Text Box 4Levels of evidence for intervention studies

LevelType 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
1Meta-analyses, systematic reviews of RCTs or RCTs with a high risk of bias*
2++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
2Case–control or cohort studies with a high risk of confounding bias or chance and a significant risk that the relationship is not causal*
3Non-analytic studies (for example, case reports and case series)
4Expert opinion, consensus methods
*

Studies with a level of evidence ‘−’ should not be used as a basis for making a recommendation. Reproduced with permission from SIGN.

From: 4, METHODS USED TO DEVELOP THIS GUIDELINE

Cover of Drug Misuse
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Leicester (UK): British Psychological Society; 2008.
Copyright © 2008, The British Psychological Society & The Royal College of Psychiatrists.

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