| Grade A | – | Requires level 1 evidence |
| Grade B | – | Requires level 2 or 3 evidence or extrapolations from level 1 trials |
| Grade C | – | Requires level 4 evidence or extrapolations from level 2 or 3 trials |
| Grade D | – | Requires level 5 evidence or conflicting evidence from trials of any other level |
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