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 (for example case reports, case series)
4Expert opinion, formal consensus

From: 2, Introduction

Cover of Constipation in Children and Young People
Constipation in Children and Young People: Diagnosis and Management of Idiopathic Childhood Constipation in Primary and Secondary Care.
NICE Clinical Guidelines, No. 99.
National Collaborating Centre for Women's and Children's Health (UK).
London: RCOG Press; 2010.
Copyright © 2010, National Collaborating Centre for Women's and Children's Health.

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