Centre for Evidence-based Medicine Levels of Evidence
| Level | Types of study |
| 1 | Randomised controlled trials |
| 2 | Cohort studies |
| 3 | Case–control studies |
| Poor quality estimates of data that include sensitivity analyses incorporating clinically sensible variations | |
| 4 | Case-series studies |
| 5 | Case reports Studies that do not provide quantitative evidence to support a hypothesis |