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Ia
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Evidence from a systematic review or meta-analysis of randomized controlled trials (RCT’s)
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Ib
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Evidence from at least one RCT
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IIa
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Evidence from at least one well-designed controlled study without randomization
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IIb
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Evidence from at least one well-designed quasi-experimental study, such as a cohort study
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III
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Evidence from a well-designed non-experimental descriptive studies, such as comparative studies, correlation studies, case–control studies and case series
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IV
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Expert committee reports, opinions and/or clinical experience of respected authorities
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