Table 1

Levels of evidence

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
3Non-analytical studies (for example, case reports, case series)
4Expert opinion, formal consensus

From: 3, Methods

Cover of Low Back Pain
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NICE Clinical Guidelines, No. 88.
National Collaborating Centre for Primary Care (UK).
Copyright © 2009, Royal College of General Practitioners.

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