Royal College of General Practitioners (RCGP) Three Star System
*** Strong evidence – provided by generally consistent findings in multiple, high quality scientific studies.
** Moderate evidence – provided by generally consistent findings in fewer, smaller or lower quality scientific studies.
* Limited or contradictory evidence – provided by one scientific study or inconsistent findings in multiple scientific or narrative studies.
- No scientific evidence – based on theoretical considerations.
Revised Scottish Intercollegiate Guidelines Network (SIGN) Grading System: Levels of Evidence
1++ High quality meta analyses, systematic reviews of randomised controlled trials or randomised controlled trials with a very low risk of bias
1+ Well conducted meta analyses, systematic reviews of randomised controlled trials or randomised controlled trials with a low risk of bias
1- Meta analyses, systematic reviews of randomised controlled trials or randomised controlled trials with a high risk of bias
2++ High quality systematic reviews of case-control or cohort or studies. High quality case-control or cohort studies with a very low risk of confounding, bias, or chance and a high probability that the relationship is causal
2+ Well conducted case control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal
2- Case control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal
3 Non-analytic studies (e.g. case reports, case series)
4 Expert opinion