Table 1

The Oxford Centre for Evidence Based Medicine (CEBM) 2011 Levels of Evidence

Level 1*Level 2*Level 3*Level 4*Level 5
Question:
 How common is the problem?Local and current random sample surveys (or censuses).Systematic review of surveys that allow matching to local circumstances.†Local non-random sample.†Case series.†n/a
 Is this diagnostic or monitoring test accurate? (Diagnosis)Systematic review of cross-sectional studies with consistently applied reference standard and blinding.Individual cross-sectional studies with consistently applied reference standard and blinding.Non-consecutive studies, or studies without consistently applied reference standards.†Case–control studies, or ‘poor or non-independent reference standard’.†Mechanism-based reasoning.
 What will happen if we do not add a therapy? (Prognosis)Systematic review of inception cohort studies.Inception cohort studies.Cohort study or control arm of randomised trial.*Case series or case–control studies, or poor quality prognostic cohort study.†n/a
 Does this intervention help? (Treatment benefits)Systematic review of randomised trials or n-of-1 trials.Randomised trial or observational study with dramatic effect.Non-randomised controlled cohort/follow-up study.†Case series, case–control studies or historically controlled studies.†Mechanism-based reasoning.
 What are the common harms? (Treatment harms)Systematic review of randomised trials, systematic review of nested case–control studies, n-of-1 trial with the patient you are raising the question about or observational study with dramatic effect.Individual randomised trial or (exceptionally) observational study with dramatic effect.Non-randomised controlled cohort/follow-up study (postmarketing surveillance) provided there are sufficient numbers to rule out a common harm. (For long-term harms, the duration of follow-up must be sufficient).†Case series, case–control or historically controlled studies.†Mechanism-based reasoning.
 What are the rare harms?
 (Treatment harms)
Systematic review of randomised trials or n-of-1 trial.Randomised trial or (exceptionally) observational study with dramatic effect.
 Is this (early detection) test worthwhile? (Screening)Systematic review of randomised trials.Randomised trial.Non -randomised controlled cohort/follow-up study.†Case series, case–control or historically controlled studies.†Mechanism-based reasoning.
  • This method was used to grade the levels of evidence of the papers discussed in this systematic review. Levels of evidence range from 1 to 5, with 1 as having the highest quality and 5 as the lowest quality.

  • Adapted from Oxford CEBM Levels of Evidence Working Group.* ‘The Oxford 2011 Levels of Evidence’. Oxford Centre for Evidence-Based Medicine. http://www.cebm.net/index.aspx?o=5653.

  • *OCEBM Levels of Evidence Working Group = Jeremy Howick, Iain Chalmers (James Lind Library), Paul Glasziou, Trish Greenhalgh, Carl Heneghan, Alessandro Liberati, Ivan Moschetti, Bob Phillips, Hazel Thornton, Olive Goddard and Mary Hodgkinson.

  • *Level may be graded down on the basis of study quality, imprecision, indirectness (study PICO does not match questions PICO), because of inconsistency between studies or because the absolute effect size is very small; level may be graded up if there is a large or very large effect size.

  • †As always, a systematic review is generally better than an individual study.