Scottish Intercollegiate Guidelines Network(SIGN) grading system for recommendations in evidence based guidelines
| Levels of evidence | ||
| 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 of RCTs, or RCTs with a low risk of bias | |
| 1 | Meta-analyses, systematic reviews or RCTs, or RCTs with a high risk of bias | |
| 2++ | 1) High-quality systematic reviews of case–control or cohort studies, or 2) 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 | Nonanalytical studies, e.g. case reports and case series | |
| 4 | Expert opinion | |
| Grades of recommendations | ||
| A | 1) At least one meta-analysis, systematic review, or RCT rated as 1++ and directly applicable to the target population, or 2) a systematic review of RCTs or a body of evidence consisting principally of studies rated as 1+ directly applicable to the target population and demonstrating overall consistency of results | |
| B | 1) A body of evidence including studies rated as 2++ directly applicable to the target population and demonstrating overall consistency of results, or 2) extrapolated evidence from studies rated as 1++ or 1+ | |
| C | 1) A body of evidence including studies rated as 2+ directly applicable to the target population and demonstrating overall consistency of results, or 2) extrapolated evidence from studies rated as 2++ | |
| D | 1) Evidence level 3 or 4, or 2) extrapolated evidence from studies rated as 2+ |
RCT: randomised controlled trial. Reproduced from 1 with permission from the publisher.