Not all evidence is the same.  Clearly, results from a systematic review of well conducted double-blind randomised controlled trials are much more reliable than anecdotal opinion.

NHMRC Levels of Evidence

The following is the designation used by the Australian National Health and Medical Research Council (NHMRC)^[1]:

Level I

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

Level II

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

Level III-1

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

Level 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.

Level 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.

Level IV

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

Oxford Centre for Evidence Based Medicine

This is the system used by the UK National Health Service (NHS).  The following has been divided into a simplified version of the “grade of recommendation” system first (the oft seen grades A to D) and then the more detailed levels of evidence^[2].

1. Consistent Randomised Controlled Clinical Trial, cohort study, all or none, clinical decision rule validated in different populations.
2. Consistent Retrospective Cohort, Exploratory Cohort, Ecological Study, Outcomes Research, case-control study; or extrapolations from level A studies.
3. Case-series study or extrapolations from level B studies.
4. Expert opinion without explicit critical appraisal, or based on physiology, bench research or first principles.

Levels of Evidence for Therapy/Prevention, Aetiology/Harm

Level 1a

Systematic review with homogeneity(*) of randomised control trials

Level 1b

Individual randomised control trial with narrow confidence interval (studies with wide confidence interval should be tagged with a “-” at the end of their designated level).

Level 1c

All or none (met when all patients died before Rx became available, but some now survive on it; or when some patients died before the Rx became available but none now die on it).

Level 2a

Systematic review with homogeneity(*) of cohort studies

Level 2b

Individual cohort studies;

Low quality randomised control trials (e.g., < 80% follow up)

Level 2c

“Outcomes” Research; Ecological studies

Level 3a

Systematic review with homogeneity(*) of case-control studies

Level 3b

Individual case-control studies

Level 4

Case series;

Poor quality cohort studies (failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same, objective way in both exposed and non-exposed individuals and/or failed to identify or appropriately control known confounders and/or failed to carry out a sufficient long and completely follow up);

Poor quality case control studies (failed to clearly define comparison groups and/or failed to measure exposures and outcomes in the same, objective way in both cases and controls and/or failed to identify or appropriately control known confounders)

Level 5

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

(*) A systematic review free of worrisome variations in the directions and degrees of results between individual studies. Not all systematic reviews with statistically significant heterogeneity need be worrisome, and not all worrisome heterogeneity need be statistically significant. Studies displaying worrisome heterogeneity should be tagged with a “-” at the end of their designated level.

Grade of Recommendation

A: consistent level 1 studies

B: consistent level 2 or 3 studies or extrapolations from level 1 studies

C: level 4 studies or extrapolations from level 2 or 3 studies

D: level 5 evidence or troubling inconsistent or inconclusive studies of any level

References

1. A guide to the development, implementation and evaluation of clinical practice guidelines.  NHMRC, 1999.
2. Levels of Evidence (March 2009).  Centre for Evidence Based Medicine.  Retrieved from www.cebm.net on 5 October 2010.