Table 2.1Criteria for grading evidence and recommendations

Note that diagnostic study levels of evidence and classification of recommendations were also included.16

Levels of evidenceClassification of recommendations
LevelType of evidenceClassEvidence
1++High-quality meta-analysis (MA), systematic reviews (SR) of randomised controlled trials (RCTs), or RCTs with a very low risk of bias.ALevel 1++ and directly applicable to the target population

or

level 1+ and directly applicable to the target population AND consistency of results.
Evidence from NICE technology appraisal.
1+Well-conducted MA, SR or RCTs, or RCTs with a low risk of bias.
1−MA, SR of RCTs, or RCTs with a high risk of bias.Not used as a basis for making a recommendation.
2++High-quality SR 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.BLevel 2++, directly applicable to the target population and demonstrating overall consistency of results.

or

extrapolated evidence from 1++ or 1+.
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 causalNot used as a basis for making a recommendation.
3Non-analytic studies (for example case reports, case series).CLevel 2+, directly applicable to the target population and demonstrating overall consistency of results

or

extrapolated evidence from 2++.
4Expert opinion, formal consensus.DLevel 3 or 4

or

extrapolated from 2+

or

formal consensus.
D (GPP)A good practice point (GPP) is a recommendation based on the experience of the GDG.

Diagnostic study level of evidence and classification of recommendation was also included.2

From: 2, Methodology

Cover of Atrial Fibrillation
Atrial Fibrillation: National Clinical Guideline for Management in Primary and Secondary Care.
NICE Clinical Guidelines, No. 36.
National Collaborating Centre for Chronic Conditions (UK).
Copyright © 2006, Royal College of Physicians of London.

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