Evidence is arranged into levels or a hierarchy (sometimes displayed as a pyramid), and studies are assigned to different levels based on the methodological quality of their design, and validity. There are several evidence rating scales available to use, including:
| Evidence Level | Study Type |
| One | Systematic review or meta-analysis of all relevant randomized trials (RCTs) |
| Two | Well-designed RCTs |
| Three | Well-designed controlled trials without randomization |
| Four | Well-designed case-control and cohort studies |
| Five | Systematic review of descriptive and qualitative studies |
| Six | A single descriptive or qualitative study |
| Seven | The opinion of authorities and/or reports of expert committees |