(Click here to see the levels of evidence and grades of recommendation table)

Levels of Evidence and Grades of Recommendation Table

Study criteria Level of evidence Criteria for grading of recommendation Grade of recommendation
Systematic review (with homogeneity) of randomised, controlled trials 1a Consistent level 1 studies A
Individual, randomised, controlled trials with statistically significant results 1b
All or none, i.e. prior to availability of new therapy, all died, now with therapy some survive; or, prior to therapy some died, now with therapy none die 1c
Systematic review (with homogeneity) of cohort studies 2a Consistent level 2 or 3 studies (or extrapolations* from level 1 studies) B
Individual cohort study (including low quality randomised controlled trial, e.g. <80% follow up) 2b
Outcomes research 2c
Systematic review (with homogeneity) of case-controlled studies 3a
Individual case-controlled study 3b
Case-series, and poor quality cohort and case-controlled studies 4 Level 4 studies (or extrapolations* from level 2 or 3 studies) C
Expert opinion without explicit critical appraisal, or based on physiology, bench research or first principles 5 Level 5 evidence (or troublingly inconsistent or inconclusive studies of any level) D
*Extrapolations are where data are used in a situation that has potentially clinically important differences to the original study situation. In the case of PROSPECT, extrapolation largely refers to transferable evidence.