Ia Meta-analysis of RCTs | | A Category I evidence |
Ib RCT | | |
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IIa Controlled study without randomisation | | B Category II evidence or extrapolated from category I evidence |
IIb Quasi-experimental study | | |
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III Non-experimental descriptive studies, such as comparative, correlation, and case-control studies | | C Category III evidence or extrapolated from category I or II evidence |
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IV Expert committee reports or opinion or clinical experience of respected authorities, or both | | D Category IV evidence or extrapolated from category II or III evidence |