Key to evidence statements

The level of evidence provided by a piece of research is determined by the combination of the study design and its methodological and analytical quality.

The hierarchy of evidence outlined in the table below is modified from that used by the Scottish Intercollegiate Guidelines Network (SIGN). During the appraisal process, we assigned a level of evidence to each study appraised. 

Level

Description of studies meeting this level

1++

High quality meta-analyses, systematic reviews of RCTs*, or RCTs with a very low risk of bias.

1+

Well-conducted meta-analyses, systematic reviews, or RCTs with a low risk of bias.

1-

Meta-analyses, systematic reviews, or RCTs with a moderate to high risk of bias.

2++

High quality systematic reviews of case control, cohort or analytical cross-sectional studies.
High quality case control, cohort and analytical cross-sectional studies with a very low risk of confounding or bias and a high probability that the relationship is causal.

2+

Well-conducted case control, cohort or analytical cross-sectional studies with a low risk of confounding or bias and a moderate probability that the relationship is causal.

2-

Case control, cohort or analytical cross-sectional studies with a high risk of confounding or bias and a significant risk that the relationship is not causal.

3

Non-analytic studies, e.g. case reports, case series and descriptive cross-sectional studies.

4

Expert opinion

 *RCT: Randomised controlled trial