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Table 2.2Levels of evidence for intervention studies

LevelSource of evidence
1++High-quality meta-analyses, systematic reviews of randomised controlled trials (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
3Non-analytical studies (e.g. case reports, case series)
4Expert opinion, formal consensus

From: 2, Development of the guideline

Cover of Hypertension in Pregnancy
Hypertension in Pregnancy: The Management of Hypertensive Disorders During Pregnancy.
NICE Clinical Guidelines, No. 107.
National Collaborating Centre for Women's and Children's Health (UK).
London: RCOG Press; 2010 Aug.
Copyright © 2011, Royal College of Obstetricians and Gynaecologists.

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