of case-control studies

3b: Individual case-control study

4: Case series (and poor quality cohort and case-control studies)

5: Expert opinion without explicit critical appraisal, or based on physiology, bench research or “first principles”