Evidence Grades

Organisations like NICE and the Royal Colleges make guidelines for clinicians and produce recommendations after reviewing all the available evidence. Any recommendation should be graded so you can judge how strong the evidence behind it is. The level of evidence behind this should also be specified.

A Grade A recommendation with level 1 evidence means that the evidence behind a recommendation is very strong. A Grade D recommendation with level 5 evidence means that the evidence is very weak.

Grade of Recommendation A Consistent randomised controlled clinical trial evidence or better
Grade of Recommendation B Consistent cohort, or extrapolations from level A studies
Grade of Recommendation C Case series or extrapolations from level B studies
Grade of Recommendation D Expert opinion without explicit critical appraisal


There is an important difference between strength of evidence and strength of effect.

It is perfectly possible to have a grade A evidence based recommendation (based on high quality systematic reviews and meta-analyses) for a treatment with a very modest clinical effect. Equally, there are some things that we do that are supported by only very weak evidence, but we would never change.