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KEY TO EVIDENCE STATEMENTS AND GRADES OF RECOMMENDATIONS
LEVELS 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, or RCTs with a low risk of bias
1 - Meta-analyses, systematic reviews, 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 or bias and a high probability that the
relationship is causal
2 + Well conducted case control or cohort studies with a low risk of confounding or bias and a moderate probability that the
relationship is causal
2 - Case control or cohort studies with a high risk of confounding or bias and a significant risk that the relationship is not causal
3 Non-analytic studies, eg case reports, case series
4 Expert opinion
GRADES OF RECOMMENDATION
Note: The grade of recommendation relates to the strength of the evidence on which the recommendation is based. It does not reflect the
clinical importance of the recommendation.
A
B
C
D
At least one meta-analysis, systematic review, or RCT rated as 1 ++ ,
and directly applicable to the target population; or
A body of evidence consisting principally of studies rated as 1 + ,
directly applicable to the target population, and demonstrating overall consistency of results
A body of evidence including studies rated as 2 ++ ,
directly applicable to the target population, and demonstrating overall consistency of results; or
Extrapolated evidence from studies rated as 1 ++ or 1 +
A body of evidence including studies rated as 2 + ,
directly applicable to the target population and demonstrating overall consistency of results; or
Extrapolated evidence from studies rated as 2 ++
Evidence level 3 or 4; or
Extrapolated evidence from studies rated as 2 +
GOOD PRACTICE POINTS
Recommended best practice based on the clinical experience of the guideline development group
NHS Evidence has accredited the process used by Scottish Intercollegiate Guidelines
Network to produce guidelines. Accreditation is valid for three years from 2009 and is
applicable to guidance produced using the processes described in SIGN 50: a guideline
developer’s handbook, 2008 edition (www.sign.ac.uk/guidelines/fulltext/50/index.
html). More information on accreditation can be viewed at
www.evidence.nhs.uk
Healthcare Improvement Scotland (HIS) is committed to equality and diversity and assesses all its publications for likely impact on the
six equality groups defined by age, disability, gender, race, religion/belief and sexual orientation.
SIGN guidelines are produced using a standard methodology that has been equality impact assessed to ensure that these equality
aims are addressed in every guideline. This methodology is set out in the current version of SIGN 50, our guideline manual, which
can be found at www.sign.ac.uk/guidelines/fulltext/50/index.html. The EQIA assessment of the manual can be seen at www.
sign.ac.uk/pdf/sign50eqia.pdf. The full report in paper form and/or alternative format is available on request from the Healthcare
Improvement Scotland Equality and Diversity Officer.
Every care is taken to ensure that this publication is correct in every detail at the time of publication. However, in the event of errors
or omissions corrections will be published in the web version of this document, which is the definitive version at all times. This version
can be found on our web site www.sign.ac.uk.
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