| A1 | Meta-analysis containing at least some trials of level A2 and of which the results of individual trials are consistent |
| A2 | Randomized comparative clinical trials of good quality (randomized double-blind controlled trials) of sufficient size and consistency |
| B | Randomized clinical trials of moderate (weak) quality or insufficient size or other comparative trials (non-randomized, cohort studies, patient–control studies) |
| C | Non-comparative trials |
| D | Expert opinion |
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