Categories of evidence
| Characteristics | Confidence | |
| High | Based on consistent results from well-performed randomised controlled trials | Further research is very unlikely to change the estimate of the effect |
| Moderate | Based on randomised controlled trials where there is evidence of bias, or from other well-conducted study types (eg, well-executed observational studies) | Further research is likely to have an impact on the estimate of the effect |
| Low | Based on observational evidence, or from controlled trials with several serious limitations | Further research is likely to have an important impact |
| Very low | Based on case studies or expert opinion | Estimates of effect are far from certain and more research is needed |