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Table 3 Classification of evidence

From: Antibiotic prophylaxis in foot and ankle surgery: a systematic review of the literature

Level Source of Evidence
1++ High-quality meta-analyses, systematic reviews of RCTs, or RCTs with very low risk of bias
1+ Well-conducted meta-analyses, systematic reviews of RCTs, or RCTs with a low risk of bias
1− Meta-analyses, systematic reviews of RCTs, or RCTs with a high risk of bias
2++ High-quality systematic reviews of case-control or cohort studies; high-quality case-control or cohort studies with a very low risk of confounding, bias, or chance and a high probability that the relationship is causal
2+ Well-conducted case-control or cohort studies with a low risk of confounding, bias, or chance and a moderate probability that the relationship is causal
2− Case-control or cohort studies with a high risk of confounding, bias, or chance and a significant risk that the relationship is not causal
3 Non-analytical studies (e.g., case reports, case series)
4 Expert opinion, formal consensus