Table 1.1Levels of evidence for intervention studies13

LevelSource of evidence
1++
1+
1−
2++
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: 1, Introduction

Cover of Long-acting Reversible Contraception
Long-acting Reversible Contraception: The Effective and Appropriate Use of Long-Acting Reversible Contraception.
NICE Clinical Guidelines, No. 30.
National Collaborating Centre for Women’s and Children’s Health (UK).
London: RCOG Press; 2005 Oct.
Copyright © 2005, National Collaborating Centre for Women’s and Children’s Health.

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