Table 3-1Evidence Levels

Level of evidenceType of evidence
1++
  • High-quality meta-analyses, systematic reviews of RCTs, or RCTs with a 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 (for example, case reports, case series)
4
  • Expert opinion, formal consensus

From: 3, Methods

Cover of Postnatal Care
Postnatal Care: Routine Postnatal Care of Women and Their Babies [Internet].
NICE Clinical Guidelines, No. 37.
National Collaborating Centre for Primary Care (UK).
Copyright © 2006, National Collaborating Centre for Primary Care.

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