Table 4 NICE hierarchy of evidence and recommendation grading scheme

1

Evidence obtained from a single randomised controlled trial or a meta-analysis of randomised controlled trials

A

At least one randomised controlled trial as part of a body of literature of overall good quality and consistency addressing the specific recommendation (evidence level I) without extrapolation

2

Evidence obtained from at least one well-designed controlled study without randomization or Evidence obtained from at least one other well-designed quasiexperimental study

B

Well-conducted clinical studies but no randomised clinical trials on the topic of recommendation (evidence levels II or III); or extrapolated from level I evidence

3

Evidence obtained from well-designed non-experimental descriptive studies, such as comparative studies, correlation studies

4

Evidence obtained from expert committee reports or opinions and/or clinical experiences of respected authorities

C

Expert committee reports or opinions and/or clinical experiences of respected authorities (evidence level IV). This grading indicates that directly applicable clinical studies of good quality are absent or not readily available